An operational amplifier circuit need help - Page 1
Forum section: electronics / beginners

author: daisizhou
author_rank: Super Contributor
timestamp: June 23, 2025, 09:50:24 pm
content: Hallo everyoneI am trying to calibrate and repair a pressure sensor differential amplifier circuit board.The PCB board and schematic diagram are shown in the following figure.Question 1-R1116(ZERO)， What does “zero” mean here? How should I calibrate correctlyQuestion 2-I measured the voltage at both ends of C1105 and found significant fluctuations,After removing LM307, the voltage at both ends of C1105 stabilized. Does this indicate that LM307 has been damaged?

author: dietert1
author_rank: Super Contributor
timestamp: June 24, 2025, 05:53:56 am
content: If you had an image of the PCB bottom we could check whether U1103 really drives Ground. And some other things.

author: daisizhou
author_rank: Super Contributor
timestamp: June 24, 2025, 06:48:42 am
content: Quote from: dietert1 on June 24, 2025, 05:53:56 amThanks.I tried to draw a circuit diagram, but I'm not very good at analyzing circuit principles.The model of the pressure sensor is 125pc05d1. The operating voltage is 0-10V, the maximum is no more than 12V, the operating current is 0-2ma, the maximum is no more than 2.4ma, please refer to the data sheet for other parameters

author: dietert1
author_rank: Super Contributor
timestamp: June 24, 2025, 08:30:53 am
content: So the green ground appears to be a virtual ground. And R1120 determines its relation to the supply rails. As soon as you connect a meter or a scope to the virtual ground, unexpected things may happen. Wondering why they spent an extra opamp to define a virtual ground: Maybe to make re-engineering a bit more difficult. Or it's a complicated way of trimming the bridge voltage. I don't know.If i wanted to measure this circuit, i would add a capacitor of 1 uF between supply ground and virtual ground, maybe with a small resistor in series, like 22 Ohm.The voltage between supply ground and virtual ground should be less than 2 or 3 V. The bridge supply voltage of the pressure sensor should be stable relative to virtual ground.Regards, Dieter

author: daisizhou
author_rank: Super Contributor
timestamp: June 24, 2025, 08:43:59 am
content: Quote from: dietert1 on June 24, 2025, 08:30:53 amThe green ground is actually a common terminal.The actual circuit connection diagram is shown in "RT-203 FLOW MODULE-SCH_Schematic1_2025-05-24".The "allesCaput" schematic was drawn for easier readingI don't know why, as long as there is LM307 in the circuit, the voltage across C1105 will always fluctuate.How can I measure? To ensure accurate analysis

author: daisizhou
author_rank: Super Contributor
timestamp: June 24, 2025, 09:05:06 am
content: This is an RT-200 gasflow analyzer, the A3A3A2 motherboard inside the RT-203 module.manual:https://archive.org/details/manual_Timeter_RT-200_Calibration_Analyzer_Operator_and_Service_ManualThe fault phenomenon is that the error code "HELP-0" is displayed,In other words, the V/F converter cannot stabilize in the range of 250-500.HI-LO voltage is always fluctuating,I don't know where the problem isPlease watch my test video

author: dietert1
author_rank: Super Contributor
timestamp: June 24, 2025, 09:41:59 am
content: Did you try the proposed method to support measurements using a 1 uF+22 Ohm snubber?Can you show your measurement?What is the voltage difference between supply ground "LO" and the virtual ground? The reference can fluctuate if it bounces into the positive supply, just as an example.Are the supply voltages noted in the schematic measured values relative to "LO"?Regards, Dieter

author: daisizhou
author_rank: Super Contributor
timestamp: June 24, 2025, 10:11:41 am
content: Quote from: dietert1 on June 24, 2025, 09:41:59 amDo you mean to connect the 1UF capacitor and the 22 ohm resistor in series, then connect them to the negative terminal of the digital multimeter, and then connect them to the common terminal of C1105, and connect the positive terminal of the digital multimeter to the other port of C1105, and measure it in this way?I will retake the video, is it better to measure with a digital multimeter or a digital oscilloscope?Quote from: dietert1 on June 24, 2025, 09:41:59 amPlease see the picture below

author: Terry Bites
author_rank: Super Contributor
timestamp: June 24, 2025, 10:31:36 am
content: I'm still not sure that I believe this schematic is entirely correct. Or which version is correct.Also, this discussion seems to be running in two parallel threads. What's going on with that?+19.8 and -8.2 supply rails? Yet another crazy complication and it limits the negative swing of U1103.1 Is it struggling to reach its goal. It's marginal given the worst case output swing specifications of this opamp.Where does "LO" come from and where is the rest of the schematic? What does the signal "LO" measure with respect to ground, what is the voltage, is it stable, is it free of noise and AC?If you dont have a scope, set your DVM to AC mV.Try this:Carefully bend up pin 3 of U1103.1 so you can plug it back into it's socket without it being connected to LO. Now conect pin 3 to ground. Pin 6 of U1103.1 should read a stable -5V and pin 6 of the REF01 should read a stable +5V. If this so, then the REF01 and U1103.1 are working correctly.If this is not so, the issue is either with this opamp or with the source of the mysterious "LO".Does it connect to the VCF?A circuit with two reference sources seems like a recipe for error.The LM346 is not a conventional opamp, it's a current controlled amplifier. What does it do here I wonder, is it part of a temperature compensation loop?Then again, It is usually not necessary to temeprature compensate a resistive bridge sensor as all elements drift by the same amount.You suggested that this opamp may be dammaged so get a new opamp. Its not unkown for opamps to go bad over time.The LM307 is seriously obsolete , maybe upgrade to an OP77 or OP177, this will not alter or adversely affect any circuit functions.If that doesn't fix your drift problems then the issue is probably in the source of the signal "LO" and it needs to be investigated. Where does it come from?Does it look clean on an oscilloscope trace?Let’s redraw a little to clarify how this this circuit operates:I've ignored the effect of the voltage LO for now.R1, R2 and X1 create a virtual ground equal half of the REF01 output voltage, ie ±5V.I’ve called these voltages REF+ and REF- I've used my labels and component names. LO shifts the balance.You can check this function. REF+ should be 5V+LO and REF- should be -5V+LO. eg if LO=0.1V then they would read -4.9V and +5.1VAll above voltages referenced to ground (not public ground).Why create a pair of balanced voltages offset by "LO" and then sum them at U1102, that just regenerates LO plus some gain.My head hurts."Trim" allows for adjustment of the REF01 output voltage. From the datasheet this is about ±6%Say, 9.94 to 10.06V. The current into the REF01 ground or supply pin should be about 5mA plus what ever the sensor requires.It is an overly complex circuit for sure, designed by an accademic and not a real world engineer I'd bet. All those trimpots are a give away!!

author: dietert1
author_rank: Super Contributor
timestamp: June 24, 2025, 10:43:40 am
content: If i look at the OPs schematic i think your R4 should be connected to the virtual ground (Ref-), not to ground. The same applies to your R10 (R1112).The OP posted a measurement of virtual ground (Ref-) being 8.86 V above ground. Something is broken.The virtual ground is driven by opamp U1103. If you connect a 1 meter measurement cable there or the ground clip of a scope, that may be enough to cause instability or even oscillation. In order to get valid measurements i proposed a snubber between ground and virtual ground. The snubber may help anyway, i mean it can remain.

author: daisizhou
author_rank: Super Contributor
timestamp: June 24, 2025, 11:09:45 am
content: Quote from: Terry Bites on June 24, 2025, 10:31:36 amYes。The circuit is exactly as you said.The "Trim" button can adjust the output voltage of REF01, the range is very small, that is, R1123 (VB SET) in the original circuit. I think this potentiometer is used to adjust the working voltage setting of the pressure sensor."Zero" control setting,That is how to adjust R1116 (ZERO):Connect the black probe of the digital multimeter to the real ground,The red test lead measures Ref+ and Ref- respectively,Adjust R1116 (zero) until the REF+ and REF- voltages are equal but opposite in polarity, is that right?The second amplifier I'm sure is a LM346N.This is the configuration the manufacturer shipped with, I did not change itThe final signal output voltage HI-LO is always fluctuating, which looks like thermal drift. Do you know where the problem is? Thanks

author: daisizhou
author_rank: Super Contributor
timestamp: June 24, 2025, 11:42:19 am
content: Hallo.I measured REF+ and REF- as you said,Connect the negative terminal of the digital multimeter to the real ground,REF+ is 17.5v, REF- is 8.78v。I tried adjusting R1116 (ZERO), which is R5 (zero) in your diagram, and I found that REF+ and REF- changed at the same time. Is this normal?

author: iMo
author_rank: Super Contributor
timestamp: June 24, 2025, 11:52:12 am
content: .. if you will create a new thread on the same topic in a different section, you (and people trying to help you) will start again and again from scratch. You made some investigation here already:https://www.eevblog.com/forum/projects/is-it-normal-for-the-voltage-fluctuation-of-ref01-reference-circuit-to-be-150mv/msg5948848/#msg5948848

author: dietert1
author_rank: Super Contributor
timestamp: June 24, 2025, 11:54:12 am
content: As i wrote before your measurements may all be invalid as long as you don't check the circuit for instability/oscillation.Anyway: It seems Ref+ is bouncing into the positive supply and it cannot output its 10 V, but only 17.5 - 8.78 = 8.72 V. The reference chip needs more headroom to work properly. If it cannot be adjusted to +/- 5 V something is broken.

author: daisizhou
author_rank: Super Contributor
timestamp: June 24, 2025, 11:59:54 am
content: Quote from: dietert1 on June 24, 2025, 11:54:12 amWhere do I start checking the steps? I'm a bit confused

author: iMo
author_rank: Super Contributor
timestamp: June 24, 2025, 12:02:20 pm
content: From the above link he measured Vcc = 29.18V against "the real ground".So the REF01 should get enough voltage, imho (provided his measurement is ok).I recommended him to focus on the first issue - why the REF01 gives 8.54V output.. One reason could be 6-7mA load..PS: the below voltages are against the "virtual ground" (green - the LM307 output)..

author: daisizhou
author_rank: Super Contributor
timestamp: June 24, 2025, 12:08:24 pm
content: Quote from: iMo on June 24, 2025, 12:02:20 pmThanks.As I captured in the video above,The large load appears to be coming from the LM307,I can't tell if the LM307 is damaged.

author: iMo
author_rank: Super Contributor
timestamp: June 24, 2025, 12:13:21 pm
content: REF01's load - look at your schematics - 8.54V ref01 output loaded with 4.22k and 1.95k and 20k and 18.3k in parallel. Calculate the current..And measure the voltages between pin 2 and pin 4 as well as between pin 6 and pin 4 of the REF01. Triple check the measurement..

author: daisizhou
author_rank: Super Contributor
timestamp: June 24, 2025, 12:30:51 pm
content: Quote from: iMo on June 24, 2025, 12:13:21 pmThe calculated result is 7.28ma, right?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 24, 2025, 12:33:30 pm
content: I made a little start to put this thing in KiCad, but I also quickly got a bit bored and distracted.What I did was:Download the images, did some perspective correction in Gimp (It's better to start with "flat" images).Mirrored the bottom PCB in Gimp. (Also with the perspective correction tool).Start a KiCad project, imported images of schematic, front and back of the PCB.Partially drew over the schematic image, Assigned some footprints.Put the footprint on the PCBImported the images of the PCB into the PCB editor.Placed some of the footprints and roughly scaled the images (I don't have the real PCB size).It's not much, just enough to get an idea of what can be done in KiCad. Doing the whole thing would be quite a lot of work. If you want to have a go at it, I attached the whole project (In KiCad V8 format). It looks like you only drew about half of the schematic. The advantage of the method as shown in the KiCad project below is that you can switch between schematic & PCB, and you can see what has been done, and what still has to be done. Mistakes are also easily caught and corrected.

author: iMo
author_rank: Super Contributor
timestamp: June 24, 2025, 12:36:30 pm
content: Quote from: daisizhou on June 24, 2025, 12:30:51 pmAnd measure the voltages between pin 2 and pin 4 as well as between pin 6 and pin 4 of the REF01 again. Triple check the measurement..

