Voltage reference not working properly #SOLVED# - Page 1
Forum section: electronics / beginners

author: FotatoPotato
author_rank: Regular Contributor
timestamp: February 26, 2019, 01:25:02 am
content: Hello everyone,I'm currently building a programmable DC electronic load and for the most part, it works perfectly. But there is one problem and it is quite a big one. In my schematic (Linked below) I use an LTC1046 to generate -5v for the calibration pots and for the AD8630 opamp. Feeding on this is a TL431 voltage reference IC that is set to output -2.5v for the current calibration. But for some reason, it is outputting 0.36v which is causing the reported current draw from the Load to be higher than the actual current draw. In addition to that it is impossible for me to match the reported and actual current draw and no matter what, it is always about 100mA high on the reported current draw than it is actually drawing.I have double checked all of my connections and I cannot seem to figure out what is wrong with the TL431, any suggestions would be greatly appreciated!ThanksHere is a PDF of the schematic. The -5v generation and -2.5v schematics are in the top right corner of the schematic sheet.

author: Shock
author_rank: Super Contributor
timestamp: February 26, 2019, 05:06:42 am
content: When in doubt test it out (of circuit).

author: BNElecEng
author_rank: Regular Contributor
timestamp: February 26, 2019, 07:33:26 am
content: Agreed. Remove the TL431. Measure the -5V and check it is ok. Also, check the -2.5V line on the board even though you've removed the voltage reference IC. There may be a short or something which is pulling the voltage higher than expected.

author: Arznei
author_rank: Contributor
timestamp: February 26, 2019, 08:07:54 am
content: Another idea: the datasheet states there must be a minimum of 1mA going through the LM431 for proper operation of the internal reference voltage. With R1=2k2 you have only 1.136mA going into the node of 2.5V though you are loading it with 0.65mA via R29. That leaves less than 0.5mA for the LM431. Maybe try decreasing R1 to guarantee the 1mA?

author: iMo
author_rank: Super Contributor
timestamp: February 26, 2019, 11:01:23 am
content: Try this:

author: StillTrying
author_rank: Super Contributor
timestamp: February 26, 2019, 11:53:52 am
content: "But for some reason, it is outputting 0.36v"I don't think the too low 0.5mA through the 431 explains it, sounds like the 431 is wired wrong, perhaps backwards.imo'slookslike a good idea, but it's no longer a steady -2.5V reference from 0V.Time travel:Quote from: imo on February 26, 2019, 12:54:59 pmThe -5V itself isn't regulated.

author: iMo
author_rank: Super Contributor
timestamp: February 26, 2019, 12:54:59 pm
content: Quote from: StillTrying on February 26, 2019, 11:53:52 amYes, it is "stable" -2.5V from -5V.

author: iMo
author_rank: Super Contributor
timestamp: February 26, 2019, 01:23:46 pm
content: Quote from: FotatoPotato on February 26, 2019, 01:25:02 amOn the first glance it looks like you have got it reversed. Double check the wiring again..

author: Arznei
author_rank: Contributor
timestamp: February 26, 2019, 03:40:18 pm
content: That was also my first thought, though it shouldn't really matter. From my understanding of the block diagram of the device it should still keep its Anode-Cathode voltage at 2.5V the way it is wired up in the schematic. It actually makes more sense this way, since the desired outcome is a -2.5V reference from ground, not 2.5V from the -5V rail.And since the device itself doesn't have a GND reference it shouldn't matter wether you decrease the Cathode voltage or increase the anode voltage with a resistor.The current range for stable operation is 1mA to 100mA through the device. If a current below 1mA leads to the internal voltage reference to decrease that would lead to the internal transistor beeing overdriven. A fully driven BJT would explain the ~0.4V across the device which are beeing measured.

