Improving Adjustable Dual Voltage Bench Power Supply - Page 1
Forum section: electronics / projects

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 12, 2020, 01:30:58 am
content: Greetings,This is an adjustable dual voltage power supply that I've been putting together for quite a while.I am using a salvaged center tapped transformer and rectifier and adding my circuit to the mix. The final product will have a single multi turn 4k7 pot. Basically, I don't want a dual gang pot and I also don't want a coarse a fine adjustment pot pair.The schematic is actually something that some folks on this forum help me put together a couple of years ago and it works fine. But I can't get the negative voltage to go to 0V. The voltages are nicely matched when I turn the pot up and down, but only when I get down to 3.2V. After that, the positive voltage keeps going down all the way to zero, but the negative stays stuck on -3.2V.Any ideas how I can fix this?Thanks...

author: xavier60
author_rank: Super Contributor
timestamp: April 12, 2020, 02:16:53 am
content: Measure the voltage on all of the op-amp's pins, in the fault state.

author: mikerj
author_rank: Super Contributor
timestamp: April 12, 2020, 10:12:54 am
content: The 10uF capacitor from the op-amp output to ground doesn't look ideal. The 741 is not suitable for driving into such high value capacitors so it could be oscillating under some conditions.It is used to compensate the loop since the feedback path has the LM337 in it, but it's not a nice way to do it IMO.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 12, 2020, 02:03:19 pm
content: Thank you both.I measured the voltages at various points.The exact incoming voltage is +24.4 ; 0 ; -24.4The max output voltage is +22.8 ; 0 ; -22.8In fail state, the voltages at the OP Amp pins are as follows.1: -24.42: -1.63: 04: -24.45: -24.46: 4.17: 58: 0What I discovered that's interesting is the voltage at pin 2. In fail state the voltage is -1.6V but that voltage is 0V in the entire range when the output voltages are nicely matched. So, when I keep decreasing the output voltage by turning the pot CCW, the pin 2 voltage starts going below 0V right at the point when the negative outgoing voltage stops decreasing below the -3.2V. There's obviously a connection.Regarding the C9 10uF cap. Should I just remove it?Thanks...

author: xavier60
author_rank: Super Contributor
timestamp: April 12, 2020, 02:48:47 pm
content: Quote from: Adinol on April 12, 2020, 02:03:19 pmThe op-amp is working properly. With +4.1V on the ADJ pin of U3, its output should be zero. the regulator could be faulty or something else is causing the -3.2V to be on the output. Also try putting a small load on the negative output.

author: xavier60
author_rank: Super Contributor
timestamp: April 12, 2020, 03:01:15 pm
content: Actually, those types of regulator ICs need some minimum load. Try various value resistors between the Output and ADJ pin.Try 270Ω or higher. Lower value resistors put more load on the LM741.C9 will be reversed biased at low voltage settings. Try removing C9 and putting something like a 1uF ceramic capacitor between pins 6 and 2 of the LM741.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 12, 2020, 04:07:56 pm
content: Quote from: xavier60 on April 12, 2020, 03:01:15 pmThat solved the problem. Thank you very much for that.To be clear, I put a 270Ω resistor between the output and adj pins of U3.Quote from: xavier60 on April 12, 2020, 03:01:15 pmI'm not sure if there is typo in your instructions. You said to use a 1uF ceramic capacitor. Are there 1uF ceramic capacitors?Thanks.

author: xavier60
author_rank: Super Contributor
timestamp: April 12, 2020, 05:22:19 pm
content: Yes, or any type of non-polar capacitor. The ceramic type are often called "monolithic capacitor".

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 12, 2020, 06:14:27 pm
content: OK, this works perfectly, now, all the way down to 0V.I made an updated schematic for prosperity, and I'll definitely will be posting pictures of the finished product.There are a few other details that I am hoping to resolve.One is the fuse blowing up. So, the salvaged transformer came with an EMI filter and bridge rectifier. The EMI filter board has a fuse. The 2A/250V fuse flows when I turn off the AC power switch. I am using a variable transformer to slowly power up/down, but when I just turn off the AC switch the fuse blows and there is no load on the power supply.Does that have something to do with the big 4700uF caps on the bridge rectifier?Thanks...

author: David Hess
author_rank: Super Contributor
timestamp: April 13, 2020, 03:40:53 am
content: I would probably try to find a way to get both outputs to track each other but I am not sure how to do it easily and also support output voltages down to zero. The circuit you have is the common way to handle it.Quote from: mikerj on April 12, 2020, 10:12:54 amIf the capacitor ESR is high enough to prevent emitter follower oscillation, like with a common aluminum electrolytic or solid tantalum, then the the capacitance combined with the output resistance of the operational amplifier can result in dominant pole frequency compensation and stable operation. This is sometimes done when a low AC output impedance is required and it is not a bad idea in this case since the LM337 is designed to operate with a low AC impedance at its adjustment pin.Quote from: Adinol on April 12, 2020, 04:07:56 pmThat was going to be my suggestion. Floating regulators like the LM317 and LM337 have their quiescent current flow into their output pin instead of adjustment pin so if the load current is not sufficient, then the voltage between the adjustment pin and output pin climbs higher than 1.25 volts. 1.25 volts / 270 ohms will produce about 5 milliamps which the 741's output can handle although I would still buffer it with a bipolar emitter follower or JFET source follower.

author: rdl
author_rank: Super Contributor
timestamp: April 13, 2020, 04:01:05 am
content: I don't know why you'd blow a fuse when turning it off, but if it only happens when turning on, it's probably the inrush current that's blowing the fuse. The 4700uF caps are around 2 to 3 times what you actually need. If the fuse is a fast blow type, you could try switching it to slow blow.Quote from: Adinol on April 12, 2020, 06:14:27 pm

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 13, 2020, 05:04:51 am
content: Quote from: David Hess on April 13, 2020, 03:40:53 amSo, should I put that 10uF cap back?Quote from: David Hess on April 13, 2020, 03:40:53 amI don't know how to do that. Can you point me to some resources?In any case, I worked all day on this and pretty much put the thing together on a board. I just drew it by hand, drilled and removed copper with a dentist's drill on my Dremel tool. I even went through the trouble of installing a salvaged multi pin connector for the ribbon cable.When I was testing my board I noticed that the volt meter was reading -0V on the positive output, when I turned the pot all the way down. I switched the voltmeter to mV range and I now see that I am getting a negative reading, when it's supposed to be zero. So, I am getting about -34mv on the positive output and a matched +34mV on the negative output, after it passes 0V.I guess I could just install a trim pot on the adjustment pot wiper and dial it in to hit zero. Or is there a better way to solve this?Thanks...

author: David Hess
author_rank: Super Contributor
timestamp: April 13, 2020, 10:26:00 am
content: Quote from: Adinol on April 13, 2020, 05:04:51 amIt can work either way but there is no good reason to include it; with the capacitor, the 741's frequency response is very limited so output impedance at medium frequencies depends on the LM337.QuoteThe example shown below uses a JFET but for a negative regulator, an NPN transistor could be used.

author: xavier60
author_rank: Super Contributor
timestamp: April 13, 2020, 11:22:17 am
content: The -34mV minimum on the positive output and mirrored on the negative output is very likely caused by a small difference of the reference voltages of U2 and U4. I can't see how it could ever be a practical problem. Just ignore it.Or a small resistor could be placed in series with R3.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 13, 2020, 12:22:48 pm
content: Quote from: xavier60 on April 13, 2020, 11:22:17 amThat's what I was suspecting, too. The reference voltage is not exactly -1.25V. It actually measures -1.85V. But I guess I can just place that resistor in series when I mount the pot into the enclosure.Quote from: David Hess on April 13, 2020, 10:26:00 amThat looks like an interesting project to experiment with down the line for me to learn from. Perhaps at some point in the future I'll upgrade this bench power supply further and include this buffer. Thank you for including this.So, today I plan to work further on the enclosure and I also want to connect the panel meter(s).Ideally I would want to have one panel meter for positive and one for negative voltage as well as current.The transformer has a secondary output winding and there's already a separate bridge rectifier on the board. I plan to use that output to power up the panel meters. So my question is this.If I use this separate power source, is is possible to use one of the panel meters to measure the negative voltage and current? And also to have the negative current wires for both meters connected to the same 0V output on the enclosure?I believe the thick black wire and thin black wire of the panel meter are internally connected.Thank you.