author: daisizhou
author_rank: Super Contributor
timestamp: June 24, 2025, 12:54:28 pm
content: Quote from: iMo on June 24, 2025, 12:36:30 pmREF01 2-4 is 19.70v 6-4 is 8.59v (slowly rising)

author: iMo
author_rank: Super Contributor
timestamp: June 24, 2025, 12:58:25 pm
content: Is the REF01 getting hot? The power loss could be 80mW therefore it could cause its output voltage moves by 150mV after start.. (the issue from the previous thread -https://www.eevblog.com/forum/projects/is-it-normal-for-the-voltage-fluctuation-of-ref01-reference-circuit-to-be-150mv/msg5948848/#msg5948848).Unless you will fix the REF01 issue it has none sense to mess with other stuff.Your REF01 should show nice and stable 10.00V (my current understanding of its very usage there)..PS: also triple check your schematics and the real values of the parts on the pcb..

author: daisizhou
author_rank: Super Contributor
timestamp: June 24, 2025, 01:25:00 pm
content: Quote from: iMo on June 24, 2025, 12:58:25 pmREF01 I didn't feel any heat when I touched it with my fingers.Do you mean try replacing a REF01?By the way, I tested REF01 alone, and the voltage can only reach 9.97vDo you mean the resistor values ​​shown in the picture below? Do you need me to measure the exact values ​​offline? I will disassemble them for measurement if necessary.

author: iMo
author_rank: Super Contributor
timestamp: June 24, 2025, 04:04:51 pm
content: If you have got an oscilloscope try to watch the output of the REF01, it may oscillate.Resistors - no need to desolder them, just measure them aproximately when the board is switched off (no need to have exact values - only check them for a larger deviation, a short or a disconnect, etc.).How did you check the REF01 alone? Have you tried with 19.7V input and say 6mA output load? Have you used that 20k trimmer for setting the output voltage?

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 24, 2025, 07:28:52 pm
content: Quote from: dietert1 on June 24, 2025, 05:53:56 amIt's well know that you just have to have a long enough lever - and you can move even the Earth. :-)QuoteLever history

author: daisizhou
author_rank: Super Contributor
timestamp: June 24, 2025, 10:26:48 pm
content: Quote from: iMo on June 24, 2025, 04:04:51 pmPlease see the C1105 test video below,The REF01 output is checked using an oscilloscope and does not fluctuate.Quote from: iMo on June 24, 2025, 04:04:51 pmI did a rough measurement of the resistance and found nothing unusual.Quote from: iMo on June 24, 2025, 04:04:51 pmI conducted two types of REF01 tests.One is to disconnect REF01 from the PCB and connect an external 18.1V power supply to the second pin as shown in the video above,Use a digital multimeter to measure the output voltage of pin 6, and the voltage tested is 9.963V.The other is to connect as shown in the figure "66" below, "88" is the physical connection diagram,and then adjust the adjustable resistor, the output voltage is between 9.74v-9.96v

author: daisizhou
author_rank: Super Contributor
timestamp: June 24, 2025, 11:48:32 pm
content: Quote from: Terry Bites on June 24, 2025, 10:31:36 amThe schematic "RT-203 FLOW MODULE-SCH_Schematic1_2025-05-24" is reverse-engineered from the actual PCB. The connections are correct, but it is difficult to read.So someone helped me draw the "allesCaput" schematic, which is easier to read, but some details are confusingly labeled, such as: resistor numbers, resistance values...The two parallel threads are because I still can't completely solve the problem, so I create a new thread againQuote from: Terry Bites on June 24, 2025, 10:31:36 amAs shown in the picture on my 7th floor, the interface J1102, the 2nd pin is the real ground, that is, the negative pole. The 3rd pin is the power supply 29.18v. The 4th and 5th pins are differential voltage output signals.If you need an oscilloscope test waveform please tell me the exact pin definition and I will test the waveformQuote from: Terry Bites on June 24, 2025, 10:31:36 amI tried to replace an LM307N, but it didn't improve, the problem was still the same.What do you mean by VCF?Quote from: Terry Bites on June 24, 2025, 10:31:36 amPlease see RT-200 Service Manual, pages 8-5 and 8-6.The above describes the function of the RT-203 module and describes the relationship between flow and temperature coefficient. I think this is why R1109 (TC) exists.

author: dietert1
author_rank: Super Contributor
timestamp: June 25, 2025, 05:10:21 am
content: You could add nominal and measured resistor values into the schematic oamp.png that you had at the beginning of your thread.

author: daisizhou
author_rank: Super Contributor
timestamp: June 25, 2025, 06:52:40 am
content: Quote from: dietert1 on June 25, 2025, 05:10:21 amBlack indicates the resistance value with a color ring, while green represents the actual online measurement value

author: dietert1
author_rank: Super Contributor
timestamp: June 25, 2025, 09:01:16 am
content: Your photo of the PCB bottom shows that the ground ring around the circuit is connected to the circuit in five places:- Two on the large connector,- two of the positive and negative supply buffer caps and- R1120 1K.The only way i can imagine the voltage levels 17.5 V and 8.8 V you measured on the reference minus and plus is that a fault current enters into the circuit from the large connector. One can confirm this by temporary wiring the PCB only to ground and plus and minus supply. I mean if the fault is outside of this PCB, you can remove the problem and the reference should sit at about -5 and +5 V relative to ground as Terry Bites and the manual said.I would also measure the sensor itself. All four resistors of the bridge should be similar and isolated from ground.

author: daisizhou
author_rank: Super Contributor
timestamp: June 25, 2025, 09:15:00 am
content: Quote from: dietert1 on June 25, 2025, 09:01:16 amThanks.I don't quite understand what you mean.Can you tell me the simple steps, I will follow your instructions and tell you the test resultsI tested the pressure sensor offline separately, and the zero drift voltage of s+ and s- is about 0.38mv, which is consistent with the ±1mv specified in the data sheet.Please see the video below for a separate test of the pressure sensor

author: dietert1
author_rank: Super Contributor
timestamp: June 25, 2025, 09:45:12 am
content: Does the sensor have good isolation from ground?You take off the big plug that normally connects the PCB to the rest of the circuit. Now you need to make three temporary wires to supply the PCB with ground, -9 V supply and +19 V supply. Now you can test the PCB independent of other circuits in the instrument. Then you turn the instrument on and measure Ref- and Ref+ relative to ground.You can also measure the voltage across R1120.

author: daisizhou
author_rank: Super Contributor
timestamp: June 25, 2025, 10:03:26 am
content: Quote from: dietert1 on June 25, 2025, 09:45:12 amI drew a connection diagram, is this how I should connect it for testing?

author: tooki
author_rank: Super Contributor
timestamp: June 25, 2025, 10:28:51 am
content: Quote from: daisizhou on June 23, 2025, 09:50:24 pmdaisizhou, what was wrong with the FOUR threads you’ve already started about this board? Seriously, stop making unnecessary new threads. The prior questions and answers give important context.

author: daisizhou
author_rank: Super Contributor
timestamp: June 25, 2025, 10:40:34 am
content: Quote from: tooki on June 25, 2025, 10:28:51 amAs I said before, RT-203 Module the V/F converter cannot stabilize at 387, which means it cannot stabilize in the range of 250-500.That is, the HI-LO voltage value cannot be stabilized at ±0.005vdc.So I drew a schematic diagram, and followed the HI-LO voltage value clues to find the cause of the voltage fluctuation. At present, it is here at REF01 and REF+ and REF-related clues, and the fault has not been eliminated.

author: tooki
author_rank: Super Contributor
timestamp: June 25, 2025, 10:43:43 am
content: Quote from: daisizhou on June 25, 2025, 10:40:34 amAnd why couldn’t you just ask that in one of the existing threads?

author: daisizhou
author_rank: Super Contributor
timestamp: June 25, 2025, 10:47:57 am
content: Quote from: tooki on June 25, 2025, 10:43:43 amI don't know why, but after I submitted the test results, there was no new progress, so I had to start a new process to continue following up on the clues.

author: tooki
author_rank: Super Contributor
timestamp: June 25, 2025, 10:57:12 am
content: Quote from: daisizhou on June 25, 2025, 10:47:57 amThe more threads you start, the more people get confused about where the discussion is happening, which leads to questions being abandoned. Making new threads doesn’t help the problem, it makes it worse. (And it annoys people, which may cause them to not reply simply because you’ve annoyed them.)

author: daisizhou
author_rank: Super Contributor
timestamp: June 25, 2025, 10:59:21 am
content: Quote from: tooki on June 25, 2025, 10:57:12 amOK.I understand. I won't do that again next time.Thanks for the reminder.

author: dietert1
author_rank: Super Contributor
timestamp: June 25, 2025, 10:59:37 am
content: Quote from: daisizhou on June 25, 2025, 10:03:26 amDoes the sensor have good isolation from ground?Yes, that circuit is a good test setup, if you pull the large connector from the PCB.

author: daisizhou
author_rank: Super Contributor
timestamp: June 25, 2025, 11:27:37 am
content: Quote from: dietert1 on June 25, 2025, 10:59:37 amThe insulation between the sensor and the ground is very good, please see the picture below, I did not find any defectsGive me some time I will connect as per the picture above and test it

author: daisizhou
author_rank: Super Contributor
timestamp: June 25, 2025, 11:46:58 am
content: I completed the test.ref+:2.883v ref—:-5.74v R1120：-1.434vPlease see the pictures belowIf there are any other tests you need please let me know,thanks

author: daisizhou
author_rank: Super Contributor
timestamp: June 25, 2025, 12:14:43 pm
content: Could it be that R1117 (VB SET) needs a special setting value to satisfy the pressure sensor operating voltage and current, and to balance REF+ and REF-?Because the R1117 (VB SET) setting range seems very arbitrary so far

author: dietert1
author_rank: Super Contributor
timestamp: June 25, 2025, 01:05:25 pm
content: That looks better now.Yes, try to adjust the reference to 10 V using R1117_VB_SET.After that center it around ground using R1116_ZERO to have +5 V and - 5 V as suggested before?Edit: It's sad that you lost the resistor values. And i saw a green -4,90 V measurement label at U1102 pin 3. That means the sensor bridge pin DUP2 is already near -5 V. You should check that. It means the reference won't be centered at -5 V and +5 V when the bridge is centered at -5 V and + 5 V. The bridge outputs DUP1 and DUP3 should be near 0V relative to ground and DUP4 = U1102 pin 1 should be +5V.

author: iMo
author_rank: Super Contributor
timestamp: June 25, 2025, 04:08:14 pm
content: aprox

author: daisizhou
author_rank: Super Contributor
timestamp: June 26, 2025, 12:45:47 am
content: Quote from: dietert1 on June 25, 2025, 01:05:25 pmSorry, I didn't know the resistor value was important to you.I re-measured it and marked all the resistance values ​​and voltages at each test point. I hope it will be helpful for your analysis.Please let me know if there are other locations that need to be measured. I haven't made any adjustments yet.By the way, the voltage across C1105 is still fluctuating.The REF01 housing is slightly hot to the touch

author: daisizhou
author_rank: Super Contributor
timestamp: June 26, 2025, 03:11:48 am
content: There are some errors in the picture of layer #46. There are some jumpers on the back of J1102. I measured the data without connecting the jumpers. I will remake the jumpers and remeasure all the data.