author: FotatoPotato
author_rank: Regular Contributor
timestamp: February 26, 2019, 11:55:59 pm
content: QuoteI have checked the -5v rail and all is good on that end so no problems there. I also checked to see if there are any shorts that would pull the voltage up and I can't find any, so I don't think this is the problem.QuoteAgain, I double checked the schematic and board and If you look at the schematic that I posted, the 431 is wired the same as IMO's LTSpice simulation which shows that it should be outputting -2.5. One thing about his Simulation that does confuse me tho is that he has the positive 5v going to the cathode and gnd which seems like it shouldn't work...QuoteI think you may be onto something here, the -2.5v is only used on a potentiometer to set the current calibration potentiometer, and that doesn't pull much if any current. So that might be one of the problems.What I actually think is the problem (and this is super stupid on my part) is that the LM431 and the TL431 may be the "same" IC, but after googling it, it turns out they are very different. For one, the TL431 has a 1.25v internal reference while the LM431 has an internal 2.5v reference. Also, the voltage ranges and minimum current ranges are very different for the two parts. I'll post a pic of the chart I found. Anyway, I think I just pulled a big goof and used the wrong chopI already ordered the LM431 and It should be here in the coming day or 2. Once it arrives I'll test it and post back if the issue has been resolved or not. Thanks so much for the feedback guys, I really appreciate it!

author: not1xor1
author_rank: Frequent Contributor
timestamp: February 27, 2019, 06:31:38 am
content: Quote from: FotatoPotato on February 26, 2019, 11:55:59 pmTL431 and LM431 are both 2.5V and are both specified for a minimum current of 1mA (the 1.25 100µA version is LMV431)I think the problem is wrong connections, wrong potentiometer value (lower than 10k) or resistor value (i.e. 22k instead of 2.2k) or dead/fake TL431. BTW I would also decrease the resistor value to 1-1.5k just in case ofcurrent greedyout-of-spec TL431.

author: iMo
author_rank: Super Contributor
timestamp: February 27, 2019, 07:27:23 am
content: Quote from: FotatoPotato on February 26, 2019, 11:55:59 pmThe TL431's voltage at cathode (the bar) should always be positive against its anode (the triangle). In order to get 2.5V you have to connect the "REF" pin to the cathode. The regulated voltage is at the cathode (measured against the anode).

author: StillTrying
author_rank: Super Contributor
timestamp: February 27, 2019, 08:50:47 am
content: Quote from: FotatoPotato on February 26, 2019, 11:55:59 pmYes, all the schems and sims are correct, it's more a case of checking the actual package's pins.Could it be oscillating, at 2.5V and very low current it needs a cap on its output, 4u7 to 10u depending on which data sheet you look at. "Stability Boundary Conditions"Edit: While I remember from yesterday visit,the DAC's data sheet says it should have a 100n very close to its power pins.

author: FotatoPotato
author_rank: Regular Contributor
timestamp: March 01, 2019, 03:48:27 am
content: Ok, So I have spent the last 4 hours trying to debug this thing and I just can't seem to figure out what's wrong. When I set up the LM431 on it own on a breadboard it works fine and outputs -2.5v but when I connect it to the PCB the output changes again.The original design was made by Louis over at Scullcom Hobby Electronics (He makes fantastic videos, check his channel out). I will add the PDF of his schematic to this post so that you can see it. The only differences I have in mine from his is that I opted not to have a trigger input, otherwise my schematic is (or should be) identical to his. I watched through his series of videos covering this design, particularly the one where he calibrates the load. In that video, he starts by calibrating the 4.096v reference (R31 on my schematic, RV5 on his), then the voltage readout (R5 on my schematic and RV2 on his) then the current null calibration (R29 on mine, RV3 on his), and lastly the current draw calibration (R26 on mine, RV4 on his). I tried to calibrate my load in the exact same order but, every time I change R29 then the 4.096v reference goes down to around 3.1v, and I can't get it back to 4.096v unless I change R29. Also, the -2.5v goes down to around -0.6v when I do this. In addition to that my voltage readout slowly goes out as the voltage increases, so at 5v it is perfectly accurate but at 30v it is 0.7v higher which is a big problem and no matter how much I adjust R5, nothing changes. Lastly, the current calibration doesn't seem to work on my board. Once I try to change the current null calibration, it throws off the current calibration and even if I don't touch the current null calibration, at low currents (500mA - 100mA) the readout on my load is very off. If I set it to draw 100mA it will only draw 70mA but the load will display that it is drawing 120mA, and when I adjust R26 the actual current draw only goes up by about 7mA and the reported current draw doesn't change.At this point I really don't know what to do, I have gone through both my schematic and His schematic to look for differences and I just can't seem to find any. I was hoping that you guys would be able to take a look and see if there is something that I'm missing or doing wrong. I would really appreciate it!Thanks so much!FYI: The file titled "DC Load" is my version and the other one is His.Louis's Video:

author: iMo
author_rank: Super Contributor
timestamp: March 01, 2019, 08:17:25 am
content: Try to feed the -5V rail from an external source (remove the LT1046). Double check the 4.096V rail is not overloaded (REF5040 is not a voltage regulator) by something in your schematics.

author: StillTrying
author_rank: Super Contributor
timestamp: March 01, 2019, 11:34:10 am
content: It looks like there are 2 unrelated voltage ref problems then.The REF5040's +/- 10mA output should very easily drive the 10k pot and DAC without dropping to 3.1V.Are you sure the+5V is always good.

author: Shock
author_rank: Super Contributor
timestamp: March 01, 2019, 12:12:07 pm
content: So after you have tested out of circuit the next thing to try is in circuit with the supply section disconnected from the rest of the circuit, obviously if there needs to be a minimum load take that into consideration as well.If that checks out, isolate it from the circuit it's powering and then feed that circuit from a bench supply (or alternate source) instead of using the supply circuit.Anyway that is how to isolate a supply problem and also discover if it's a load based problem.

author: FotatoPotato
author_rank: Regular Contributor
timestamp: March 02, 2019, 12:03:51 am
content: So I did some probing and the +5v is perfectly stable, 0 ripples and 0 instability. The next thing I checked was the -5v and it seems that when I adjust R29 on my schematic, not only does the 4.096v and -2.5v change but the -5v goes down to around -4.57v which isn't as severe as the other regulators but it is still enough to throw the calibration off. It seems to me that R29 is the source of the problems which is a big issue because I need it to calibrate the load. I also tested the -5v and -2.5v on a breadboard and everything seems to be working fine. Also once I remove the LM431 and the -2.5v is removed from R29, the 4.096v ref goes back to normal. Still not sure what to do because it seems that I would have to remove essential parts of the circuit which would render my load useless and wildly uncalibrated...

author: iMo
author_rank: Super Contributor
timestamp: March 02, 2019, 12:31:33 pm
content: 1. is the value of R29=10k?2. is the value of R28=4M7?3. it could be at a specific R29 position the whole stuff starts to oscillate wildly (look at it with o'scope). You may also verify that easily - remove the R28 from your circuit, and wire the 4M7 from R29's wiper to GND, for example. Turn your R29 up/down - my bet the 4.096V and -2.5V will stay stable (assuming the R29 is not defunct per se).

author: StillTrying
author_rank: Super Contributor
timestamp: March 02, 2019, 01:52:17 pm
content: "Still not sure what to do because it seems that I would have to remove essential parts of the circuit which would render my load useless and wildly uncalibrated..."When you're fault-finding on a new circuit with lots of interacting parts it's quite normal to spend nearly all of the time "not sure what to do"! You just have to eliminate details one by one by very careful checking, while keeping an overall view of how it works so that you're hopefully concentrating on the most likely."It seems to me that VR29 10k is the source of the problems"I agree with imo's checks immediately above, but unless you've got it on very long leads I don't think it's the problem, itshouldjust be a very contant 0.66 mA load between the 4.096V and -2.5V.Are you sure the output ofU2U7 OPA277 always stays ~0V or above, especially during calibration. If its output tries to go lower than about -0.3V it will start to draw a lot more than its 0.8 mA from the -5V. It's about the only place I can see where the varying load on the -5V would be coming from.