author: xavier60
author_rank: Super Contributor
timestamp: April 13, 2020, 10:15:14 pm
content: The VC288 type meter can be used normally on the negative output but so far I see no easy way on the positive output unless a separate 0V terminal is used for the positive output but this could cause other complications with the way dual polarity loads are connected.

author: xavier60
author_rank: Super Contributor
timestamp: April 14, 2020, 11:10:31 am
content: Here is one solution. It's a current to voltage converter that level shifts the signal voltage across the high-side CS resistor to the low-side.The output can be fed into the I_sense A/D pin of the meter's micro-controller.The performance is dependent on the op-amp's characteristics. Most important is that it will work properly with it's inputs at + supply voltage.I think the LM741 is a suitable starting point. TL071 also.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 14, 2020, 05:12:22 pm
content: Quote from: xavier60 on April 14, 2020, 11:10:31 amThank you for this, although I don't quite understand everything.If I interpret this schematic correctly, the OP Amp is connected to receive power (pins 7 and 4) from the output of my variable power supply. Since that is made to go down to 0V, I guess I have to connect pins 7 and 4 to an independent power source, correct?In tat case, I guess I don't need the D1 and R2, right?Also that R3 resistor is labeled as 100m. The lower case "m" confuses me as it would stand for 100 mili Ohms. Do they mean 100 Mega Ohms?Thanks...

author: xavier60
author_rank: Super Contributor
timestamp: April 14, 2020, 05:40:06 pm
content: Although it could be made to work on the output side. I intended it to be on the unregulated input to U2. The resistor and zener could be omitted depending on the type of op-amp used.The idea should be tested and developed in stages as a separate project at first rather than immediately modifying your power supply and meter.R3 is 100 milliohms, Diptrace didn't understand the Ω symbol. It can be a higher value for early testing so that tests can be done at lower load currents.

author: David Hess
author_rank: Super Contributor
timestamp: April 14, 2020, 06:06:48 pm
content: Quote from: Adinol on April 13, 2020, 12:22:48 pmI have used it several times to make fault protected and blow out proof precision high current power supplies but its minimum output voltage is the reference voltage.The reason that a transistor is used to buffer the operational amplifier's output is that high output current from the operational amplifier causes heating of the die which ruins precision. This probably does not matter for a general purpose power supply but it is still good design practice.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 14, 2020, 06:56:00 pm
content: I don't quite understand. This is an inverting OP Amp to be used to invert the negative voltage so that the panel voltmeter can read it, correct?If so, I don't understand how it will work if it's connected to the unregulated input to U2.Or perhaps I just don't understand how this circuit should be connected to my power supply.

author: xavier60
author_rank: Super Contributor
timestamp: April 14, 2020, 07:06:36 pm
content: Quote from: Adinol on April 14, 2020, 06:56:00 pmIt will allow the panel meter to read the high-side current going into U2.Or did you have other plans for metering current from the positive output?These VC288 meters normally only measure low-side current which will cause problems if used on the positive output.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 14, 2020, 07:20:46 pm
content: Quote from: xavier60 on April 14, 2020, 07:06:36 pmJust to make sure we are going int the right direction let me first explain what this power supply is for.I found a lot of schematics online that require positive and negative voltage for audio amplifiers. So, I just want to have an adjustable power supply to test those circuits.I bought 2 panel meters that measure volts and current and would like to have both of those meters installed, one showing voltage and current for the negative side and the other for the positive side.Hope this makes sense.Thanks...

author: xavier60
author_rank: Super Contributor
timestamp: April 14, 2020, 09:15:34 pm
content: That makes sense. I assumed that there would be a problem in wiring the positive meter to read current properly. Maybe I'm over thinking it.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 15, 2020, 12:15:25 am
content: Quote from: xavier60 on April 14, 2020, 09:15:34 pmIf I understand correctly, there is in fact a problem wiring up the positive for current. I found another thread on this forum...https://www.eevblog.com/forum/beginners/digital-panel-meters-measuring-negative-voltages/...with the following image.Clearly that would not be what I need because I would need the 0V output to be a single terminal.I think I am now better understanding your schematic for the current to voltage converter. Although I still don't fully understand how to wire it all up. I did some experiments, only for the voltages, but they all failed. It seemed like this was going to be a straightforward thing but once it came down to figuring out how to make the connections it's not as easy as I had assumed. And I'm really not finding any solutions through Google.

author: xavier60
author_rank: Super Contributor
timestamp: April 15, 2020, 12:32:02 am
content: Quote from: Adinol on April 15, 2020, 12:15:25 amThere should be no problem with measuring voltage and current for the negative output. The only complication is that the 5V regulator needs to be supplied with about +7V.The meter for the positive output should be ok just measuring voltage for now, powered from U1.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 15, 2020, 01:20:27 am
content: Quote from: xavier60 on April 15, 2020, 12:32:02 amOK, this time I understand everything that you just explained.I guess the question that remains is how to resolve the current for the positive output. Is there a solution?Thanks...

author: xavier60
author_rank: Super Contributor
timestamp: April 15, 2020, 02:14:58 am
content: Quote from: Adinol on April 15, 2020, 01:20:27 amYes, read from Reply #16.The current to voltage converter will likely need some tweaking depending on what type op-amps you have?Then the meter will need a simple mod. I had to do a lot of hacking to make one work properly with my bench supply.https://www.eevblog.com/forum/projects/linear-lab-power-supply/msg2409717/#msg2409717

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 15, 2020, 08:18:56 pm
content: I did some tests on the breadboard. The diagram that was posted in the other thread, drawn on the white board, did not work, because of how that 7805 is wired, by being tapped behind the 337. Even when I tapped in front of the 337 it did not work.I decided to test the voltmeters first and worry about the amp meters later.So, I powered up the positive voltmeter straight from the 7805 that was already on the board that I made (the one that's powering the 741). Then I wired a separate 7805 to provide separate powering for the negative voltmeter. I utilized the salvaged rectifier board, which in fact has two rectifiers. However, there was an internal connection on that salvaged board, which connected the ground of the rectifier 1 with the ground of the rectifier 2. I broke off that internal connection.The voltmeters worked but I'm not sure if they worked as they should. The negative voltmeter seems to have a reading that is slightly off (only by 0.1 volt). But the real question for me is that I had to connect the ground from rectifier 2 with the -Ve output from my power supply board. And I just don't know if I'm supposed to do that. In fact, that's the reason why I severed that internal ground connection, on the salvaged factory rectifier board.I made a schematic to explain what I mean.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 17, 2020, 03:59:32 pm
content: Quote from: xavier60 on April 14, 2020, 11:10:31 amI studied your circuit a bit and tried to gather components for testing.First I made a schematic that shows how I think your current to voltage converter circuit should be connected into my circuit. Is my schematic correct?Also, I can't find any BC558 transistors. But I did find a few PNP transistors in my salvage bin. Can you tell me if any of those are suitable replacements for the BC558?I found the following ones: BD140, 2N3906, S8550, 881Y, C327, B331, A 1296 (I do have more but I have to go through them).You mentioned that the amp meter needs a minor mod. Would that be removing the internal shunt?The way I understand the am meter should be connected is to connect the thick red wire to the OUT of your circuit and the thick black wire to the 0V. Is that correct?I have some .22 Ohm resistors. Would two of those in parallel be a good enough replacement for that 100m Ohm resistor?I am looking for a 12V zener. Would any 12V zener work or do I need to know any other specs for the zener?Thank you...