author: daisizhou
author_rank: Super Contributor
timestamp: June 26, 2025, 04:06:32 am
content: I added the jumper at position J1102,And retested the data of each location

author: dietert1
author_rank: Super Contributor
timestamp: June 26, 2025, 05:40:02 am
content: I would measure the output n4 and check the whole sensor circuit applying a small pressure. At this stage a calibration doesn't make sense, but one can do a simple test by closing the ends of both tubes with clamps. After both ends are closed, if you press one of the two tubes with your hand, there will be a pressure difference and it should show in the n4 voltage. The voltage must be opposite sign if you press the other tube.Once you know that your sensor PCB works, you can put it back into your instrument. I think the original problem was caused by missing -9 V supply to the sensor PCB. Re-seating PCB-to-cable connectors sometimes fixes bad contacts in old instruments.Regards, Dieter

author: daisizhou
author_rank: Super Contributor
timestamp: June 26, 2025, 06:21:25 am
content: Quote from: dietert1 on June 26, 2025, 05:40:02 amHi dietert1The initial clue to the fault was the zero voltage drift at position N4.This is the HI-LO voltage zero drift I described initially.Pressure gradient and linearity are normal.So I followed the clues and found REF01 and its related auxiliary circuits.I will once again shoot a video of the HI voltage drift using +19v and -9v external power supplies, give me some time

author: daisizhou
author_rank: Super Contributor
timestamp: June 26, 2025, 07:04:23 am
content: Quote from: dietert1 on June 26, 2025, 05:40:02 amPlease see the video below. I measured the HI voltage and REF01 output voltage again, and the fault still exists.According to the manual, the HI-LO voltage fluctuation can only be 0.005VDC--0.005VDC, which is 10mv.But you can see that it has now clearly exceeded the 10 millivolt range.Initial RT-200 host failure "HELP-0", RT-203 module failure, please see the video below, I compared the RT-202 pressure module

author: dietert1
author_rank: Super Contributor
timestamp: June 26, 2025, 09:07:30 am
content: I haven't seen from your video that the pressure sensor works. Maybe it has humidity inside causing that drift, who knows.You can try to reduce the output drift on n4 Hi by re-seating the opamp parts U1101, U1103 and U1104 in their sockets. Or try replacement parts. Or you check whether the drift is present before the Sallen-Key low pass U1101.3. I mean if you want to locate the origin of the drift, just go along the signal chain, beginning at the output n4Regards, DieterEdit:Watch out with U1101. It's a 14 pin part in a 16 pin socket. This can cause errors.In a vintage device drift can also be caused by dirt. How about cleaning the bottom of the sensor PCB with alcohol? It doesn't even have a solder mask.

author: daisizhou
author_rank: Super Contributor
timestamp: June 26, 2025, 09:43:01 am
content: Quote from: dietert1 on June 26, 2025, 09:07:30 amThanks.I tested the pressure sensor's (S+ and S-) data and HI-LO voltage separately. Please see the EXCEL table below.From the data, the pressure gradient is normal and the linearity is also normal.I have repeatedly plugged and unplugged the U1103, U1101, U1102, and R1103 you mentioned, and used a blade to clean the chip pins and the oxidized parts of the chip socket.And I have replaced LM307N, which is U1103, and the result is still the same.I could test N6 and N9 again, but I guess the effect is the same, because the REF01 output pin 6 is slowly rising, which means the pressure sensor VCC (DUP4) is also changing.I tried cleaning it and used a PCB cleaning agent, but the result was the same.I have been doing this for more than a month, but there is still no progress, which is why I keep postingBy the way, R1103 is a special resistor. I didn't find the exact data sheet. I attached a diagram that a friend on the forum helped me analyze. Please see the attachment.(https://www.eevblog.com/forum/projects/what-model-is-this-chip/msg5916090/#msg5916090)

author: daisizhou
author_rank: Super Contributor
timestamp: June 26, 2025, 10:07:29 am
content: Frankly speaking, REF01 loses its function in the circuit. According to the data sheet, REF01 is a 20PPM voltage reference chip.But in reality it is not even as good as a 10V voltage regulator transistor (I mean voltage fluctuation). I don't know what causes this result.

author: iMo
author_rank: Super Contributor
timestamp: June 26, 2025, 10:10:24 am
content: Now, the question is still the same - why the output voltage of the REF01 (pin 6 against pin 4) drifts by 150mV after a power-on.If you have a spare REF01 (my current understanding is you have not removed the original drifting REF01 from the pcb, but you measured a different REF01 - is that correct??) try following:1. input voltage 19.7V (pin 2 and pin 4)2. load 5-6mA, for example a 2k or 1k5 resistor wired between pin 6 and pin 43. measure the voltage on that resistor after power on.

author: daisizhou
author_rank: Super Contributor
timestamp: June 26, 2025, 10:30:39 am
content: Quote from: iMo on June 26, 2025, 10:10:24 amNO.The REF01 I tested is the one on the PCB circuit board.Many people say that online measurement is easily interfered by other factors, so I removed REF01 from the PCB.REF01 I only have one at the moment, no extra one.This is the only REF01 tested in a 26-story post.I have 2 LM307N and replaced them with each other, but the result is still the same.

author: iMo
author_rank: Super Contributor
timestamp: June 26, 2025, 10:40:07 am
content: The not loaded output of a REF01 should be always above 10.00V, imho, like 10.010V for example. With trimming you will go down with the output voltage, afaik.The load regulation characteristics of the REF01 is 50-90ppm/mA, thus with 6mA load max -5.4mV, imho.The TC is, say, 20ppm/C, assuming 30C increase is could be +2mV (see the DS for the graph).So when without load it will be 10.010V, but with 6mA load and some, say, 30C temperature increase it should stay at10.010 - 0.0054 + 0.002 = 10.0066V, aprox. worst case..

author: daisizhou
author_rank: Super Contributor
timestamp: June 26, 2025, 10:46:20 am
content: By the way。I have a LH0070-OH voltage reference chip in my electronics warehouse.If it is possible to replace test REF01, I can do the related testsIf you think the REF01 is likely to age, I can buy the REF01, which may take a few days to ship, and then test it.Thanks

author: dietert1
author_rank: Super Contributor
timestamp: June 26, 2025, 10:52:46 am
content: A reference is kind of a voltage regulator. They show wrong output voltage under oscillation. In order to avoid oscillation one puts buffer caps onto the input and onto the output. The circuit does have the output cap. It is C1105. The input cap is missing and one cannot simply put a cap there, since the reference has a virtual ground driven by opamp U1103. So it needs to be the snubber i was talking about in the beginning, like 1 uF+22 Ohm. It should be between ground and virtual ground. Or you can wire it across pins 2 and 4 of the REF01.Regards, Dieter

author: iMo
author_rank: Super Contributor
timestamp: June 26, 2025, 11:05:27 am
content: ..like this..

author: daisizhou
author_rank: Super Contributor
timestamp: June 26, 2025, 11:41:42 am
content: Quote from: dietert1 on June 26, 2025, 10:52:46 amIs it necessary to use a 1uf capacitor? Is a 0.1uf-104 capacitor OK?Looks like a non-polar capacitor, I've been looking for a suitable

author: dietert1
author_rank: Super Contributor
timestamp: June 26, 2025, 01:03:36 pm
content: With a 100nF cap the time constant is 2,2 usec, with 1 uF it is 22 usec.The time constant of the supply filters is 330R * 100nF = 33 usec. For safe operation capacitors C1103 and C1104 should be bigger than C1105.You can also use electrolytic capacitors of 2.2 or 4.7 uF if you them. Need to observe the polarity.Regards, Dieter

author: daisizhou
author_rank: Super Contributor
timestamp: June 26, 2025, 01:13:18 pm
content: Hi dietert1I did the connection test as you said, please see the video, it seems that there is no obvious improvement, the HI voltage still fluctuates

author: iMo
author_rank: Super Contributor
timestamp: June 26, 2025, 01:46:45 pm
content: If I were you I would do again the measurement around the REF01 as I described below.If the REF01 is faulty you will see the drift in that configuration as well.Also do not use the trimmer this time.Quote from: iMo on June 26, 2025, 10:10:24 amMy bet would be you should see the output voltage above 10.0V and none drift when the REF01 is ok.PS: I would try with 1k, 1k5 and 2k2 loads and write down the voltages on that loads.PPS: your on-board 20k trimmer could be faulty as well..

author: daisizhou
author_rank: Super Contributor
timestamp: June 26, 2025, 02:14:32 pm
content: Quote from: iMo on June 26, 2025, 01:46:45 pmDo you mean to connect as shown in the figure below, replace resistors with different resistance values, and record the voltage value across the resistor. Is this the way to test?

author: iMo
author_rank: Super Contributor
timestamp: June 26, 2025, 02:18:48 pm
content: yes.. also the output voltage without a resistor (no load)..

author: daisizhou
author_rank: Super Contributor
timestamp: June 26, 2025, 02:25:01 pm
content: OK.Give me some time, I will find the resistor and test it.

author: dietert1
author_rank: Super Contributor
timestamp: June 26, 2025, 03:27:28 pm
content: I looked into the LM307 datasheet and there is no spec on the maximum output current, But they say it will drive 10 V into a 2 KOhm load. In our circuit the voltage divider R1115, R1116 and R1117 is a load of about 2 KOhm, so the opamp operates near its limit.But the reference also needs a quiescent current of 1 to 1.4 mA.That means the opamp may not be able to drive its load to more than 8.6 V or so. You can improve the situation with a 1K resistor from -9 V supply to the opamp output (virtual ground). The resistor provides some mA of current and helps the opamp.Regards, Dieter

author: daisizhou
author_rank: Super Contributor
timestamp: June 26, 2025, 09:02:21 pm
content: Quote from: dietert1 on June 26, 2025, 03:27:28 pmHi dietert12 "1uf+22ohm" test devices, if there is no improvement, can I remove them?Do you mean, the IKOHM resistor is connected in the circuit as shown below? And then proceed to test the N4 voltage fluctuation?

author: dietert1
author_rank: Super Contributor
timestamp: June 26, 2025, 10:28:40 pm
content: Yes, the resistor between pins 4 and 6 of the opamp will supply the REF01 and the voltage divider made of R1115, R1116 and R1117 with some more current, so it becomes easier for the opamp. As a result the reference output between REF01 pins 4 and 6 or on C1105 should rise from 8.7 V to 10 +/- 0.1 V and remain stable within spec, to about +/- 100 uV. As a result the output drift at N4 should disappear, as imo proposed before.One snubber between REF01 pins 2 and 4 is sufficient. You can leave the snubber in and test later whether it is necessary or not.Regards, Dieter

author: daisizhou
author_rank: Super Contributor
timestamp: June 26, 2025, 10:35:34 pm
content: Quote from: iMo on June 26, 2025, 01:46:45 pmHi iMoI tested it according to your idea and found that the voltage measured by 1K, 1.5K, and 2K is almost the same(9.959v).I use 19.0v to power REF01. With different resistors, only the power supply sees some difference in current.Is there anything that needs to be modified?

author: iMo
author_rank: Super Contributor
timestamp: June 26, 2025, 10:52:51 pm
content: ..the REF01 looks stable, even the voltage is below 10V..The next step could be to investigate whether the 20k trimmer is ok, or it is faulty.Afterwards try dieter1's hack with the 1k from the LM307's output to the real ground.

author: daisizhou
author_rank: Super Contributor
timestamp: June 26, 2025, 11:52:41 pm
content: Quote from: iMo on June 26, 2025, 10:52:51 pmPlease see the 20K trimmer resistor test picture.Is there anything else I need to test?If not, I will restore REF01 and the 20K trimmer resistor and then do the test suggested by dieter1

author: daisizhou
author_rank: Super Contributor
timestamp: June 27, 2025, 02:07:22 am
content: Quote from: dietert1 on June 26, 2025, 10:28:40 pmHi dietert1I added a 1K resistor, but C1105 still has a voltage of 8.53v,And as always, it rises slowly.The output voltage of N4, which is the HI voltage, still fluctuates.(You can compare it with the video I shot before.)Watch the video.I kept the buffer between REP01 pins 2 and 4 as you said

author: iMo
author_rank: Super Contributor
timestamp: June 27, 2025, 04:53:23 am
content: Take the U1102.1 off the socket, and look at the output voltage of the REF01.You may also put the other chips (except the REF01 and LM307) off the sockets and watch the REF01 output.Trimmer 20k: remove it from the pcb and watch the output of the REF01.Always do one single change per step.