author: iMo
author_rank: Super Contributor
timestamp: March 02, 2019, 03:20:41 pm
content: Quote from: StillTrying on March 02, 2019, 01:52:17 pmGood point - when the OPA277' output goes below -0.3V the AD8630's input lower side clamping diode starts to conduct, and the current from the -5V source flows via OPA277's output to AD8630's input, then via the clamping diode to GND (assuming the V- of the AD8630 is GND). It could be many mAmps.Double check - put a 100k (for example) resistor between the OP277 output and the AD8630 input as a test.PS: if it helped you may leave the 100k resistor there as the input current of that AD is less than 100pA - that creates <10uV offset.

author: FotatoPotato
author_rank: Regular Contributor
timestamp: March 02, 2019, 04:22:28 pm
content: So I just did some probing and here are my results:OPA277 output: 0.645v ± 0.01vLM7805 out: 5.027v ± 0.02vLTC1046 out: -4.882v ±0.24vLM431 out: -2.654v (not connected to anything) and -0.186v (Connected to the rest of the circuit, voltage depends on the value of R29)REF5040 out: 3.479v ± 0.8v depending on what the value of R29 isValue of R29: 9.952KValue of R28: 22.85KR28 is very weird considering it IS a 4.7M resistor, I triple checked the color bands on it and it is a 4.7M resistor but for some unexplainable reason, it is 22.8K...I think we are getting close to a solution and I definitely seem to be the circuitry surrounding R29 and R28. I am going to replace R28 and see what happens but I doubt it will make a difference.

author: iMo
author_rank: Super Contributor
timestamp: March 02, 2019, 04:44:11 pm
content: Quote

author: FotatoPotato
author_rank: Regular Contributor
timestamp: March 02, 2019, 04:53:16 pm
content: Just replaced R28, no difference

author: StillTrying
author_rank: Super Contributor
timestamp: March 02, 2019, 05:07:04 pm
content: Something else to try.What value have you reduced R1 2k2 supplying the LM431 to, I don't think ~1k is low enough.The -2.5V supply needs 1mA for the 431(they usually work fine @ 0.7mA),0.8mA for the OPSA277and 0.66 mA for the VR29 10k pot = 2.46 mA.Assuming the -5V is only -4.8V, R1 has to be(4.8-2.55)/2.46 = 915R, 820R should do.(4.8-2.55)/1.66 = 1.4k or slightly lower for R1.Edit: While you're there add a 1u to 10u to the -2.5V supply."REF5040 out: 3.479v ± 0.8v depending on what the value of R29 is"It's still a mystery why the REF5040's 10mA 4.096V output isn't more than enough to power the DAC, unless the DAC is oscillating, or it's output is shorted.

author: iMo
author_rank: Super Contributor
timestamp: March 02, 2019, 05:14:27 pm
content: Above my hint on an external -5V (hard one, ie an external floating 5V source), and you may pass 20mA via the 431 (125ohm) during the troubleshooting..The LM431 voltage must be 2.5V +/-0.05V always.

author: FotatoPotato
author_rank: Regular Contributor
timestamp: March 02, 2019, 05:16:06 pm
content: I still have a 2.2k resistor on the 431, If I change it the output voltage changes as well, I'll try it though and update you on the results.

author: iMo
author_rank: Super Contributor
timestamp: March 02, 2019, 05:18:09 pm
content: And doublecheck (measure!) all 40 resistor's values on your pcb..BTW, what is the value of R18? I would remove it during the troubleshooting.Also doublecheck the polarity of the capacitors in your LT1046 -5V generator. I would also try to load it with a resistor to get an idea how much current it gives you actually while having -4.88V on it.

author: StillTrying
author_rank: Super Contributor
timestamp: March 02, 2019, 05:39:03 pm
content: "I still have a 2.2k resistor on the 431"I've had to change my previous calculation for R1, I was adding the OPA277's-5Vcurrent into the -2.5V.https://www.eevblog.com/forum/beginners/voltage-reference-not-working-properly/msg2235837/#msg2235837Now (4.8-2.55)/1.66 = 1.4k or slightly lower for R1.