author: xavier60
author_rank: Super Contributor
timestamp: April 17, 2020, 07:38:32 pm
content: The load resistor R9 converts the current signal coming out of the Collector of the PNP transistor into a voltage just for testing for now.When the circuit is eventually connected to the meter, the load resistance will be in the meter.This project shouldn't be too difficult to get working in a fashion. The hard work will be in refining it to get acceptable accuracy.Does your meter look like this?https://www.sunrom.com/media/content/2260/vc288-500x500.jpgThere are a few ways of getting the current signal into the meter. One safe way involving no mods is to make a connection to pin 5 of the LM358 via a 100K resistor.The PCB is tricky to get out of the housing without breaking off a retainer tab. At the shunt end, I wedge small screw driver between the edge of the PCB and the tab and gently pry open the gap until the protrusion at the end of the PCB is just clear of the slot. Another screw driver is then used to lever up the PCB by the shunt. I then file down the protrusion to make the PCB easier to remove next time.Get the circuit working well enough first.Only the 3 wires are needed to be used on the meter plus the current signal wire.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 18, 2020, 01:00:14 am
content: Thank you, again, for the information.Correct, my meter is the same model. I went ahead and installed that 100k resistor on pin 5 of the LM358. So the meter is prepped.I first glued the resistor in place with rubber cement and when it was dry I soldered the connection.I also found all the necessary components for the circuit. I researched transistors a bit and found out that the 2N3906 should be a suitable replacement for the BC558. I also found a 1W zener diode 1N4742A. The only component I do not have is the 100m Ohm current sensing resistor. But I do have two .22 Ohm resistors that I connected in parallel. So the circuit is now built on the breadboard.If I understand correctly I'll have to cut the trace on my circuit board, in front of the LM317 and tap the board there to run this circuit. I am not 100% clear on how to proceed with the testing, though.Thanks...

author: xavier60
author_rank: Super Contributor
timestamp: April 18, 2020, 02:19:26 am
content: Ideally the voltage across R9 should be 0V and then increase linearly with increasing voltage drop across Rcs.This depends all on the op-amp's specs. The output needs to be able to swing high enough to completely turn off the transistor. If it cant, it will need a 10K pull up resistor from its output pin to + rail.Next is that the op-amp can work normally with its input pins at + rail voltage. I know that the LM741 can have its inputs very close to + rail but I don't exactly know how close. If a problem is suspected,a temporary fix can be applied until a more suitable op-amp can be found.Final possible problem is if the op-amp has too much input offset error. An offset null Pot will need to be added. Most single channel op-amps have provision for nulling.http://www.freeclassnotesonline.com/Ideal-741-Operational-Amplifier-Lab.php

author: xavier60
author_rank: Super Contributor
timestamp: April 18, 2020, 03:41:35 am
content: The voltage drop across D2 will ensure that the LM741's inputs are within their proper common mode range.Early testing can be done with a higher value Rcs so that lower load current can be used for a given output.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 18, 2020, 04:09:47 pm
content: Quote from: xavier60 on April 18, 2020, 02:19:26 amI just tested the circuit and I get 0V across the R9 resistor (reads 0V even when multi meter is in mV mode). However it stays at 0V regardless how much I increase the value of Rcs.Since this voltage is not increasing when I increase the value of Rcs, does that mean the the first test failed?Thanks...

author: xavier60
author_rank: Super Contributor
timestamp: April 18, 2020, 06:41:19 pm
content: What value is Rcs and how much load current is flowing through it? And what voltage drop is it causing across Rcs?Did you add the extra diode D2 in my last schematic? It will ensure that the LM741 is happy with the input common mode voltage.

author: xavier60
author_rank: Super Contributor
timestamp: April 18, 2020, 10:39:15 pm
content: For example if a 1 ohm resistor is used for Rcs for testing, the load current only needs to be 100mA to be equivalent to testing with 100 milliohms and 1A of load current.The circuit will behave in the same way in both cases.BTW: Typical audio power amplifiers are actually like a big op-amp with inverting and non-inverting differential inputs.http://www.ecircuitcenter.com/Circuits_Audio_Amp/Basic_Amplifier/Basic_Audio_Amplifier.htm

author: David Hess
author_rank: Super Contributor
timestamp: April 18, 2020, 11:45:33 pm
content: Quote from: xavier60 on April 18, 2020, 03:41:35 amThe LM301A has an input common mode range which includes the positive supply so it is a little easier to use for high side current sensing. Many old JFET input operational amplifiers also have an input common mode range which includes the positive supply.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 19, 2020, 01:23:48 am
content: Quote from: David Hess on April 18, 2020, 11:45:33 pmI was actually looking for an LM301 for my main circuit and could not find one. That's why I ended up using the LM471 for the main circuit. But I think I just made some progress with the current to voltage converter circuit.Thanks...

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 19, 2020, 01:39:45 am
content: Quote from: xavier60 on April 18, 2020, 06:41:19 pmYes, I did in fact use the version with the diode, 10K resistor and 10k pot and I also tested the earlier version of the circuit, although I only included one image in my post.In any event, I decided to pull everything off of the breadboard and rebuild from scratch and I do think that I made progress, as the circuit behaves differently. The very obvious difference is that I do get a reading in mV across the R9 resistor and when I dial the 10k pot I can make that reading go down to 0V. Also, when I increase the resistance of Rcs that reading increases for a few mV's, just as you said it should.Since I do not have a 100m Ohm resistor I am using a 110m Ohm value there by connecting two .22 Ohm resistors in parallel. When I said that I increased the value of Rcs I just took one of these resistors out of circuit.I also made the measurements that you asked for, so here is the data.The unregulated incoming voltage is +24.3V.When the power supply pot is at its max the outgoing voltage is now +22.0VWith +22.0V outgoing voltage the readings are as follows:6.61mA current going through Rcs0.7mv across .11 Ohm Rcs1.4mV across .22 Ohm RcsWith output voltage dialed down to 0V the readings are:4.73mA current going through Rcs0.5mv across .11 Ohm Rcs1.0mV across .22 Ohm RcsI hope these readings are as expected and that the circuit behaves properly.Please let me know what you think.Thank you very much...PS - Thank you also for that link.http://www.ecircuitcenter.com/Circuits_Audio_Amp/Basic_Amplifier/Basic_Audio_Amplifier.htmThis is very interesting and educational for me. Although I think there is a typo there. The output buffer stage has Q4 and Q5, but the schematic has them labeled as Q11 and Q12.

author: xavier60
author_rank: Super Contributor
timestamp: April 19, 2020, 01:58:22 am
content: The behavior of the null Pot and other readings is an indication that the circuit is working.The purpose of the circuit is so that current can be measured. Simply put a load on the PSU's output, anything from 1 to 10 ohms.Adjusting the output voltage will also vary the current flow. Beware of the heatsink temperature.Also: The LM301 seem to be getting scarce and expensive. There are some reasonably priced JFET(P-CH) input TL071's on ebay if not fakes.

author: xavier60
author_rank: Super Contributor
timestamp: April 19, 2020, 03:38:28 am
content: We want to see a useful relationship between PSU output current and the voltage across R9

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 19, 2020, 05:28:37 pm
content: I just finished some testing. I have a bunch of salvaged low Ohm resistors that I was connecting to the output load and I had a multimeter connected to measure voltage across R9. I measured voltage across R9 for each load resistor that I connected to the output.I could get a stable reading across R9 for each load resistor that was 10Ohm or more, but the lower the values of load resistors I was using, the more unstable the readings were. I guess it was due to the heat buildup on the LM317 and on the load resistor itself.Here are some readings I've recordedLoad Resistor - voltage across R91 Ohm .................390mv1.5 Ohm ..............401mV1.8 Ohm ..............411mV2 Ohm ................406mV2.2 Ohm .............431mV3.9 Ohm ..............471mV4 Ohm ................462mV5 Ohm ................494mV8 Ohm ................360mV10 Ohm ..............306mVI also did some readings above 10 Ohm12 Ohm ..............261mV36 Ohm .............106mV50 Ohm .............77.4mv100 Ohm ...........40.5mV200 Ohm ...........20.7mVAgain, it was difficult to decide which reading to write down when connecting lower value load resistors because the readings on the DMM kept dropping as it was heating up. But I tried to capture the first number that showed up.Also, I just wrote down whatever value was written on the resistor. Perhaps I should have measured each resistor first, to make sure they are on spec.Please let me know what your thoughts are.Thank you very much.