author: daisizhou
author_rank: Super Contributor
timestamp: June 27, 2025, 05:04:54 am
content: Quote from: iMo on June 27, 2025, 04:53:23 amI disconnected R1130 in the circuit and removed jumper J1101,Continue to measure the voltage value of C1105.The circuit is shown in Figure 22 below,Please watch test video 1I removed the LM307N and LM346N, see test video 2Finally I removed the LM307N, see test video 3

author: iMo
author_rank: Super Contributor
timestamp: June 27, 2025, 05:22:41 am
content: Put the LM307 into the socket (it has to stay there as I wrote above). You cannot remove it from the socket as it creates virtual ground for the REF01.Remove the LM346 - U1102.1 from the socket.Watch the REF01 output (pin 6 vs. pin 4).Watch the LM307 output against real gnd.Do not make any other change.Resistor 1130 - wire it back afterwards.

author: dietert1
author_rank: Super Contributor
timestamp: June 27, 2025, 06:03:24 am
content: Quote from: daisizhou on June 27, 2025, 02:07:22 amIn your video i see that you added the resistor in the wrong place. It should be between U1103 LM307 pins 4 and 6. Output of opamp to negative supply.Regards, Dieter

author: daisizhou
author_rank: Super Contributor
timestamp: June 27, 2025, 06:38:13 am
content: Quote from: iMo on June 27, 2025, 05:22:41 am27/fm measures the voltage between the LM307 pin 6 output and the ground.187 measures the voltage across c1105

author: daisizhou
author_rank: Super Contributor
timestamp: June 27, 2025, 06:58:00 am
content: Quote from: dietert1 on June 27, 2025, 06:03:24 amIs this ok? I connected the common end to the front through the blue wire,Because of the PCB installation location, the distance may be a bit longIf it works, I tested the HI voltage and C1105 voltage again.As mentioned before, the HI voltage still fluctuates and decreases slowly, and the c1105 voltage still rises slowly around 8.5V.

author: iMo
author_rank: Super Contributor
timestamp: June 27, 2025, 07:01:36 am
content: ..no video..

author: daisizhou
author_rank: Super Contributor
timestamp: June 27, 2025, 07:08:02 am
content: Quote from: iMo on June 27, 2025, 07:01:36 amWait a minute, I'm uploading, the YouTube network speed is very slow

author: iMo
author_rank: Super Contributor
timestamp: June 27, 2025, 08:56:29 am
content: Try the same measurement with the 1k resistor as dietert1 has suggested..

author: daisizhou
author_rank: Super Contributor
timestamp: June 27, 2025, 09:01:21 am
content: Quote from: iMo on June 27, 2025, 08:56:29 amI have tried, and the HI voltage still fluctuates (slowly decreases), while the C1105 voltage (8.5~v)slowly increases, which is the same as before and has not improved

author: iMo
author_rank: Super Contributor
timestamp: June 27, 2025, 09:08:52 am
content: The perhaps last issue could be the 20k trimmer setting.Your is 18k/16k so perhaps the REF01 is set off its regulation.Try to remove the 20k trimmer.And make the same measurement.And do not put the trimmer back after the measurement, plz.

author: daisizhou
author_rank: Super Contributor
timestamp: June 27, 2025, 09:20:40 am
content: Quote from: iMo on June 27, 2025, 09:08:52 amDo you mean to ask me to remove the 20K fine-tuning resistor and test the voltage across C1105 and HI-GND?Do I need to measure the ground voltage of pin 6 of LM307?Should LM346N be removed?(And do not put the trimmer back after the measurement)？？？？what do you mean?

author: iMo
author_rank: Super Contributor
timestamp: June 27, 2025, 09:29:53 am
content: 1. remove the 20k trimmer off the pcb2. do the exactly same measurement as above in your latest video3. do not solder the 20k trimmer back until we will tell you.

author: daisizhou
author_rank: Super Contributor
timestamp: June 27, 2025, 10:45:20 am
content: Quote from: iMo on June 27, 2025, 09:29:53 amOK.Please watch the video and let me know if there are any adjustments that need to be made

author: iMo
author_rank: Super Contributor
timestamp: June 27, 2025, 10:50:20 am
content: Ok, so the REF01's output was drifting because the setting of the 20k trimmer was incorrect, it seems (somebody set it at the edge of REF01's trimming range)..What I would do - I would wire a different trimmer (like 10k - 50k) from the pcb bottom side and play with the REF01's trimming settings, while doing the exact measurement as you do now.Find the range where the REF01 works stable without drifting.Afterwards you may return the LM368 there and again repeat the measurements.Also you may leave the 1k resistor there as dietert1 has suggested.At the very end you may return the 20k trimmer back on the pcb and set it accordingly..

author: daisizhou
author_rank: Super Contributor
timestamp: June 27, 2025, 11:06:16 am
content: Quote from: iMo on June 27, 2025, 10:50:20 amAre you suggesting that I replace a 3006P model 20K adjustable potentiometer with a variable range?Do I need to purchase 3006P in 30K? (Because 20K is usually less than 20K, I purchased 30K, which happens to fit 20K)Do adjustable resistors have any special requirements for temperature drift? And precision

author: iMo
author_rank: Super Contributor
timestamp: June 27, 2025, 11:15:54 am
content: Read my posts after a while as I am doing some improvements/additions often.In order to fix your equipment you have to get a feeling how the stuff works.So I suggested to use a different trimmer 10k to 50k instead of the original one (like the one you have used with off pcb measurement) and try to find the range where the REF01 works fine (it means it does not drift). Write down the voltages. Get the idea how it works best.Then step by step add the LM368, etc.FInaly, when you will get it working, put the original 20k trimmer back on the pcb (and set it once accordingly).Why I would not use the original 20k trimmer for experiments? - Because the more you turn up/down with those 10T trimmers the higher probability it gets crappy..

author: daisizhou
author_rank: Super Contributor
timestamp: June 27, 2025, 01:48:56 pm
content: I restored the LM346N and replaced the original 20k trimmer resistor with a 10k trimmer resistor.Measure the voltage across C1105,I found that 9.000v seems to be a critical value，8.900v seems to fluctuate, but as long as it is above 9.000, it will no longer fluctuate.Is this the only way I can try to measure it?Which voltage is the best and accurate voltage?

author: dietert1
author_rank: Super Contributor
timestamp: June 27, 2025, 04:54:48 pm
content: Are you going in circles?This was answered before. Read the manual. The sensor bridge should get + and - 5 V. Adjustment with reference trimpot and R1116. Surprising that you still don't use the reference trim like you had for the out-of-board reference test.

author: iMo
author_rank: Super Contributor
timestamp: June 27, 2025, 06:58:51 pm
content: Also where the +19.7V and -9V come from?The only voltages described in Manual are +15V, -15V and 5V.

author: daisizhou
author_rank: Super Contributor
timestamp: June 27, 2025, 07:20:59 pm
content: Quote from: iMo on June 27, 2025, 06:58:51 pmCurrently powered by an external experimental power supply, with input voltages of+19V and -9V respectively

author: daisizhou
author_rank: Super Contributor
timestamp: June 27, 2025, 09:19:31 pm
content: Quote from: dietert1 on June 27, 2025, 04:54:48 pmNO.Frankly speaking I don't know how to adjust R1117 (VB SET) correctly。Initially inside the host, I measured the voltage between the pressure sensors (DUP4-DUP2) to be 9.300v before adjusting R1117，Referring to the pressure sensor data sheet, I tried to adjust it to 10V, but it never reached 10V.Finally, I set the DUP4-DUP2 voltage to 9.400vR1117(VB SET)、R1123(VB SET)、20k trimmer resistor(Now replace it with a 10k trimmer resistor)--Refers to the same objectThe actual output of REF01 can only reach 9.96-9.97v，Cannot reach 10.00vThe 1K resistor remains in the circuit, right?

author: dietert1
author_rank: Super Contributor
timestamp: June 27, 2025, 09:46:47 pm
content: Read again the contribution of Terry Bites.He explained that R1117 adjusts the balance of 10 V at the sensor to be -5 V at DUP2 and +5 V at DUP4. So you measure those two voltages at DUP2 and DUP4 and trim the reference and R1117 until you get +5 V and -5 V.After the adjustment please measure the reference voltage and tell us.Likely the adjustment will result again in a reference voltage that is low and unstable. Then you can reduce the gain of U1102.1 by 10 %. The gain is proportional to R1113 6K97. If you add a 68 K resistor parallel to R1113, this will result in a lower gain. Then you repeat the adjustment of the reference trim and R1117 and measure the reference voltage again. The reference will be in its stable region near 10 V. If not you can modify the U1102.1 gain once more.Regards, DieterBing translate:再次阅读Terry Bites的贡献。他解释说，R1117将传感器的10 V平衡调整为DUP2的-5 V和DUP4的+5 V。因此，您测量这两个电压在DUP2和DUP4，并调整参考电压和R1117，直到您得到+5 V和-5 V。调整后，请测量参考电压并告知我们。很可能调整结果仍会导致参考电压低且不稳定。然后，您可以将U1102.1的增益减少10%。增益与R1113的6K97成正比。如果您在R1113并联一个68 K的电阻，这将导致更低的增益。然后，您重复参考调整和R1117的调整，并再次测量参考电压。参考电压将处于10 V附近的稳定区域。如果没有，您可以再次修改U1102.1的增益。

author: daisizhou
author_rank: Super Contributor
timestamp: June 27, 2025, 11:44:52 pm
content: Does the voltage difference between DUP2 and DUP4 really need to be 10.000v?Would it be better around 9.29?Because if you want to achieve a 10v voltage difference, R1116 (zero) seems to have to be adjusted to the limit. The figure below shows the data I have adjusted so far.Both ends of c1105 can be stabilized at 9.147VN4 still experiences fluctuations

author: daisizhou
author_rank: Super Contributor
timestamp: June 28, 2025, 12:50:56 am
content: Hi everyoneI installed the PCB back into the host and found a new problem,After removing the external power supply +19v, -9v.Use the host's internal power supply.The HI-LO voltage is too high. I tried to adjust R1102 (NULL), hoping to get it within ±0.005vdc, but unfortunately the limit is only 0.866v.I don't know if it's because of the addition of the buffer(22ohm+1uf) or the addition of the 1k resistor, but the voltage across C1105 is currently stable at 9.147v.I cannot adjust R1116 (ZERO) when powered by the internal power supply, and all measurements seem to fail.

author: dietert1
author_rank: Super Contributor
timestamp: June 28, 2025, 04:53:46 am
content: Your circuit is simple but everything is old, for example the sensor itself. I understand that you want to keep the original circuit, but it can't work until you fix it.

author: daisizhou
author_rank: Super Contributor
timestamp: June 28, 2025, 05:31:03 am
content: Quote from: dietert1 on June 28, 2025, 04:53:46 amYes.I really want to fix it.I tested the pressure sensor and it is according to the datasheet,So I think it's normal.I am a beginner and I am not familiar with circuitsPlease see the picture below, the added buffer (22 ohm + 1uf ​​capacitor) and 1K ohm resistor, is this installation feasible?The 1K ohm resistor is currently a temporary installation for experimental purposes, do I need to mount it permanently on the PCB?

author: iMo
author_rank: Super Contributor
timestamp: June 28, 2025, 06:09:01 am
content: In the Manual I do not see an info on the pressure sensor's Vcc vs. Gnd voltage, so perhaps it will work with a lower voltage too.It is a bridge, the diff amplifier wired afterwards amplifies the difference only.The Manual only writes something on setting X test points to be within 0..+0.005V and setting a value to 375 +10.I would set the REF01 output (on that C1102 capacitor) to 9.5V as at least the REF01 is stable there.Check the power sources voltages in that unit too - what are the voltages? They should be +15V, -15V, +5V, all +/-0.2V.Alsodouble check the naming("ref.designation") of the AxAyAz pcb boards in the Manual there against the reality..The Manual talks about 4 various pcb boards, 4 various testpoints and 5 various trimmers..PS: the equipment you are repairing is a medical device perhaps intended for calibration of devices for a support of breathing, imho, so hopefully you are aware of that..