author: iMo
author_rank: Super Contributor
timestamp: March 02, 2019, 05:44:55 pm
content: Quote from: imo on March 02, 2019, 12:31:33 pmDid you check this?Read the previous posts and do make a checklist

author: FotatoPotato
author_rank: Regular Contributor
timestamp: March 02, 2019, 06:02:59 pm
content: @StillTrying So I found out I had the LM431 wired wrongon the breadboard and after I fixed it and added the 1.4k resistor to the cathode the output is still -2.5v when it isn't connected to the load. But now when I do connect it to the -2.5v rail on the PCB the output drops to -0.72v and the output on the REF5040 drops to -0.7v as well. The LM431 also gets warm to the touch and I am now unable to set the current null calibration. Previously I would be able to set it to 0mA which is what I need to do but now it can only go down to 13mA. BTW, current null just means the leakage current when the load is turned off.@IMO I did and there is nothing out of the ordinary, I am gonna write a quick checklist and go through this entire post thread to look for anything I have missed

author: StillTrying
author_rank: Super Contributor
timestamp: March 02, 2019, 06:11:37 pm
content: "But now when I do connect it to the -2.5v rail on the PCB the output drops to -0.72v"You must have the 431's anode and cathode reversed, have a look a its internal diagram, it's a 2.5V zener the right way, and just a normal diode the wrong way.

author: FotatoPotato
author_rank: Regular Contributor
timestamp: March 02, 2019, 06:39:29 pm
content: @IMO The checklist was a fantastic Idea!, I was stupid not to do it earlier. I have fixed some of the problems now. First The 431 was wired completely wrongI wired it back to what the original was in my schematic and It output -2.506v with a 1.3k resistor for R1. The LTC1046 now outputs a steady -4.96v no matter what. Also, the REF5040 outputs a stable 4.0962v perfectly. The only problem that still persists is that when I try to adjust the current null, while the 4.096v stays stable, the -2.5v goes up to 0.9v. This is an improvement to the plethora of issues that I had earlier but its still not perfect. Also, the current readout on the Load is terribly wrong. I'll send some pics of the discrepancies between the current readout on my load and the actual current it draws.If you look at the pic of the LCD the set current is 0.1A, it is reporting to draw 0.132A and the actual current being drawn as shown by the 34401A is 0.93A.

author: StillTrying
author_rank: Super Contributor
timestamp: March 02, 2019, 07:24:12 pm
content: I'm still suspicious of the OPA277, are you sure it's powered from -5v and not the -2.5V."the -2.5v goes up to 0.9v."Are you sure it's +0.9V, the OPA277 could possibly pull the -5V and -2.5V up to about -0.9V.+0.9V on the -2.5V ref. should be impossible, because of the LM341's internal reverse diode.

author: FotatoPotato
author_rank: Regular Contributor
timestamp: March 02, 2019, 07:31:28 pm
content: I just checked and the OPA277 is getting all of the correct voltages. And yes, I just checked again and the output of the 431 is 0.9vI think something is overloading the 431 and pulling its output High but I may be wrong, I just checked the current draw on the -2.5v rail and it is 4mA, so I don't think it is being overloaded but hey, I have been wrong plenty of times before.

author: StillTrying
author_rank: Super Contributor
timestamp: March 02, 2019, 07:36:55 pm
content: "I just checked the current draw on the -2.5v rail and it is 4mA"What does the -2.5V supply, as far I know it's just VR29 10k = 0.66mA.Are you sure the pin out of VR29 is correct on the PCB, the 4.096V 10mA ref. could easily pull the -2.5V ref. up to ~ +0.8V.I'm running out of ideas on what's wrong with the -2.5V.

author: iMo
author_rank: Super Contributor
timestamp: March 02, 2019, 07:37:56 pm
content: You must limit the output current of the OP277 with a resistor as we told you (10k-100k for example). You should not wire an output of an opamp powered -V..+V with an input of an opamp powered 0..+V. Did you insert the resistor there? That would be an important point on my checklist.

author: FotatoPotato
author_rank: Regular Contributor
timestamp: March 02, 2019, 07:44:14 pm
content: I have not done that, I'll do it now and see what happens.