author: xavier60
author_rank: Super Contributor
timestamp: April 19, 2020, 11:04:25 pm
content: It's not clear what the actual current is for each resistor without knowing the output voltage.You could have just used a few of those resistors and simply varied the output voltage to change the current.Connect it to the modified panel meter and test it. Expect the calibration to be way off for now.You want the meter reading to accurately track the actual PSU output current.Because these meters don't read properly down to zero, the calibration should be done at some low current like 50mA and some higher current that can be sustained by the PSU, 500mA maybe.Change the value of R8 to correct the high reading, this is your Gain setting. The I-ADJ Pot in the meter can be used for fine adjustment.Adjust the Null Pot to correct the low reading, this is your Zero setting. There will be some interaction, so the adjustments will have to be done few times.Remember to remove R9.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 20, 2020, 01:10:05 am
content: Quote from: xavier60 on April 19, 2020, 11:04:25 pmOh, I see. Will do...Quote from: xavier60 on April 19, 2020, 11:04:25 pmJust to be clear how to connect the modified panel meter. I need to remove R9 and connect the meter by connecting the collector of the transistor with the 100k resistor that I installed inside the meter. Is that correct?

author: xavier60
author_rank: Super Contributor
timestamp: April 20, 2020, 02:12:02 am
content: Quote from: Adinol on April 20, 2020, 01:10:05 amThat's right.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 20, 2020, 03:01:50 am
content: Great!I actually already gathered some data, using a 10 Ohm load as well as a 4 Ohm load, and varying the voltage.current across load ..... voltage across R90.05 A ......................... 12.8mV0.10 A ......................... 24.1mv0.20 A ......................... 47.0mV0.40 A ......................... 92.1mV0.50 A ........................ 114.7mvThis all makes sense this time.Tomorrow I think I'll build a board of this circuit and do all the calibration straight on the board. I will add a 10k VR in series with that 10k R8 resistor so I can just dial in.You were really helpful and I can't find the words to express my gratitude. It is clear that I could not have done this without your help.I'll post an updated schematic and pictures of my work as I put it together.Once I have this power supply I'll finally be able to work on some projects that I wasn't able to work on.Thank you...

author: xavier60
author_rank: Super Contributor
timestamp: April 20, 2020, 03:32:44 am
content: Those readings look very good.In place of R8, make plenty of provision for adjustment. At least 2 parallel resistors, one with a Trimpot in series with it.I sometimes solder in machined IC socket pins so that I can change component values without soldering..The LM358 is actually poorly suited to its function in these meters. Later on you might decide to remove it altogether. The I-ADJ Pot will likely go also, relying totally on R8 for gain adjustment.Also: check to see if the LM741 needs the D2 diode to work properly.

author: xavier60
author_rank: Super Contributor
timestamp: April 20, 2020, 08:06:52 am
content: Also remove the 270K resistor near pin 8 of the LM358.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 20, 2020, 02:36:38 pm
content: Quote from: xavier60 on April 20, 2020, 08:06:52 amI just removed it (it was the one marked 274) but now I no longer get any amp readings on the meter.

author: xavier60
author_rank: Super Contributor
timestamp: April 20, 2020, 03:42:21 pm
content: Quote from: Adinol on April 20, 2020, 02:36:38 pmYou could try putting it back but removing it should not have stopped it from working. The resistor is there to add some positive offset the LM358 which might cause a problem now. The voltages on pins 5, 6, 7 and 8 while there is some input might tell me something.

author: xavier60
author_rank: Super Contributor
timestamp: April 20, 2020, 03:53:07 pm
content: I think I know what's wrong. The meter needs to be zeroed. First see if you can get about 50mV on pin 7 of the LM358 using the Null Pot with the PSU unloaded. Ill look for a link that shows how to zero the meter,

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 20, 2020, 03:54:12 pm
content: When I look at the datasheet for LM358 it says that pin 8 is the positive power supply for the IC. Is there a mistake?

author: xavier60
author_rank: Super Contributor
timestamp: April 20, 2020, 04:02:33 pm
content: Quote from: Adinol on April 20, 2020, 03:54:12 pmPin 8 is +3.3V supply to the LM358. The 270K resistor is connected from the +3.3V to pin 5, the Non-inverting input.

author: xavier60
author_rank: Super Contributor
timestamp: April 20, 2020, 04:24:56 pm
content: If you can't get 50mV on pin 7, put the 270K resistor back if you can still find it and then zero the meter again.I didn't think all this through properly.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 20, 2020, 08:07:26 pm
content: OK, as per your instructions, I did all the steps and I reinstalled the 270k resistor. Then I did some calibration adjustments on the breadboard.To be honest, I am actually very happy about how the circuit is performing. The readings are matching almost 100% and I guess I could continue tweaking until it's even more accurate, but I decided I might as well do it all on the final board. So, as we speak I am prepping the board then I'll populate it with all the components.I'll post photos of the progress and I'll post an updated schematic. I'm actually doing two things at the same time so it takes a while.But I have a technical question about the tapping of the 100k resistor into the #5 pin of the LM358. Is this basically bypassing the internal shut and also converting the amp meter into a volt meter?Thanks...

author: xavier60
author_rank: Super Contributor
timestamp: April 20, 2020, 10:35:46 pm
content: I'm still curious to see if D2 is still needed.It could be seen that adding of the 100K converts it into a voltage meter and it could be used for one.In this situation, the PNP transistor outputs purely current and that's what the meter is measuring.The 100K resistor just protects the input. It could be bridged out or changed to 1M without affecting the calibration.The input signal is combined with the internal CS signal and will still function normally.

author: xavier60
author_rank: Super Contributor
timestamp: April 20, 2020, 11:46:44 pm
content: On your PCB make provision for a small electrolytic capacitor in place of R9. Because you will be powering audio circuits which draw rapidly fluctuation current, some extra smoothing might be needed.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 21, 2020, 01:56:43 am
content: I didn't catch your post in time to include a place for the electrolytic cap. But it's actually very easy to add one, on this type of PCB. I already figured out how and where to place one.Should it be 10uF?Here are some pictures of how I made the PCB, using a dentist's drill bit, just like I made the main board. There are a lot of components used here that were pulled out of discarded electronics, both trim pots, the 100m Ohm resistor, the transistor, and the 100n cap. The rest are new components.I'm still working on the revised schematic, but I'll have to finish it tomorrow.I did test the circuit on the breadboard without the diode and basically the circuit worked, but I remember getting slightly different voltage measurements across R9. I'd be happy to run a test tomorrow and post the data, as I did on a previous test. I can easily take the diode out of circuit by just putting a jumper with alligator clips across the diode, right?

author: xavier60
author_rank: Super Contributor
timestamp: April 21, 2020, 02:20:32 am
content: Your PCBs turn out surprisingly ok considering.D2 can be jumpered out for the test. Try a TL071 if you get the opportunity.A 1uF or a bit less between the PNP transistor's collector and ground should be more than enough smoothing without affecting the reading settling time by too much. A monolithic cap might be better, possibly less leakage.What resistance did R8 need to be? Im also curious if you took note of how much the amperage reading could be changed with the Null Pot alone.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 21, 2020, 04:17:55 am
content: Quote from: xavier60 on April 21, 2020, 02:20:32 amIf I have one I think I will build another breadboard circuit and see how that one performs.Quote from: xavier60 on April 21, 2020, 02:20:32 amIt's a bit odd because on the breadboard I just put a 10k trim pot in series with the 10k resistor and I was able to adjust so that the reading of 500mA matched. I assumed that was going to be the case on the PCB but I still made 3 spots for resistors, just as you've suggested. Then, to my surprise, I was not able to match the reading at 500mA, even when I dialed the trim pot all the way down. So I added another 10k resistor in parallel with the existing one, so that I could dial up the trim pot. That worked out fine.I can't imagine that this was due to subtle differences in the resistance of the PCB traces compared to the internal resistance of the breadboard. There must be some other explanation.Quote from: xavier60 on April 21, 2020, 02:20:32 amWhy don't I just build another circuit on the breadboard and I can post all the readings that you are interested in. This worked out really well for me and for sure I could not have done it without your help. I'd be more than happy to post any measurements that you are interested in or test any variations of the circuit.There is no solution for this that I could find on the internet and I'm pretty sure no one ever figured out any solutions for using these panel meters on a dual power supply. I am really impressed with your skills and forever grateful for your help.Thank you very much.