author: dietert1
author_rank: Super Contributor
timestamp: June 28, 2025, 06:43:09 am
content: In our surplus corner i found an old pressure sensor Fujikura FPM07PG and its online datasheet. After unsoldering it from the circuit the bridge resistance is about 5.4 KOhm, measured at the bridge feed (in your case DUT2 and DUT4). They recommend running the bridge at a constant current of 1.5 mA. That means a bridge feed of about 8.1 V. At that voltage the bridge imbalance is 3.8 mV without any trim. And i get a signal of 20 or 30 mV when blowing into or sucking from the sensor tube. My sensor is for +/- 48 kPa = +/- 480 mBar.Why constant current? It is their method for temperature compensation of the pressure measurement. Your circuit has a constant current source, too. R1112 and R1114 are current measurement resistors. The bridge feed voltage will depend on bridge resistance and after 40 or 50 years of aging, it may be out of range in your case. You need to modify the circuit.Regards, DieterEdit:R1118 makes it a current drive, while R1113 makes it a voltage drive. In your case there is a mixture.

author: iMo
author_rank: Super Contributor
timestamp: June 28, 2025, 07:40:09 am
content: My current understanding is the REF01 voltage sets the current of the bridge, and the "R1116 zero set trimmer" has to position the DUP1/3 close to 0.000V, in other words the DUP2/4 voltages symmetrical around zero volt (assuming +/-15V power).With current schematics and values there I can not get close to that (in sim) with REF01's output from 8 to 10V and any R1116 settings.So double check the schematics..

author: daisizhou
author_rank: Super Contributor
timestamp: June 28, 2025, 07:51:52 am
content: Quote from: iMo on June 28, 2025, 06:09:01 amThank you for reading the manual carefully.See here (https://www.eevblog.com/forum/repair/micro-switch-pressure-sensor-help-needed/)。The pressure sensor model is 125pc05d1.Please see the attached picture for voltage and currentI think C1102 should be set to 9.144, because A2A4 also has similar (almost the same) PCB (at least currently stable and accurate)Please see the attached A2A4-C1105 TEST pictureQuote from: iMo on June 28, 2025, 06:09:01 amPlease see the attached +15v, -15v, +5v test picturesQuote from: iMo on June 28, 2025, 06:09:01 amPlease see "FULL"

author: daisizhou
author_rank: Super Contributor
timestamp: June 28, 2025, 07:57:54 am
content: Quote from: iMo on June 28, 2025, 06:09:01 amPlease see "RT-202 MODULE-A2A4", "RT-203 MOUDLE A3A4+A3A3", "RT-202 MODULE-A2A4+A2A3"，“A2A4”Quote from: iMo on June 28, 2025, 06:09:01 amI know for sure.Frankly speaking, RT-200 has always been the tool I use.I have been using it for more than 10 years.Thanks for your reminder

author: daisizhou
author_rank: Super Contributor
timestamp: June 28, 2025, 08:18:35 am
content: Quote from: dietert1 on June 28, 2025, 06:43:09 amPlease see the attached picture. R1112 and R1114 can be clearly seen in the picture.Resistor color ring "yellow, purple, green, black, brown, red".This is a 475 ohm, ±1%, 50ppm resistor.I can disassemble it, take it offline and measure its resistance using an LCR meter.If it is really aged as you said, I can buy a resistor with better performance to replace it (such as 0.1% accuracy, low temperature drift)Do you think it is necessary to disassemble and measure?If you have better suggestions, please tell me.Thanks.Although it is old,It may be 40/50 years old, but it is a measuring instrument, not something that is turned on and run every day.To be honest, the actual running time is very short.Just like my multimeters, they haven't been calibrated for years, but they are still very accurate

author: dietert1
author_rank: Super Contributor
timestamp: June 28, 2025, 08:35:32 am
content: I wrote about aging of the sensor bridge itself, not the resistor. It's hopeless.https://www.eevblog.com/forum/beginners/an-operational-amplifier-circuit-seeking-help/msg5960965/#msg5960965

author: daisizhou
author_rank: Super Contributor
timestamp: June 28, 2025, 08:37:57 am
content: Quote from: iMo on June 28, 2025, 07:58:45 amAre you referring to the DUP1, DUP2, DUP3, DUP4 measurements on the A2A4 in the RT-202 module?Does the "GND" here refer to the second pin of J1102? See the picture

author: iMo
author_rank: Super Contributor
timestamp: June 28, 2025, 08:44:52 am
content: Your boards are powered by +15V and by -15V, where in the "middle" is the GND (those are 2 independent sources where the GND is their common).So from now up let us reference everything against the GND.You have to adjust your schematics such it shows +15V, GND, and -15V (thus your current "gnd symbol" and "29.18V" is not "good"). See the Manual.And yes, please measure the DUP1,2,3,4 voltages on the "good board" against the GND.

author: daisizhou
author_rank: Super Contributor
timestamp: June 28, 2025, 08:54:19 am
content: Quote from: dietert1 on June 28, 2025, 08:35:32 amPlease see my actual test values ​​of the pressure sensor.The zero drift of the pressure sensor is 0.46mv(MAX), which is in line with the ±1mv in the data sheet.This is a real test video of the pressure sensor.

author: daisizhou
author_rank: Super Contributor
timestamp: June 28, 2025, 09:12:54 am
content: Quote from: iMo on June 28, 2025, 08:44:52 amI have explained how to modify it countless times, but I am completely confused by this "GND"How should I modify it correctly?Please see my test pictures

author: daisizhou
author_rank: Super Contributor
timestamp: June 28, 2025, 10:18:44 am
content: Quote from: dietert1 on June 28, 2025, 08:35:32 amI even used 4 5K ohm resistors to form a Wheatstone bridge network to simulate a pressure sensor, but the HI output voltage still fluctuates, see the videoIf you suspect that this is a voltage fluctuation on the C1105, I can repeat the test again.

author: daisizhou
author_rank: Super Contributor
timestamp: June 28, 2025, 01:52:15 pm
content: Hi dietert1Please see the picture below, the added buffer (22 ohm + 1uf ​​capacitor) and 1K ohm resistor, is this installation feasible?The 1K ohm resistor is currently a temporary installation for experimental purposes, do I need to mount it permanently on the PCB?

author: iMo
author_rank: Super Contributor
timestamp: June 28, 2025, 02:23:10 pm
content: The voltage fluctuates because you set the REF01 below 9.XXX Volt. You made the exercise and you know you should not go so low with the REF01 voltage. Set the voltage to 9.5V and it will not fluctuate with your bridge.Still waiting on your DUP1/2/3/4 voltages on the "good board" against GND.QuoteI do understand your confusion..Your analog boards are not powered by two wires, like Vcc=+30V and "gnd"=0V (total difference is 30V).Your analog boards are powered by 3 wires, like +15V, GND, -15V (total difference is 30V as well).So, your schematics should be using 3 wires.Your confusion comes from the fact, that you started with 2 wires powering and you did measurements against "gnd"=0V.So for example 19V in your 2 wires measurement (against "gnd") is +4V in 3 wires measurement (against GND).2 wires 3 wires30V +15V19V +4V15V 0V (GND)14V -1V10V -5V3V -12V0V (gnd) -15VMoreover, we had a "virtual ground" (the output of the LM307), say "VGND" the green ground symbols. But that is not important now at this stage.My recommendation for you is following - redraw your schematics such you have +15V, GND, -15V (as described in the Manual).The highest voltage you may measure in your equipment (the analog boards you mess with) is therefore +15V, the lowest -15V. You cannot measure +19V against GND, as you do not have such voltage handy.Otherwise we will have problems to communicate the voltages.

author: daisizhou
author_rank: Super Contributor
timestamp: June 28, 2025, 09:58:26 pm
content: OK.Thanks,iMoI updated the schematic and adjusted the external power supply voltage,Next, we will use +15v, -15v, and GND as shown in the figure as referenceQuote from: iMo on June 28, 2025, 02:23:10 pmDUP1:4.52VDUP2:-0.223VDUP3:-4.82VDUP4:-0.223V

author: daisizhou
author_rank: Super Contributor
timestamp: June 28, 2025, 10:44:41 pm
content: Under the new reference state, I re-measured the voltage values ​​of each pin.And according to the reference of "good board", C1105 is set at 9.13v (can be adjusted higher),R1116(ZERO) was adjusted,I found that the voltage measured by Terry Bites ("ZERO" sets = REF+ = REF-) is slightly different from that measured by the "good board".If Terry Bites' suggestion is followed, R1116 (ZERO) will be adjusted to the limit, and the N4 voltage will not be adjusted to ±0.005 VDC specified in the service manual by R1102 (NULL).This is the current status. If you have different suggestions, please tell me and I will adjust it. Currently, the voltage of N4 is still fluctuating.

author: iMo
author_rank: Super Contributor
timestamp: June 29, 2025, 05:21:06 am
content: Quote from: daisizhou on June 28, 2025, 09:58:26 pmIs that measurement on the "good board" correct? Your "bad board" shows fully different voltages..Your N4 shows 0.000V what is a perfect number.. The other voltages are good as well (even better than on the "good board"), imho.Quote from: daisizhou on June 28, 2025, 10:44:41 pmWhat is the "fluctuation" like there?

author: daisizhou
author_rank: Super Contributor
timestamp: June 29, 2025, 11:16:05 am
content: Sorry for the tagging error, I marked "good board" in red.Give me some time, I will repeat the measurement again, hoping to obtain stable and accurate data

author: daisizhou
author_rank: Super Contributor
timestamp: June 29, 2025, 12:25:44 pm
content: Hi iMoAll the data here are changing (that is what I call fluctuations), so is there any meaning in my measuring and recording?For example, the red arrow N4,When the power is turned on, N4 displays 0.175v,Then the number started to drop, from 0.175v to 0.154v, which took 15 seconds.It took 25 seconds to drop from 0.154 to 0.131v............That is to say, as the power-on time increases, the voltage drops more and more slowly.Finally, wait for about 30 minutes or an hour, the voltage will reach 0.000v. Then it will become -0.001v......-0.005v....Of course, the changes will be very slow, and the amplitude will become smaller and smaller.And the value of 0.000 can be set in R1102 (NULL), for example,I set R1102 (NULL) to 0.000v 20 minutes after power on,Then when the time reaches 21 minutes or 25 minutes, a negative number will be displayed.Of course we all know that the longer the R1102 (NULL) setting is, the better, but it is always changing. How can I set it stably?There is another question. The longer I set R1102 (NULL), the higher the initial value will be. For example, if I set it to zero for 30 minutes, it will display 0.175v after powering on. When I set R1102 (NULL) to 0.000 for 1 hour or 2 hours, the power-on voltage will become 0.375 or 0.575 (I'll give you an example).Do I need to record this data?

author: iMo
author_rank: Super Contributor
timestamp: June 29, 2025, 03:06:17 pm
content: Such slow drift may come from various sources there. I would set the REF01 to a higher voltage (like 9.5V or more) and I would watch the drift again (perhaps the 9.14V is still too low to be stable). I think the exact REF01's output voltage is not that critical, what is important is it stays rock stable.Also the pressure sensor may fluctuate (I do not know how sensitive it is and whether it is a differential or absolute sensor) due to temperature/atm.pressure. And the opamps (and any parts around - resistors/trimmers etc.) may drift with temperature as well.It is a complex DC coupled amplifier so you have to simply track down the drift contributor from the REF01 towards the N4.Btw., how the N4 of the "good board" drifts?

author: dietert1
author_rank: Super Contributor
timestamp: June 29, 2025, 08:35:37 pm
content: For a repair it isn't enough to say "everything drifts". You need to use some method to locate the origin of the drift and then fix it. Somewhere above i recommended to work from the output n4 in steps towards the voltage reference.For example you can shorten the output of the sensor bridge (sensor pins DUP1 and DUP3), then adjust R1111 XD BAL to have n4 = 0 V and observe whether it drifts or not. Then you know whether the drift happens in the instrumentation amplifier or in the bridge or bridge biasing circuit.To find out whether the drift happens in the bridge or in the bridge biasing circuit you replace the bridge by 4x 5 KOhm good resistors and test whether the drift disappeared. And so on.Somewhere above the conclusion was that the drift originated from the REF01 or rather its trimpot. After that the voltage reference should remain without trimpot and with the reference voltage stable at 9.96 V. How is it possible that you later found the reference voltage to be 9.1 V?I don't see any problem solving but a lot of confusion.

author: daisizhou
author_rank: Super Contributor
timestamp: June 29, 2025, 10:13:08 pm
content: Quote from: iMo on June 29, 2025, 03:06:17 pmSorry, forgot the description for REF01.The REF01 output can indeed be set higher, that is, the voltage across C1105,But the higher voltage seems to be related to the operating voltage of the pressure sensor, that is, the pressure sensor's (VCC (V+) and GND (V-))[DUP4-DUP2].Currently set to 9.14v, DUP4-DUP2 is already [5.02+(-4.78)≈10.03v].As I said before, C1105 no longer changes at 9.000v, Same as 9.14V. Do you think it needs to go higher?(The maximum operating current of the pressure sensor is 2.4ma, so I have to consider preventing it from burning out.)The same is true for R1116 (ZERO),Terry Bites suggestion ("ZERO" sets = REF+ = REF-),That is to say, setting it at 4.51 and -4.63 is the best adjustment value，But this is the limit setting of R1116 (ZERO).And at this time N4 cannot be adjusted to ±0.005vdc through R1102 (NULL).I had to compromise and set R1116 (ZERO) at 3.45v and -5.68v.Seems to have lost the "ZREO" effect(Please see the attached "ZERO" picture)Quote from: iMo on June 29, 2025, 03:06:17 pmPlease see the pressure sensor data sheet in the figure below (125PC05D1)Quote from: iMo on June 29, 2025, 03:06:17 pmPlease watch the video below carefully,When I enter "10" the test is "good board"，“Good board” also has “drift”，In the video you can see that the "good board" starts from 357, 358, 359..363..367..379......。The changes are not that drastic.Finally, the "good board" can be stabilized at 387.In other words, the "good board" variation is always in the 250-500 range as described in the service manual.Switch to N4 voltage value, which is ±0.005vdc corresponding to 387.I haven't tested the voltage fluctuation range of "good board" N4 in detail. If you think it is necessary to test, I will arrange the test and shoot a video.

author: daisizhou
author_rank: Super Contributor
timestamp: June 29, 2025, 11:02:25 pm
content: Quote from: dietert1 on June 29, 2025, 08:35:37 pmI measured the voltage of the pressure sensor (S+ and S-) and then adjusted R1111 (XD BAL).That is to say, adjust R1111 (XD BAL) so that (s+ and s-) display 0.000v,Is my measurement method wrong?You mentioned Terry Bites' suggestion ("ZERO" setting = REF+ = REF-)，Adjust R1116 (ZERO)，That is to say, the settings at 4.51 and -4.63 are the best adjustment values, but these are the limit settings of R1116 (ZERO).At this time, N4 cannot be adjusted to ±0.005vdc through R1102 (NULL). I had to compromise and set R1116 (ZERO) at 3.45v and -5.68v. It seems that the "ZREO" effect is lost.Quote from: dietert1 on June 29, 2025, 08:35:37 pmAs I mentioned earlier, the voltage across C1105 is a critical value at 9.000v.If it is set at 8.9v, then fluctuations will occur.So I set the C1105 to 9.14v based on the "good board",And at this time the working voltage of the pressure sensor is already (5.25+(-4.78)≈10.03v).[Considering that the operating voltage of the pressure sensor is less than 12VDC and the operating current is less than 2.4ma]Quote from: dietert1 on June 29, 2025, 08:35:37 pmGive me some time and I will test it again as you said.You haven't answered me yet, is my ("22 ohm + 1uf" buffer + 1K ohm resistor) installed in the right place(Installation by extending the lead wire)?Do I need to permanently install the 1K ohm resistor on the PCB?

author: dietert1
author_rank: Super Contributor
timestamp: June 30, 2025, 12:52:44 am
content: You ignored imo's suggestion to leave those parts in until you found a good solution. Those parts help to avoid known risks of failure.And you should really run the reference without the adjustment trimpot. It works best at 10 V and it is not made to generate 9.14 V. Removing the trimpot avoids a known risk of failure.I suggested to you a test that can determine whether the source of drift is in the instrumentation amplifier and the Sallen-Key filter or not. The test starts with installing a short between sensor pins DUP1 and DUP3. You ignored that. Don't you understand what is a short circuit?

author: daisizhou
author_rank: Super Contributor
timestamp: June 30, 2025, 02:04:18 am
content: Hi dietert1Please answer the following 4 questions:1、 My ("22 ohm + 1uf" buffer + 1K ohm resistor) installed in the right place(Installation by extending the lead wire)?2、Do I need to permanently install the 1K ohm resistor on the PCB?3、How to adjust the R1111 (XD BAL) trimmer resistor.4、Is my current value of adjusting the trimmer resistor R1116 (ZERO) correct?[You mentioned Terry Bites' suggestion ("ZERO" setting = REF+ = REF-)，Adjust R1116 (ZERO)，That is to say, the settings at 4.51 and -4.63 are the best adjustment values, but these are the limit settings of R1116 (ZERO).At this time, N4 cannot be adjusted to ±0.005vdc through R1102 (NULL). I had to compromise and set R1116 (ZERO) at 3.45v and -5.68v. It seems that the "ZREO" effect is lost.]

author: daisizhou
author_rank: Super Contributor
timestamp: June 30, 2025, 02:27:37 am
content: Quote from: dietert1 on June 30, 2025, 12:52:44 amYou mean to remove the pressure sensor from the PCB, then short S+ and S-, and then test N4, right?

author: daisizhou
author_rank: Super Contributor
timestamp: June 30, 2025, 06:10:51 am
content: Hi dietert1As you said, I used four 5K resistors to simulate the pressure sensor and installed it on the PCB for testing.Indeed, the N4 output no longer fluctuates.I don't know if it's because the voltage has reached the limit and no longer fluctuates or for some other reason. Please watch the video.So how should I adjust it with the pressure sensor installed?

author: iMo
author_rank: Super Contributor
timestamp: June 30, 2025, 06:57:03 am
content: My simulation shows the R1118 (17.8k in your schematics) and R1113(6k97 in your schematics) have the largest influence on the voltage between DUP4 and DUP2 and on the setting of ZERO.The lower the value the higher the voltage between the DUP4 and DUP2.With the 17.8k and 6.97k there I get only about 3.5V between the DUP4 and DUP2 (for any REF01 voltage) with 3k5 resistors in the bridge and I cannot set "ZERO".So perhaps their values are wrong there in the schematics (or the bridge resistors values should be much higher).Double check their values please.PS: What is the resistance between the pins of the sensor (the bridge resistors)?PPS: with 5k bridge resistors I get (in simulation) 8.3V between DUP4 and DUP2 with 9.14V REF01 output (and I can set ZERO).PPS: with 5k bridge resistors I get (in simulation) 9.05V between DUP4 and DUP2 with 9.97V REF01 output (and I can set ZERO).So, provided the R1118 (17.8k) and R1113 (6.97k) are ok, then the sensor's bridge resistors dropped down with their resistances and the ZERO setting went off the range.With 3k5 bridge resistors (for example) the R1113(6k97 in the schematics) should be 4k2, and I get in simulation the voltage between DUP4 and DUP2 10.14V with 9.97V REF01 and I can set ZERO fine.DO NOT install sensor back.DO NOT remove 1k and the snubber (22+1u) from the board.DO NOT turn with XD_BAL and NULL trimmers.

author: daisizhou
author_rank: Super Contributor
timestamp: June 30, 2025, 08:53:20 am
content: Quote from: iMo on June 30, 2025, 06:57:03 amPlease see attached pictures R1113, R1118Quote from: iMo on June 30, 2025, 06:57:03 amPlease see the attached picture 11, which is the actual measurement value of the pressure sensorQuote from: iMo on June 30, 2025, 06:57:03 amOK, I will wait for your order to proceed with the next test

author: iMo
author_rank: Super Contributor
timestamp: June 30, 2025, 09:02:14 am
content: OK, so the 6.97k is ok.And the value of the R1118 (17.8k in the schematics)?What voltages at DUP1/2/3/4 you measure against GND with the 5k resistors in the bridge with your 9.14V REF01?Can you set ZERO trimmer such you see 0V against GND on DUP1/3?DO NOT post pictures, just post the values..PS: below what I see in simulation with 4.53k resistors in the real bridge and 5.9k R1113 resistor (instead of the 6.97k (R1113)).Zero output at DUP1/3 in aprox the middle of the ZERO trimmer for REF01 from 9.14 to 9.97V..Voltage between DUP4 and DUP2 is from 8.98V to 9.80V for REF01's output from 9.14V to 9.97V.

author: daisizhou
author_rank: Super Contributor
timestamp: June 30, 2025, 10:00:28 am
content: Quote from: iMo on June 30, 2025, 09:02:14 amSorry for missing the R1118 measurement value, I will add it hereR1118-17.734K The color wheel shows: brown purple gray red brown red (17.8k ±1% 50ppm/k)Quote from: iMo on June 30, 2025, 09:02:14 amAt 9.13v,DUP1-（-0.728v） DUP2-(-4.89V) DUP3-(-0.719V) DUP4-(3.44V)Quote from: iMo on June 30, 2025, 09:02:14 amCurrently the most correct way I know of to adjust R1116 (ZERO) is Terry Bites' suggestion ("ZERO" setting = REF+ = REF-).That is the "Best condition" on the right side of "22" in the picture below. Ref+=4.51v Ref-=-4.63v.Unfortunately, in this state, the HI voltage, that is, the N4 voltage, cannot be adjusted to ±0.005 VDC specified in the service manual through R1102 (NULL).I had to compromise and use the "good board" measurements, Ref+: 3.45v Ref-: -5.68v。That is the "Current status" on the left side of "22" in the figure below.I know this is not the best R1116 (ZERO).If you need me I can reproduce the "Best condition" and measure the voltage between DUP1/3 and GND,Please tell me if you needQuote from: iMo on June 30, 2025, 09:02:14 amSorry, I am not familiar with analyzing circuit principles, I will wait for your suggestions for further testing

author: iMo
author_rank: Super Contributor
timestamp: June 30, 2025, 10:22:47 am
content: You will not get "REF+ == REF-" (what are those voltages actually??) in this circuit, I am afraid.What is important for you at this stage are the voltages at DUP1/2/3/4.To make the story short - it seems to me your resistors in the sensor changed their value a little bit, therefore your ZERO setting (you do with ZERO trimmer R1116) went off the range.The REF01's output voltage is not critical, as I wrote, what is critical are values of the R1113 and R1118 when the bridge resistor have changed.Please try to look at the two simulations I've made. Perhaps dietert1 may add more, as he indicated to you already the change of a resistor value around the current source will be needed when the bridge resistors get old (and the simulation shows that).Based on what I see here you may try to put the sensor back on the board, you may try to change the R1113 to 5.9k (aprox), set the REF01's 9.5V for example (it does not matter much), and with the ZERO trimmer R1116 set DUP1 or DUP3 voltage to 0V against GND.

author: daisizhou
author_rank: Super Contributor
timestamp: June 30, 2025, 10:33:36 am
content: I will continue to wait for you and dietert1, and look forward to the next test.I can only tell you the most realistic measurement values, hoping to avoid errors as much as possible.ThanksPS:Do you need to see the video of the "Good board" N4 voltage value, that is, the HI voltage value? If so, please let me know and I will take a video of the RT-202 module (voltage/frequency converter) and the N4 voltageQuote from: iMo on June 30, 2025, 10:22:47 amI will look for the 5.9k resistor first, which will save time. I noticed that R1113 is a special resistor, not the ordinary 1% precision。If you really need me to test, please tell me and I will start the real test

author: iMo
author_rank: Super Contributor
timestamp: June 30, 2025, 12:55:50 pm
content: You may add a resistor in parallel to the 6k97 one in order to decrease its value.Simply calculate the exact value of the resistor, find the closest value, and solder it in there parallel to that 6k97 from the bottom side.It could be the value will not be exactly 5.9k in your case. You have to experiment.Anyhow, with proper value you have to be able to set the ZERO (DUP1/3 to 0V against GND) almost in middle of the ZERO trimmer R1116 (at least my simulation shows that) with any REF01 voltage.The instrumentation amplifier (the opamps right from DUP pins in the schematics) - double check whether the ground of that R1104 resistor is the Virtual Ground (green symbol) or not.