author: xavier60
author_rank: Super Contributor
timestamp: April 21, 2020, 04:54:15 am
content: Eventually when you understand how the circuit functions, you will be able to calculate the effect of component values such as Rcs and R8.The transistor current can be calculated from the voltage across the 100K.I got the idea from the way constant current sink circuits work, often discussed in this forum.Although it has a different purpose, the operating principles are the same.https://electronics.stackexchange.com/questions/129609/how-does-this-constant-current-sink-actually-workIll stand by.

author: xavier60
author_rank: Super Contributor
timestamp: April 21, 2020, 12:10:04 pm
content: I did some tests on a TI branded LM741 I found here. The Null Pot gave a rather symmetrical input offset range of +/- 10mV.This would result in +/- 100mA change in current reading.The LM741 isn't happy to have its input pins at + supply voltage, 200mV below is enough for it to work properly.Some rough calculations indicate a value for R8 of about 6.8K.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 21, 2020, 12:50:40 pm
content: I plan to do more testing today and post the results. It should be interesting.Perhaps once this is completely finalized and after I've used the power supply for a while I might build another one and put all the circuits one a single PCB. It can't hurt to have two power supplies.There is one question from an earlier post that I did not resolve. Here is how I connected the volt meters.My question was about that connection that appears in the lower right corner of my schematic. As you can see there is an isolated power supply that I am using to power up the panel meter that is used to measure the negative side of the main power supply. In order for that meter to be powered up the thin black wire had to be connected to the ground of the secondary power supply. However, in order for that same meter to measure negative voltage of the main power supply that same wire had to be also connected to the negative output rail. It works and I don't think there's a problem, but I'm not really sure, so I ask.Are there any issues to doing it that way?Thanks...

author: xavier60
author_rank: Super Contributor
timestamp: April 21, 2020, 02:54:10 pm
content: I would use the thin red and black wires just to power the meter from the isolated supply.Connect the thick Black to the main negative output from the PCB.And connect the thick Red to the negative output terminal on the front panel.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 21, 2020, 04:24:51 pm
content: Quote from: xavier60 on April 21, 2020, 02:54:10 pmSure, but since the thick black and thin black are internally connected, the ground from the secondary supply will still be connected to the negative output of the main supply, correct?Is there a technical issue with that?Also, I did some preliminary tests, jumping the diode, using the PCB I just built. Let me explain what the photo shows, first.So, I basically connected the second panel meter to its independent power supply and then I connected that secondary meter with my DMM in series with an 8 Ohm resistor. The modified panel meter is connected through the PCB and there is an alligator jumper connected across the diode, making it our of circuit.The modified meter does not track the readings. If I adjust the gain trim pot to match a higher reading (at 500mA) then when I dial down the output, the meter will not track. It will zero out around 330mA. Also, no amount of turning the null trim pot will fix that.I will do more tests and also build a new circuit, but I have to fist find all needed components.

author: xavier60
author_rank: Super Contributor
timestamp: April 21, 2020, 04:42:40 pm
content: Although the thin and the thick black wires are connected internally, in some situation it's best not to join them externally. It can alter readings.Tracking can be tricky. Try this procedure. disconnect the input to the 100K resistor. Use the instructions in the video to zero the meter's current range. I had one that wouldn't keep its new zero calibration so test it by seeing how much voltage has to be applied to the 100K resistor to get readings of 0.02, 0.05 and 0.50, then let me know.If you cant do this test easily, just connect it back up again and see what effect the Null Pot has on the reading. Dont load the PSU.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 21, 2020, 04:59:04 pm
content: Quote from: xavier60 on April 21, 2020, 04:42:40 pmWill do.Quote from: xavier60 on April 21, 2020, 04:42:40 pmYou mean, when I put R9 back into the circuit, correct?Also, I found a 1uF film cap. Should I just place that cap in the circuit even when R9 is connected?Thanks...

author: xavier60
author_rank: Super Contributor
timestamp: April 21, 2020, 05:08:36 pm
content: Quote from: Adinol on April 21, 2020, 04:59:04 pmDont bother with the capacitor for now. R9 was used only for initial testing of the circuit and was supposed to be left out when the modified meter got connected. The Collector of the PNP transistor should only connect to the 100K resistor in the meter.It's important that when the meter's current range is zeroed, there is no input to the 100K resistor.

author: xavier60
author_rank: Super Contributor
timestamp: April 21, 2020, 10:47:24 pm
content: I might have misunderstood something, it was the middle of the night when I read your posts.The poor tracking was being caused by the bridging out of D2 test. So there is nothing really wrong except for R9 still being in circuit?

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 22, 2020, 02:48:09 am
content: Quote from: xavier60 on April 21, 2020, 10:47:24 pmI think both of us misunderstood each other. And it was in fact me that first misunderstood your post when you said this...QuoteFor whatever reason I thought you wanted me to put R9 back in circuit and do the calibrations with the diode, then do the test. That's why I mentioned R9 and then you thought that I forgot to remove it. But I did in fact remove it as you can see if you look back at the photo I posted in my reply #46. Yo can see how the R9 resistor was cut off, right under the transistor.In any event, I reread you posts and now I understand what you meant. So, to start with I made a log of how many volts I need to apply to a 8 Ohm load to get certain mA readings, done with the diode in circuit and both trim pots properly calibrated. Here is the data.0.00 A @ 0.0 V0.01 A @ 0.0 V (I guess the meter doesn't register less than 40mV change)0.02 A @ 0.1 V0.05 A @ 0.3 V0.10 A @ 0.7 V0.20 A @ 1.6 V0.30 A @ 2.4 V0.40 A @ 3.3 V0.50 A @ 4.1 VAll of the above Amp readings matched perfectly what I was simultaneously reading off of my DMM.Next test I jumped the diode D2 and logged how many volts I need to apply to the 8 Ohm load to make 0.02, 0.05 and 0.50 Amps show up on the meter. I did not change the calibration of the trim pots. I also logged the actual amp readings off of my DMM.0.02 A @ 1.1 V (reading from DMM 0.14 A)0.05 A @ 1.2 V (reading from DMM 0.15 A)0.50 A @ 4.5 V (reading from DMM 0.56 A)Then for the next test I disconnected the jumper across the diode and re-checked that if I was going to get the exact same readings as I did on my first test. At this point I still did not touch the trim pots. The readings were dead on, as on the first test.Then for the next test I jumped the diode again, but this time I calibrated the gain trim pot R8 to display 0.50 A when the DMM was also displaying 0.50 A. Of course, since this was the correct current and I have not changed the resistor it needed 4.1 Volts. Here is the data.0.02 A @ 1.4 V (reading from DMM 0.18 A)0.05 A @ 1.4 V (reading from DMM 0.19 A)0.50 A @ 4.5 V (reading from DMM 0.56 A)You will notice that both 0.02 A and 0.05A needed 1.4 Volts. Well, I guess the change was just a few milivolts and the meter didn't register. In fact, I had to breadboard a fine adjustment 1K pot in series with my 4.7K pot, so that I could hit the fine adjustments between 0.02 and 0.05. It was simply impossible to dial in that level of accuracy with the coarse 4.7 K pot. However, in the final product I will be using a multi turn pot on the front panel.I hope those measurements are helpful.I still didn't have time to breadboard a new circuit cause I got pulled away from my workshop. I'll do it tomorrow. I'm also curious to see how the 071 Op Amp will behave.Thanks...

author: xavier60
author_rank: Super Contributor
timestamp: April 22, 2020, 03:52:55 am
content: All that looks reasonable and expected. There might be a typo in the first table, 2nd line. I notice with all the meters that I have received, they are incapable of displaying 0.01A, they jump directly from 0.00 to 0.02. Whatever the programmer has done also causes an inherent zero offset error.If an unmodified meter is properly zeroed then calibrated at some low current, at higher currents it will read higher than the actual current.You wont have this problem as you are correcting the zeroing error with the Null Pot.You might get some drift caused by the LM358. Try gently flexing the meter's PCB to see what happens to a current reading.With your present experience level, it is too risky to remove the LM358 for now.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 22, 2020, 03:54:05 pm
content: Quote from: xavier60 on April 22, 2020, 03:52:55 amI was actually able to get 0.01A on the display. Here I am including a photo that shows the 0.01 display. I was only able to hit this number because I asses a fine adjustment pot in series with the output voltage adjustment pot (seen in second picture). But even so, perhaps the programmers updated the firmware for the unit. It's possible they were receiving complaints and decided to debug the code.Quote from: xavier60 on April 22, 2020, 03:52:55 amI actually feel quite comfortable removing that chip. I have an SMD heat gun and I've been practicing on discarded electronics for years.What chip would be better suited for this? And should it be replaced in both meters?Also, I finished the updated schematic.Now I'm working on another schematic showing how the meters are hooked up.And I'm also still working on breadboarding a second circuit for testing.Thanks...