author: dietert1
author_rank: Super Contributor
timestamp: June 30, 2025, 02:49:36 pm
content: Quote from: daisizhou on June 30, 2025, 02:27:37 amYou propose to remove the pressure sensor for this test? You can not do this test without the pressure sensor. Think about it.

author: iMo
author_rank: Super Contributor
timestamp: June 30, 2025, 03:36:10 pm
content: As there is no chance to get a new sensor (as the OP indicates) what he can do is following:1. put the sensor back on the board2. do not remove the 1k and do not remove the snubber (22R+1uF), double check those are wired properly!3. solder a 42k (or 39k) resistor in parallel with R1113 (6k97) from the bottom side of the pcb, this should give aprox 5k94. set REF01's output to 9.2V (measured on C1105)5. measure the voltage between DUP4 and DUP26. set the DUP1 to 0.000V (measured against GND) with R1116_ZERO trimmer7. set REF01's output to 9.9V (measured on C1105)8. measure the voltage between DUP4 and DUP29. set the DUP1 to 0.000V (measured against GND) with R1116_ZERO trimmer10. report the results (no pictures but text with data/values) of 5. 6. 7. 8. 9.11. do not turn with the R1104_XD_BAL and R1102_NULL trimmers12. do not measure N4 at this stage.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 30, 2025, 04:57:34 pm
content: All sagas must end.....

author: daisizhou
author_rank: Super Contributor
timestamp: June 30, 2025, 10:25:40 pm
content: Quote from: iMo on June 30, 2025, 03:36:10 pmLet me describe what happened.This is a differential pressure sensor produced by "MICRO SWITCH",Model is 125PC05D1,Produced in 1989."MICRO SWITCH" seems to be currently acquired by "HONEYWELL".Currently I only see sellers selling brand new 125PC05D1 on EBAY (https://www.ebay.com/itm/153522618703)Unfortunately, I cannot buy from EBAY in my country (customs refuses to import personal items)If I need to replace the pressure sensor, I can only look for a compatible replacement model to use.I tried contacting "HONEYWELL", and they suggested using 26PCBFA6D instead.An important parameter of pressure sensor is sensitivity（sensitivity=Δv/Δp=Full scale voltage/Full scale pressure）Then the sensitivity of 125PC05D1 is(70mv/5psi=14),Then the sensitivity of 26PCBFA6D is(50mv/5psi=10),They have different sensitivities,If you force the use of 26PCBFA6D, you need to change the parameters in the RT-200 system configuration (I don't know how to modify it yet)And from the actual test data of the pressure sensor, the linearity of the pressure sensor meets the standard parameters in the data sheet.Please see the attached picture "test-data",sensitivity=Δv/Δp=(2.36-0.31)mv/10cmH2o=2.05mv/0.14223psi≈14.413And from the actual zero drift test video of the pressure sensor, the zero drift of the pressure sensor is actually between 0.31mv and 0.43mv,±1mv range in accordance with data sheet standards.There is no actual evidence that the pressure sensor has aged.I have not yet found a perfect replacement for the 125PC05D1 pressure sensor.If you have better suggestions, please let me know.Thanks

author: daisizhou
author_rank: Super Contributor
timestamp: June 30, 2025, 10:40:25 pm
content: Quote from: iMo on June 30, 2025, 03:36:10 pmput the sensor back on the board.Adjust R1116 (ZERO), do I need to measure DUP1 /DUP3 set to 0.000V (measured with GND as reference)?Then do the 3, 4, 5, 6........10, 11, 12 tests again

author: iMo
author_rank: Super Contributor
timestamp: July 01, 2025, 04:53:56 am
content: Yes, because we want to see whether the setting of DUP1 to 0.000V could be done easily with the ZERO trimmer for any REF01's output voltage. Simply follow the points there..

author: daisizhou
author_rank: Super Contributor
timestamp: July 01, 2025, 06:34:48 am
content: I mounted the pressure sensor back onto the circuit board。I have not installed the 42k (or 39K) resistor yetIn other words, the entire circuit board is restored to its original state.I adjust R1116(ZERO),DUP1 and DUP3(relative to GND) can be adjusted to 0.000v，As the power-on time increases, DUP1 and DUP3 (relative to GND) 0.000 will slowly increase, for example, 0.001, 0.002...Intermittent adjustment of R1116 (ZERO) is required to keep DUP1 and DUP3 at 0.000vI measured the voltage nearby, hope it helps youREF01 output=3.29v+(-5.84v)=9.13vI will wait for your reply to proceed to the next step of adding a 42k (or 39k) resistor

author: iMo
author_rank: Super Contributor
timestamp: July 01, 2025, 06:54:19 am
content: What was the REF01's output voltage and was it drifting during that measurement?

author: daisizhou
author_rank: Super Contributor
timestamp: July 01, 2025, 07:59:31 am
content: See my updated post.Did I go ahead and add 42k (39k) resistors to test 3, 4, 5...12?

author: iMo
author_rank: Super Contributor
timestamp: July 01, 2025, 08:19:49 am
content: You have measured 9.13V REF01, but you cannot see the drift there at the single mV level.As it has been said few times, the REF01's output 9.13V is too low, it would be better to have it close to 9.97V (your max).That requires to play with the 6.97k resistor and with ZERO trimmer setting , IMHO.The idea is to decrease the 6.97k resistor's value (with a parallel resistor) such the DUP1 will be 0.000V when the ZERO trimmer is around the "middle" (if it is a 10 turn trimmer then the "middle" means 5 turns).Also the voltage between DUP4 and DUP2 should not be larger than 10-11V.Again - it could be your REF01 drifts when set to the 9.13V, as well as your sensor may drift because of aging, etc.So the first step is to eliminate the REF01's drift fully - this is what must be done.Otherwise, when you are happy with the situation as it is now, do simply set N4 to be 0V after say 60minutes your measurement box is powered on (closed box warm up) and always wait 60minutes before you start to do your pressure measurements.You have decided to repair your measurement box DIY - thus you have to take the decision what to do next.

author: daisizhou
author_rank: Super Contributor
timestamp: July 01, 2025, 09:47:27 am
content: Quote from: iMo on July 01, 2025, 08:19:49 amPlease see the 2 videos below about REF01. The C1105 voltage seems to be related to DPU4-DPU2.C1105 is set to 9.14v. The operating voltage of the pressure sensor (DUP4-DUP2) is already 10.05v.If it is increased to 9.97v, I am worried that the pressure sensor will burn out.And in the video you can see that the C1105 is stable enough at 9.000v, is it necessary to increase it to 9.97v?Quote from: iMo on July 01, 2025, 08:19:49 amNext step is to add the 42k (39k) resistor, give me some time.Do you know the correct adjustment method for R1111=R1104 (XD BAL)?I hope you will give me a reply after watching the 2 REF01 videos, whether to continue to add 42k (39k) resistors

author: dietert1
author_rank: Super Contributor
timestamp: July 01, 2025, 06:44:44 pm
content: Somewhere above i wrote about a pressure sensor i found in our surplus and that it was recommended to run at constant current.Now in this case there is no constant current source but something more complicated. When we look closer at the circuit around U1102.1 LM346:Without R1113 6K97:It is a constant current source for the sensor bridge like in my own example. Current is determined by the resistor R1118 17K8, driven by reference REF01. If the reference is 10 V the current is10 V / (17800 + 475) Ohm = 0.55 mA. U1102.1 compares the voltage across the two current measurement resistors R1112 475 Ohm and R1114 475 Ohm and drives the same current into the sensor bridge. This is a constant current source, i.e. biasing the sensor bridge with "infinite" source resistance.Constant current drive doesn't seem to be enough for this pressure sensor. Now add R1113 6K97:This resistor adds apositivevoltage feedback into the bridge biasing circuit. And as R1113 6K97 is smaller than R1118 17K8 this feedback isn't a small correction but a large contribution. The result is a sensor bridge drive with a negative source resistance. The higher the bridge resistance, the higher the drive current! This creates a certain risk of malfunction as the relationship between R1113 6K97 and the sensor bridge resistance is quite critical.Regards, Dieter

author: daisizhou
author_rank: Super Contributor
timestamp: July 01, 2025, 09:55:22 pm
content: Quote from: dietert1 on July 01, 2025, 06:44:44 pmDo you mean that the pressure sensor output can be stabilized only by continuously adjusting R1113 (6K97) and finally stabilizing the working point at the red arrow in the figure below?Should I continue to adjust R1116 (zero point)?Follow Terry Bites' suggestion ("ZERO" setting = REF+ = REF-)

author: daisizhou
author_rank: Super Contributor
timestamp: July 01, 2025, 10:42:19 pm
content: Hi iMo3. Install a 40k ±1% resistor in parallel with R1113 (6k97)4. Set the output of REF01 to 9.14V (measured on C1105)5. The voltage between DUP4 and DUP2 is already 15.05v*****This testing scheme carries serious risks,!!!!!Pause execution!!!!!*******

author: iMo
author_rank: Super Contributor
timestamp: July 02, 2025, 06:08:20 am
content: This is what I see based on your data/values (see below, X axis is the position of the ZERO trimmer).So the data inputs into my simulation are not precise.Yes, stop the execution and try something different.

author: daisizhou
author_rank: Super Contributor
timestamp: July 02, 2025, 06:17:16 am
content: Quote from: iMo on July 02, 2025, 06:08:20 amOK. I am waiting for your analysis results,Looking forward to the next test

author: daisizhou
author_rank: Super Contributor
timestamp: July 02, 2025, 07:39:03 am
content: Hi iMoI would like to ask, assuming the worst case is the ultimate root cause of the pressure sensor aging,Can I choose a pressure sensor (also differential pressure type) that is more easily available on the market to replace 125PC05D1?In other words, keep the A3A4 PCB, replace the pressure sensor, reconnect the pipe interface, recalibrate, and finally make it suitable for the RT-202 host and meet the parameters specified in the service manual. Of course, it is better to exceed the original technical performance parameters.

author: dietert1
author_rank: Super Contributor
timestamp: July 02, 2025, 11:48:38 am
content: Quote from: daisizhou on July 01, 2025, 09:55:22 pmI just wanted to show you that there is some understanding of what the circuit is meant to do. Looking into it, you will notice a minor inconsistency: I included the R1112 475 Ohm current measurement resistor in the bridge voltage, but not in the current calculation.If i had to verify the circuit i would probably replace the bridge by some different resistors in the 4.7 KOhm to 5.3 KOhm range and measure the bridge bias for each resistance. Then one should get the blue curve "Actual circuit" in my diagram or something similar. During such a test one needs to monitor supply voltages and the like in order to avoid invalid results. Also the result depends on the 9.97 V reference voltage from the REF01.I think it will be very difficult to do for somebody who wants to post a youtube video for each multimeter measurement. A replacement of the pressure sensor will not be that easy. Another sensor may require constant current drive, that means a modified bridge biasing circuit.Regards, Dieter

author: daisizhou
author_rank: Super Contributor
timestamp: July 02, 2025, 12:28:16 pm
content: Quote from: dietert1 on July 02, 2025, 11:48:38 amIn the picture, the "R bridge" (4700, 4850, 5000, 5150, 5300) you refer to, are you referring to any of the four 5K network resistors that I used to simulate the pressure sensor?And where is the current that needs to be monitored?I just want everyone to see the real measurement results, so I took a video and uploaded it to YOUTUBE to avoid mistakes.As I said, the voltage of 9.000v across C1105 is stable enough and will not fluctuate.9.14v is definitely more stable.But many people always ask me to adjust the C1105 voltage to 9.97v, so I can only shoot YOUTUBE to illustrate that 9.000v is enough

author: dietert1
author_rank: Super Contributor
timestamp: July 02, 2025, 06:11:13 pm
content: No, the four resistors that simulate a bridge are a different thing.For the biasing circuit the bridge looks like one load resistor of about 5 KOhm between pins DUP2 and DUP4. I don't remember whether you measured that resistance on your sensor with an ohm meter. With temperature variation that bridge resistance will also vary a little and the biasing circuit uses that variation for temperature compensation of the pressure measurement. The pressure sensor is a pressure sensor and at the same time a temperature sensor.For my diagram i assumed the resistance is in the range of 4700 to 5300 Ohm. The sensor biasing current flows through the current measurement resistor R1112 475 Ohm and the bridge resistance. The sensor current can be measured as a voltage on R1112 (I = U / 475 Ohm). One can start with a 4700 Ohm resistor and add 150 Ohm resistors. One needs at least 3 measurements to pin down the biasing circuit characteristic around the resistance value of the real sensor.

author: daisizhou
author_rank: Super Contributor
timestamp: July 02, 2025, 10:54:29 pm
content: Quote from: dietert1 on July 02, 2025, 06:11:13 pmShould I do the following?1、Remove the pressure sensor and PCB connection.2、Connect a 4.7K resistor to DUP4 and DUP2,DUP1 and DUP3 are left floating.3、Turn on the power supply, measure the voltage across resistor R1112, and record.4、Disconnect any pin of 4.7k, connect a 150 ohm resistor, and continue to measure the voltage value on R1112 and record it.5、Repeat the above steps and finally record the voltage values ​​of 5 R1112.*Please see the diagram in picture “11”

author: daisizhou
author_rank: Super Contributor
timestamp: July 03, 2025, 09:06:15 am
content: Hi dietert1If there is no error in the test steps and methods of the 156th floor, the "data" in the figure below is the test result.I tested the 4.7k resistor accurately and it is actually 4.8003k.The actual value of 150 ohm resistor is 149.31~149.93, please refer to the mark for details.In addition to measuring the voltage value of R1112, I added the voltage value of C1105 and the voltage value of DUP4-DUP2.If you require other measurements please let me know. I will await your response.Thanks

author: iMo
author_rank: Super Contributor
timestamp: July 03, 2025, 10:35:15 am
content: FYI - I get almost identical results in the sim.Btw., in my previous sim above there are the bridge's resistances I calculated based on his picture with his sensor's resistances (between the adjacent pins around 3.4k) he measured and marked in yellow.

author: dietert1
author_rank: Super Contributor
timestamp: July 03, 2025, 09:35:49 pm
content: I also compared the measurement to the model. For this i corrected an inconsistency in the model. Then i scaled the measurements up by 10/9, as the model assumed a reference voltage of 10 V. The measured curve agrees very well to the model. So that part of the circuit seems to be working as expected.Was there a resistance measurement of the sensor between DUT2 and DUT4?

author: iMo
author_rank: Super Contributor
timestamp: July 03, 2025, 09:42:36 pm
content: ..

author: daisizhou
author_rank: Super Contributor
timestamp: July 03, 2025, 10:46:52 pm
content: Quote from: dietert1 on July 03, 2025, 09:35:49 pmDUT2 and DUT4 5.215K.I once again measured the actual object and attached the standard pressure sensor bridge arm resistance data from the data sheet.As you said, this part of the circuit is normal, so where exactly is the fault? That is why the voltage of N4 fluctuates.

author: dietert1
author_rank: Super Contributor
timestamp: July 04, 2025, 02:20:31 am
content: Looking at your sensor resistance measurements, the 5.215 KOhm between DUP2 and DUP4 is within datasheet spec (5 KOhm +/-10 %).Your other resistance measurements are surprising, especially between DUP1 and DUP3 where i expected about 5 KOhm, too. But if i read the yellow numbers correctly, the measurement was 3.3042 KOhm. Please check that measurement.And if the bridge had four resistors of 5.2 KOhm then i would expect measurements of1/(1/5,2+1/15,6) = 3.9 KOhmfor the other measurements. The lower values of 3.48 and 3.39 KOhm you saw are strange. Maybe the sensor has internal leakage paths, e.g. from humidity or corrosion. The data sheet says: "Media compatibility - contact MICRO SWITCH". I think the sensor might be sensitive to humidity. You could try to remove humidity in an oven, for example some hours at 60 to 80 °C. The datasheet allows -55 to 125 °C.I would repeat the resistance measurements after drying the sensor.Regards, Dieter

author: daisizhou
author_rank: Super Contributor
timestamp: July 04, 2025, 02:39:20 am
content: Quote from: dietert1 on July 04, 2025, 02:20:31 amOK.It is summer in my area, the temperature is 27-35 degrees Celsius.The weather is fine and the sun is shining.I will place the pressure sensor in the sun.Keep exposed to sunlight for about 5-6 hours, then re-measure,Give me some time and I will tell you the results.Thanks

author: daisizhou
author_rank: Super Contributor
timestamp: July 04, 2025, 08:09:50 am
content: Quote from: dietert1 on July 04, 2025, 02:20:31 amFollowing your suggestion, I placed the pressure sensor in the sun for 6 hours.I then used a household hair dryer with a heated wire to bake the pressure sensor for 30 minutes.After cooling, the resistance of DUP2-DUP4 is 5.217 kOhm.DUP1-DUP3 resistance 3.3045 kOhmThe resistance values ​​of other pins are still the same as in the picture on the 161st floor, and there is no improvement.

author: dietert1
author_rank: Super Contributor
timestamp: July 04, 2025, 09:01:00 pm
content: Now i looked once more at the sensor datasheet posted on the first page of the thread. It says that you can clean the sensor with alcohol. Also it shows the bridge biasing circuit that you have and verified. They explain that it provides better temperature compensation than a constant current source.When looking at their circuit i found that they use pins 2 and 4 of the sensor as output. There may be more confusion. Up to now i understood that pins DUP2 and DUP4 are the bridge input pins. For the biasing circuit it makes a difference whether the load is 3.3 KOhm or 5.2 KOhm.Regards, Dieter

author: daisizhou
author_rank: Super Contributor
timestamp: July 04, 2025, 09:49:48 pm
content: Quote from: dietert1 on July 04, 2025, 09:01:00 pmYou mean for me to soak the pressure sensor inside the alcohol solution?Please see the attached picture "pressure sensor",Even if I remove the transparent extension pipe, the inside of the P1 and P2 interfaces are sealed,It is also difficult for the alcohol solution to flow into P1 and P2.Alcohol solution can only flow into the location (purple arrow)Quote from: dietert1 on July 04, 2025, 09:01:00 pmPin 1 of the pressure sensor corresponds to DUP4 (VCC) or (V+) on the schematic diagram.Pin 3 of the pressure sensor corresponds to DUP2 (GND) or (V-) on the schematic diagram.Pin 2 of the pressure sensor corresponds to DUP1 (S+) on the schematic diagram.Pin 4 of the pressure sensor corresponds to DUP3 (S-) on the schematic diagram.The resistance of DUP2-DUP4 is 5.217 kOhm, and the resistance of DUP1-DUP3 is 3.3045 kOhm.The output DUP1-DUP3 is slightly lower than the 5K ohm ± 10% (4500ohm ~ 5500ohm) marked on the data sheet.This is the first time in my career that I have encountered such a failure phenomenon of a pressure sensor.In my opinion, pressure sensors are usually not easy to damage.If it is really damaged, the fault phenomenon is, for example, --- ohm resistance in the bridge arm, which is understandable.(Because it's disconnected)Please watch the 5th floor video carefully,From the video you can see that input "30" is the PCB we have been discussing and the 125PC05D1 pressure sensor,The V/F converter changes very drastically,That is, N4 (HI-GND voltage) fluctuates very violently.But if you look closely at the "10" converter,That is the RT-202 pressure module,This is what I called a “good board”.There are fluctuations, but they are very small and within the range specified in the manual.I have not yet confirmed that they are the same model of pressure sensors,But in appearance, they are almost identical.In summary, I speculate that this may be an inherent defect of this pressure sensor.(It may be caused by defects in the production process of pressure sensors in the 1990s.)So it makes no sense to replace the pressure sensor with the same model. In other words, if my guess is true, the new pressure sensor will inevitably have the same drift fluctuation after being used for a period of time.Only by replacing the pressure sensor of another model can this fault be completely eradicated and appear againThe jumper in the lower part of J1102, the resistor R1106 (10K), R1103, and R1109 (TC) together form the adjustable output of the final voltage N4.It looks like a temperature compensation adjustment.The lower part of J1102 is a jumper (may be a coarse adjustment), R1109 (TC) (may be a fine adjustment).Please see attached pictures "tc" and "tc-2"

author: daisizhou
author_rank: Super Contributor
timestamp: July 06, 2025, 03:12:47 am
content: Hi dietert1At present, the only way to completely eliminate the N4 fluctuation fault is to replace the 125PC05D1 pressure sensor, right?If so, I have thought of 2 solutions, which one do you think is better?Solution 1: Replace only the pressure sensor (125PC05D1) and keep the differential signal amplifier board A3A4.(This solution is relatively simple)The specific content is to replace the pressure sensor with 26PCAFA6D,Adjust the resistance value of R1113 (6.97k) appropriately (appropriately reduce 6.97K),It is also possible to adjust R1101 (100 ohms) appropriately.Finally, N4 (that is, HI-GND voltage) is made to conform to the one-to-one correspondence curve between pressure value and voltage value shown in "picture 1".In other words, I input the gas pressure value on the left side of "picture 1" into any gas pressure input port of the 26PCAFA6D pressure sensor, and the voltage value on the right side of "picture 1" is outputted by the 4th pin (HI-GND) of J1102 on the A3A4 board (N4).Why choose 26PCAFA6D pressure sensor:The sensitivity of the original pressure sensor 125PC05D1 is 14.0±0.3mv,The sensitivity of the 26PCAFA6D pressure sensor is 16.7±2mv.Frankly speaking, the working pressure range of 26PCAFA6D is (0-1PSI), the working pressure range of 26PCBFA6D is (0-5PSI), and 26PCBFA6D is more suitable for 125PC05D1 (0-5PSI).But the sensitivity of 26PCBFA6D is 50. FullScale Output Voltage=Sensitivity x Excitation Voltage/1026PCAFA6D needs to set the Excitation Voltage to 11.93v,The sensitivity is exactly 14.0.That’s why I chose 26PCAFA6D.This is also the reason for adjusting R1113 (6.97k) (the voltage is now about 10v).Solution 2:Redesign A3A4 circuit board + 26PCBFA6D pressure sensor(This solution is more complicated, but the effect is expected to be better than Solution 1)Redesigned the circuit, made the PCB circuit board, and used the MAX1452+26PCBFA6D pressure sensor.MAX1452 integrates temperature compensation, amplification and other functions. It is necessary to program the voltage to match the voltage value on the right side of "picture 1"For details, please see the application manual "sps-siot-008066-1-ciid-49885"Looking forward to your replythanks sizhou

author: daisizhou
author_rank: Super Contributor
timestamp: July 14, 2025, 02:30:13 am
content: Hello everyoneDuring the holidays, I went to a friend's house. He also had an RT-200 host. I disassembled his RT-203 module.I saw another PCB version of the hardware and it looks like the pressure sensor has been changed to a new model.Please see the attached picture.It seems that the manufacturer has updated the hardware design

author: daisizhou
author_rank: Super Contributor
timestamp: July 16, 2025, 03:20:41 am
content: Hi dietert1The conclusion we discussed a few days ago is that the aging of the 125PC05D1 pressure sensor leads to a decrease in output impedance.https://www.alldatasheet.com/datasheet-pdf/pdf/282914/SENSORTECHNICS/SCX15DNC.htmlI randomly found 2 SCX15DNC pressure sensors in the electronic warehouse,And measured the input and output impedances,I found that the actual measurement is different from what is indicated in the data manual,That is to say, a decrease in output impedance does not necessarily mean that the pressure sensor is damaged.Please take a look at my test images.Can this indicate that 125PC05D1 is also in the same situation?

author: daisizhou
author_rank: Super Contributor
timestamp: July 16, 2025, 11:38:47 pm
content: Can anyone help me?I replaced 125PC05D1 with SCX15DNC, but the N4 output still fluctuates

author: daisizhou
author_rank: Super Contributor
timestamp: July 19, 2025, 11:07:13 pm
content: Can anyone help me?The differential amplifier circuit outputs the voltage N4 with zero drift and cannot display zero position.

author: daisizhou
author_rank: Super Contributor
timestamp: July 22, 2025, 11:11:46 am
content: Can someone help me analyze the circuit? My problem still exists.