author: xavier60
author_rank: Super Contributor
timestamp: April 22, 2020, 05:38:36 pm
content: Where did your meters come from? I would like to try some. I did notice that some of the resistor values are different to mine.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 22, 2020, 08:13:09 pm
content: Quote from: xavier60 on April 22, 2020, 05:38:36 pmI bought this one...https://www.banggood.com/Mini-Digital-Voltmeter-Ammeter-DC-100V-10A-Panel-Amp-Volt-Voltage-Current-Meter-Tester-0_56-Blue-Red-Dual-LED-Display-p-1422461.html?rmmds=search&cur_warehouse=CN...however, one never knows if they send you the exact same model.

author: xavier60
author_rank: Super Contributor
timestamp: April 22, 2020, 10:03:20 pm
content: I put an OPA2192 into mine. It's an expensive part and because it needs a minimum of 4.5V, I did a mod to supply it with power from before the internal 3.3V regulator. It gets complicated.I would use the TLV2186IDR next time. It is auto-zeroing and cheaper but still needs 4.5V.There could be a suitable 3.3V op-amps out there.With your modified meter, the op-amp isn't needed at all. The mod could get messy and difficult to reverse if it doesn't work out.See how it all works for now.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 23, 2020, 01:08:38 am
content: This took me forever, but I finally did finish breadboarding the circuit from scratch.The results are very good. I first used the 741 and did all the alignments and the circuit is tracking 100% with the DMM. The voltages needed to produce certain currents are the same, since I did not change the 8 Ohm load resistor. What's impressive is that the panel meter hits the same amp readings at the same time as the DMM.After I tested the circuit with the 741 I switched it to the 071. There was new calibration needed but after it was calibrated the results were 100% the same as with the 741.I'm including 2 photos of the new breadboard in action.I am not sure if there would be any advantage to using one OP Amp over the other.If you need me to run any specific tests and collect date please let me know. It's all wired up and ready to go, so it would be easy to run additional tests. So, do let me know one way or another.Quote from: xavier60 on April 22, 2020, 10:03:20 pmHow does one go about searching for a suitable OP Amp, anyway? Is there a site where you can input certain parameters and then the search returns a short list of suitable OP Amps?Quote from: xavier60 on April 22, 2020, 10:03:20 pmYou're right. This is working really well and I also don't see any need to get messy. Perhaps one day out of curiosity I might tackle this intricate work, but as far as I see that would be more for educational purposes, as there doesn't seem to be any technical issue left to be resolved.I'll start finalizing the project. I am thinking that perhaps the heat sinks are not adequate. When I run a load at 0.50A they get quite hot quite quickly. I wonder if I even need a fan?Thanks...

author: xavier60
author_rank: Super Contributor
timestamp: April 23, 2020, 02:08:04 am
content: The main advantage of the TL071 is that its input common mode range includes its + supply, so D2 should not be needed.So the only test needed is to confirm that the calibration isn't affected by D2 being bridged out if not done so already.Op-amp manufactures do have parametric searches to help pick parts with a certain sets of specs but is tedious.Or sometimes other members make suggestions.A fan will make a big difference to the heatsink temps. Use a thermocouple probe to confirm.

author: xavier60
author_rank: Super Contributor
timestamp: April 23, 2020, 02:16:55 am
content: One experiment you can do for fun is to measure the range of input offset directly. The voltage difference between pin 2 and 3 can be measured directly with a DMM usually.Edit: there needs to be at least about 20mV drop across the CS resistor for the test to be valid.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 23, 2020, 04:54:14 pm
content: Quote from: xavier60 on April 23, 2020, 02:08:04 amI tested the circuit with the 071 OP Amp and the diode removed.You are right. The circuit with the 071 and no diode works just the same as the circuit with the 741 and diode. The tracking is right on.Quote from: xavier60 on April 23, 2020, 02:08:04 amI do not have a thermocouple probe, but I do have some salvaged heat sinks. I think I'll just put the 317 and 337 on long wires and install them on a larger heat sink at the back on the enclosure. I'll put a fan if possible.The regulators warm up when I go to 500mA. But what kind of current can I expect to draw through my amp test circuits that I plan to work on?Also, I am attaching an updated schematic that shows how the meters are connected.Thanks...

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 27, 2020, 10:08:32 pm
content: It took me a few days to make progress.I ended up using another salvaged transformer (and rectifier). This other one is already inside of a metal enclosure and I will build a wooden cabinet around it.The transformer that I am using now hat a high voltage and a low voltage output. After rectification the high voltage output is +/-45V and the low voltage output is +/-20V. I hooked up my circuit to the low voltage output.I am wondering if I could utilized the +/-45V in some meaningful way. Perhaps I could just have it as a separate high voltage output. Any suggestions?I am attaching a diagram of the new version.I am also attaching a photo of the work I've completed so far.Something odd happened when I hooked it all up. The current to voltage converter circuit did not work properly. The readings were too low and I could not adjust it to be right with the trim pots. After long troubleshooting I jumped the diode and it worked. This is odd because this circuit is using the 471 chip. I guess I'll just put a 071 IC instead, when I finish the project.I do have some additional questions:1 - On my main circuit there is a 7805, the U1, that is getting a full 20V. Would it be a good idea to add a voltage divider, so that it doesn't work so close to maximum specs?2 - Also, I have a salvaged EMI filter. Is there any advantage to installing it?3 - Would it be a good idea to include a Current Limiter Circuit, perhaps a foldback limiting circuit, such as the one explained herehttps://www.electronics-notes.com/articles/analogue_circuits/power-supply-electronics/current-limiter-circuit.php?4 - I installed a separate 12V switching power supply for the fan. I am using it also to power up the negative side panel meter. Are there any potential issues with that?Thanks...

author: xavier60
author_rank: Super Contributor
timestamp: April 27, 2020, 11:53:40 pm
content: Quote from: Adinol on April 27, 2020, 10:08:32 pmThe +/-45V could be used directly to power larger amplifier designs.The behavior of the current to voltage converter doesn't make sense, usually means that oscillation is occurring because of too much wiring in the input side of U2. It could do with an input bypass capacitor.The SMPS, fan and meter arrangement is fine.20V into an LM7805 is more than safe enough and also the current is very low.The EMI filter isn't really needed for non switching power supplies and the SMPS should have its own. There is no harm in having the extra EMI filter.I can't think of an easy way to add extra current limiting except for adding an extra set of regulator ICs configured as constant current sources.

author: rdl
author_rank: Super Contributor
timestamp: April 28, 2020, 04:38:48 am
content: Quote from: Adinol on April 27, 2020, 10:08:32 pmThere's an LM317HV that can handle up to 60 volts.

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 28, 2020, 11:46:39 am
content: Quote from: rdl on April 28, 2020, 04:38:48 amYes, I actually do know about the LM317HV and it did cross my mind, but if I built a HV adjustable supply hooked up to the +/-45V then I would still be left with the un utilized +/-20V from the salvaged rectifier. And in this enclosure there is no space to have two simultaneous adjustable supplies. Perhaps the simplest solution is to just do what xavier60 suggested and just have an extra +/-45V output for powering larger amplifier designs.But I need to resolve a safety issue, first.I noticed that one of the 3300uF caps on the +/-45V rectifier circuits stays charged up, so I could potentially have a dangerous situation on the front panel, even if my bench power supply is unplugged.I don't know what is draining the other 3300uF cap, though. There must be something on that board. I think there are some diodes (and resistors?) under the bridge rectifier heat sink. I have to trace that board and see what's there.It is odd that they would design a circuit that drains both caps on the +/-20V circuit and only one of the two caps on the +/-45V circuit. Perhaps there is a faulty component???I also want to get back to some of the issues that xavier60 replied to, but I first want to resolve this safety issue.Thanks...

author: Frankentronics
author_rank: Regular Contributor
timestamp: April 28, 2020, 04:24:08 pm
content: OK, an SMD resistor was connecting one of the caps to the motherboard that had some leftover components on it. The motherboard is now stripped and there is nothing left to drain the 3300uF caps on the +/-45V rectifier output.What resistor values (Ohms and Watts) should I install there to make sure the caps are drained each time I turn off the power supply?Thanks...

author: xavier60
author_rank: Super Contributor
timestamp: April 28, 2020, 11:42:09 pm
content: Quote from: Adinol on April 28, 2020, 04:24:08 pm10K resistors will dissipate about 200mW, Power = Voltage squared divided by Resistance.If the discharge is too slow, add extra resistors or a lower value with suitable wattage rating.The Time constant = 3300uF X 10K = 33 seconds. The voltage will drop to 37% after 33 seconds.The voltage will drop to 37% of the previous value every 33 seconds.https://www.eecs.tufts.edu/~dsculley/tutorial/rc/rc3.html

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 10, 2020, 01:28:55 am
content: Quote from: xavier60 on April 28, 2020, 11:42:09 pmThank you for this information. I put it to use on a few experiments I was doing.A long time has passed since my last post. I actually thought I would wrap up the whole project in a day or two. I've been tweaking this project every day and I did make some good progress.I am almost done building the cabinet (see photos).Since there was room in the cabinet I decided to add a secondary, isolated, power supply with three set output voltages, 5V, 9V and 12V. I'll be developing battery powered amps using 9V batteries and I figured 5V and 12V will come in handy, too.I'm still working on the front panel layout and I am trying to decide if I should include digital panel meters for every available output. Basically, what I'll have is this:Adjustable from 0V to +/-18Vregulated 5Vregulated 9Vregulated 12Vunregulated +/-45VI've been running the unit with various loads for 1/2periods and it is running nice and cool.I'm attaching a bunch of photos to show where it's all at.Once again, thank you for all your help on this project. It really made a difference.Thanks...

author: kallek
author_rank: Regular Contributor
timestamp: May 10, 2020, 06:22:55 pm
content: For me boards and layout looks good.Cabinet is also beautiful, how you are going to finish it?My choice would be panel meters only for adjustable voltages, but having meters for all channels have also advantages.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 11, 2020, 04:44:32 am
content: It took me all day to finish up the front panel and do all the final wiring (the +/-45V is not hooked up, yet).The front panel is a temporary cardboard "prototype" just to see if the layout works for me. At some later time I will probably use some piece of scrap metal to do the final panel. But before that time comes I will probably mount all the panel meters to the cardboard panel. I just have to buy them.The knob is a recycled bottle cap and the entire cabinet is built from scraps. All the wires are salvaged and most of the electronic components also came from various pieces of discarded electronic equipment.I printed the Frankentronics logo on a Post-it note and glued it to a piece of thin plywood. I cut around it with a scroll saw and gave it a coat of lacquer.I already used this power supply on a project, tonight, and it works great. I was actually already using one of the secondary power supplies (the 9V one) for my component tester as the main supply was hooked up to the breadboard. So I'm really glad I added that secondary supply.Once again, many thanks to everyone who helped, especially xavier60. Without xavier60 I would really not have been able to put this together in this way.I guess once I completely finish it I'll post some more photos.Thanks...

author: xavier60
author_rank: Super Contributor
timestamp: May 15, 2020, 11:41:35 am
content: Quote from: Adinol on April 22, 2020, 08:13:09 pmI bought some of the same.The current readings do start at "0.01" and the tracking accuracy is spot on after a calibration.But the voltage tracking is poor, If I calibrate at 5V, it will read "20.5" when I apply 20V.I have to apply 0.25V to make it read "0.1". There is a permanent negative offset in the programming.I could easily work around this but the deal breaker is the very slow 800mS display update, advertisedas 300mS.It's frustrating that an otherwise useful item has been ruined by poorly developed firmware.The seller has received some feedback from me.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 16, 2020, 02:28:44 am
content: Quote from: xavier60 on May 15, 2020, 11:41:35 amThat is really frustrating. I wonder how many variations of this unit exist and I wonder if any of the variations are actually spot on.Since you now mentioned this I did some testes with my units. The unit that is installed on the positive side seems to be more off than the unit on the negative side.So, to make the meters change from 5.9V to 6.0V I had to increase the voltage to what my DMM displayed as 6.02V for the positive side meter and 6.01V for the negative side meter. I'm not sure if I did that test right since both units were behaving slightly differently from each other. Is it perhaps due to the tolerance of the internal components?Also, I was wondering how you measured the display update to be 800mS. I bought my units at a different time and I wonder if they are identical to the unit you purchased. So, if it's a test I can do, perhaps you can let me know how to do it and I can do the test and report the results. I do have a 20MHz oscilloscope (that I still have to learn how to use) and I'll be soon swapping it for a 100Mhz one that I just bought on eBay, that's supposed to arrive soon. I assume you used a scope for the test, so if that's all it takes I might be equipped.It would just be good to know if the units they sold at different times are in fact the same and if not, how could we tell them apart before purchasing.Also, I was wondering if your old meter(s) have a better display update rate.Please let me know what you think.Thanks...

author: xavier60
author_rank: Super Contributor
timestamp: May 16, 2020, 03:07:17 am
content: Super frustrating when the seller assumes it's a one off fault rather than design flaws that should be reported back to the supplier so that improvements might be made. I encourage others who notice correctable design problems to report them to the seller also.I will not likely respond to all points in this post.I'm curious about how much voltage tracking error yours have.I set the actual voltage to 5.00V then very slowly turned the V_ADJ Pot until the reading just changed from 4.9 to 5.0.Then changed the actual voltage to 20.00V, mine read 20.5. I don't know how to quantify this error but it's something much more than the advertised 1%.

author: xavier60
author_rank: Super Contributor
timestamp: May 16, 2020, 03:32:11 am
content: My original meter is one of a bunch I bought in 2 orders from Aliexpress sellers. The PCB looks almost the same as the BangGood seller's except for some resistor value differences and an extra resistor in series with the reverse protection diode near the regulator. The processor is unmarked.They all appeared to have the same refresh rate which was slow but acceptable. I just measured one at 500mS.I used my DSO set to a slow sweep rate to look at the LED drive signals while causing the display readings to change as often as possible.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 16, 2020, 02:58:46 pm
content: Quote from: xavier60 on May 16, 2020, 03:07:17 amIt's hard to know how many complaints would be needed to make them go back to the drawing board. I'm sure they would need to be flooded with complaints and also see some kind of drop in sales, so address this issue. It almost feels like we would have to coordinate the world to send these complaints to the sellers.Quote from: xavier60 on May 16, 2020, 03:07:17 amI can easily do this test later today and report the findings. I actually ordered 6 more units, so I can do this test (or any tests) on other units, when they arrive. It will actually be easier to do more testing on units that are not already installed.But I ordered the new units from eBay this time, not really knowing if the units they'll send are the exact ones they show in the listings photos.Quote from: xavier60 on May 16, 2020, 03:32:11 amThis is the part that I don't understand. I Googled "DSO" and I'm not sure if that's an abbreviation for oscilloscope (I'm guessing it is) and I don't quite understand how to set it up and connect it, and do the readings.If this is an important test, please let me know and I'll be happy to do it.Thanks...

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 16, 2020, 05:54:23 pm
content: I performed the voltage test with both panel meters. I discovered that the meter used for the negative side does not perform identically to the meter on the positive side. The meter on the negative side matches the readings on my DMM all the way to 18V (my power supply does not go above that).But the meter on the positive side only matches what I see on the DMM up to 13V. At 14.00V the meter will display 14.1V. Then when I adjust my output voltage so that the meter displays 14.0V, the DMM shows 13.98V.I also discovered that there is a discrepancy when I decrease the voltage. The meter will match the DMM only down to 4.5V. After that, when I decrease more, it is off.Here is my log.DMM --- Panel Meter1.00 --- 0.91.02 --- 1.02.00 --- 1.92.03 --- 2.03.00 --- 2.93.02 --- 3.04.00 --- 3.94.02 --- 4.0from 4.5V to 13V OK13.89 --- 14.014.00 --- 14.114.91 --- 15.015.00 --- 15.117.87 --- 18.018.00 --- 18.1That's the log for the meter at the positive side. The meter at the negative side matches the DMM all the way to 18V but it does not match the DMM when decreasing below 4.5V. The discrepancy between 1V and 4.5V matches the discrepancy that the other panel meter is displaying.Also, let's say if I wanted to find out at which point the meter would change from 17.9 to 18.0. That would happen when the DMM would change from 17.94 to 17.95 and I got the same result for some other spot tests. Such as meter changing from 14.9 to 15.0 would happen when DMM changed form 14.94 to 14.95. That part actually suggests that that particular panel meter was pretty accurate. But could it have anything to do with how I set that V-adj pot? Or anything to do with tolerances of internal components?My DMM, BTW, is a BK Precision 2707B which I always assumed should be an accurate one. I only have one DMM at this time (I blew something on the other one and hope to fix it one day).What do you think of these test results?Thanks...

author: xavier60
author_rank: Super Contributor
timestamp: May 16, 2020, 07:19:29 pm
content: Those results are much better than mine and very acceptable, the "4.02 --- 3.0" would be a typo. They must have screwed things up between batches.Even using a Digital storage oscilloscope, it took a lot of fiddling to capture a gap in the drive pulses on a display segment line to find the refresh period. So far I can't think off any easy way for you to determine what yours is.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 16, 2020, 07:37:13 pm
content: Quote from: xavier60 on May 16, 2020, 07:19:29 pmWoops, yes, I just fixed it.Quote from: xavier60 on May 16, 2020, 07:19:29 pmThere must be a lot of variations of this unit out there. Clearly there are many units that look the same from the front but are visibly different when you see the PCB.I guess there's no easy way to access and edit the firmware.So, I have 6 units on their way and I'll do the same tests with those. I'm really curious to see the results.I'm also wondering if there are any other details that could be used to tell the units apart, before purchasing.

author: xavier60
author_rank: Super Contributor
timestamp: May 16, 2020, 08:07:59 pm
content: Quote from: Adinol on May 16, 2020, 07:37:13 pmOne would have to start from scratch, writing new firmware and burning it to a new micro-controller of the same type, very involved and not worth the effort. I would possibly go as far as doing it with a type of micro-controller that I'm already familiar with but it wont be a drop in replacement and would have to live on a small sub-board wired to the meter.The details in the listing photos can't always be trusted. With the ones from Banggood, although the PCB is the same, the meters I received have unmarked micro-controllers. I don't know if this is even relavant.https://www.banggood.com/Mini-Digital-Voltmeter-Ammeter-DC-100V-10A-Panel-Amp-Volt-Voltage-Current-Meter-Tester-0_56-Blue-Red-Dual-LED-Display-p-1422461.html?rmmds=myorder&cur_warehouse=CN

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 17, 2020, 12:08:43 am
content: It sounds like it would be more work to fix this one than to just build one from scratch.Now I just discovered that they have new models with 4 digits and some images show displays showing 3 decimal places. I wonder if that might be an indication that those models might be more accurate.RIDEN® 0-33V 0-3Ahttps://www.banggood.com/RIDEN-0-33V-0-3A-Four-Bit-Voltage-Current-Meter-DC-Double-Digital-LED-Display-Voltmeter-Ammeter-p-1060303.html?rmmds=detail-left-hotproducts__4&ID=48560&cur_warehouse=CNGeekcreit® DC 100V 10Ahttps://www.banggood.com/Geekcreit-DC-100V-10A-0_28-Inch-Mini-Digital-Voltmeter-Ammeter-4-Bit-5-Wires-Voltage-Current-Meter-with-LED-Dual-Display-p-1460060.html?rmmds=detail-left-hotproducts__3&cur_warehouse=CN

author: xavier60
author_rank: Super Contributor
timestamp: May 17, 2020, 12:49:44 am
content: Quote from: Adinol on May 17, 2020, 12:08:43 amGood accuracy reports for both. One buyer confirmed the fast refresh for the first one but I need 5 amps.There is no comment about refresh rate for the 2nd one yet and I need Green/Red displays. Blue is too dim.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 17, 2020, 02:31:53 am
content: Now I hate myself for buying those 6 additional panel meters. It would obviously be better to have 4 digit displays.Do you think those blue LEDs could easily be changed to green ones? Or, since this seems to be a new model, perhaps it's just a matter of time before the red/green option are available for the 10A model.

author: xavier60
author_rank: Super Contributor
timestamp: May 17, 2020, 02:53:41 am
content: Quote from: Adinol on May 17, 2020, 02:31:53 amAlthough I haven't searched, those LED displays would be easily available. When buying any 7 segment LED display, make sure to get the correct wiring configuration. They can be either common anode or common cathode in these meters. Ill look into it.I have been swapping the red and green because I want the colours to match the CV and CC LEDs on my PSU. It can be tricky getting them back to the same board height because they hold the filter window in place.

author: 1001
author_rank: Contributor
timestamp: June 23, 2022, 10:48:01 am
content: I bought two panel meters on aliexpress, they have one more chip, it seems to me to read 2057T, 6 pin. does anyone know which component it is?

author: xavier60
author_rank: Super Contributor
timestamp: June 24, 2022, 01:19:09 am
content: Quote from: 1001 on June 23, 2022, 10:48:01 amA 5mΩ shunt amplified by an LM358 opamp would normally have too much drift to display a useful reading on a 4 digit display.The 2057T would be some sort of CMOS analog switch controlled by the micro-controller to periodically disconnect the current shunt from the input to the opamp. During this time, the micro-controller would note the opamp's output voltage as corresponding to zero reference.I have yet to identify the past's full part number.

author: 1001
author_rank: Contributor
timestamp: June 24, 2022, 07:58:24 am
content: tnx a lot, could this be it?

author: xavier60
author_rank: Super Contributor
timestamp: June 24, 2022, 10:28:27 am
content: Quote from: 1001 on June 24, 2022, 07:58:24 amYes. Makes sense being double-throw so that either shunt or ground can be alternately selected for the opamp's input.

author: 1001
author_rank: Contributor
timestamp: July 01, 2022, 12:05:28 pm
content: ledtester asked to me to probe the "Digital Control Pin" of the RS2057, here are the pictures

author: xavier60
author_rank: Super Contributor
timestamp: July 01, 2022, 01:08:12 pm
content: I found the same looking meter in my collection. If yours also has the same slow 1 second display refresh, be sure to complain to the seller about it.The ICs control input pin is driven from one of the display's digit enable lines. The step is due the way they have reduced the drive duty cycle to reduce LED brightness.At 5 Amps, I see 20mV at pin 3 and a 0 to 20mV chopped signal at pin 4 which is then fed to the opamp.

author: 1001
author_rank: Contributor
timestamp: July 01, 2022, 09:03:12 pm
content: Specifications from seller

author: xavier60
author_rank: Super Contributor
timestamp: July 01, 2022, 09:11:28 pm
content: Mine was advertised as 300mS refresh also.

author: David Hess
author_rank: Super Contributor
timestamp: July 01, 2022, 10:18:23 pm
content: Digital panel meters typically operate at a sample rate of a multiple of 100 milliseconds so that they can reject 50 and 60 Hz noise and harmonics, but 100 milliseconds is too fast to follow so they update slower.

author: xavier60
author_rank: Super Contributor
timestamp: July 01, 2022, 11:52:48 pm
content: These panel meters used to have satisfactory refresh rates. I have some 3 digit types that are 500mS.Some I got from BangBad last year were 2000mS although they were advertised as 300mS. I sent them all back.The only reason for slowing the refresh will be a stupid one.

author: David Hess
author_rank: Super Contributor
timestamp: July 02, 2022, 12:43:56 am
content: Quote from: xavier60 on July 01, 2022, 11:52:48 pmI remember panel meters from 20+ years ago where the update rate was adjustable, either with an RC circuit or by jumpers, but they were usually never faster than 100 milliseconds for the reason I gave. The exceptions were meters that only reject 50 or 60 Hz so they did not have to rely on the least common denominator of 50 and 60 Hz to cover both.I guess we need a good "universal" panel meter kit, but it is difficult to make such a thing economical.