author: StillTrying
author_rank: Super Contributor
timestamp: March 02, 2019, 07:49:20 pm
content: Quote from: imo on March 02, 2019, 07:37:56 pmI'd agree, but the -5V is now always good at -4.96V. Still a good idea to protect the next op amp's input from -0.3V or more with a series 1k-10k.

author: iMo
author_rank: Super Contributor
timestamp: March 02, 2019, 07:59:39 pm
content: Put there 100k for a test as it may show something. When all works fine you may start to lower the resistor when necessary.Hint: It is not a good methodology to wire entire complex system together, power it on and watch various meters and displays. Especially when you mess with a prototype.Always wire it part by part, module by module while you have to always test the parts or modules individually. Take notes on voltages and currents.

author: FotatoPotato
author_rank: Regular Contributor
timestamp: March 02, 2019, 08:06:42 pm
content: Ok, so I added a 100K and 10K on the output of the OPA277 and for both the LTC1046 outputs exactly -5.006v, so I guess that helped. The output of the 431 is 0.99v now....

author: iMo
author_rank: Super Contributor
timestamp: March 02, 2019, 08:18:59 pm
content: Have you wired the resistor exactly as I depicted in the schematics above?

author: FotatoPotato
author_rank: Regular Contributor
timestamp: March 02, 2019, 08:21:16 pm
content: Yes, from the output of the OPA to the input of the AD8630, Like this right?

author: iMo
author_rank: Super Contributor
timestamp: March 02, 2019, 08:24:24 pm
content: ok, that is right.

author: iMo
author_rank: Super Contributor
timestamp: March 02, 2019, 08:27:34 pm
content: When you turn/set with the R29 - does the -2.5V voltage at 431 change "linearly" or "abruptly"?

author: FotatoPotato
author_rank: Regular Contributor
timestamp: March 02, 2019, 08:29:33 pm
content: So when I disconnect the 431 from the -2.5v the 431 actually does output -2.5v and when I probe the -2.5v rail with the 431 disconnected there is 4.096v on that rail... And once I re-connect the 431 the -2.5v rail goes to 0.99vIt doesn't change rapidly, just linearly as I change the pot.

author: StillTrying
author_rank: Super Contributor
timestamp: March 02, 2019, 08:31:57 pm
content: "The output of the 431 is 0.99v now"LOL.There should be only 3 things connected to the -2.5V line, the LM341, the 1.3k supply resistor, and one end of VR29 10k. So, either VR29's wiper is wrong, or there's something else connected to the -2.5V.

author: FotatoPotato
author_rank: Regular Contributor
timestamp: March 02, 2019, 08:38:40 pm
content: Well, it must be R29, cause that's the only thing connected to -2.5vI'll replace it and see what happens

author: iMo
author_rank: Super Contributor
timestamp: March 02, 2019, 08:40:51 pm
content: There is something wrongly connected which is able to pull -2.5V up to +0.99V, something like 800ohm - 1kohm from +5V.EDIT: +4.6mA pulls it up to 0.99V

author: FotatoPotato
author_rank: Regular Contributor
timestamp: March 02, 2019, 08:52:59 pm
content: So I just replaced the old pot with a brand new one and now the voltage on the -2.5v rail is 0.77v, so something still isn't right. The resistance between pin 1 and 3 on R29 is 10.3k and here is a screenshot of the -2.5v trace on the board. This is the only spot where -2.5v is present. If you want the Altium files I'd be happy to send them overI have a wire soldered to the center pin of U4 and that goes to a breadboard with the LM431 and R1

author: iMo
author_rank: Super Contributor
timestamp: March 02, 2019, 09:01:15 pm
content: Is the -5V still -5V when -2.5V is pulled up to +0.99 or +0.77V??PCB: your -2.5V is connected to the R29's wiper, isn't it?

author: FotatoPotato
author_rank: Regular Contributor
timestamp: March 02, 2019, 09:16:35 pm
content: ....... Oh my lord I cannot believe thisall this time spent, and I connected -2.5v to the damn wiper. SMHI'll go fix that and take a look at the results

author: FotatoPotato
author_rank: Regular Contributor
timestamp: March 02, 2019, 09:27:35 pm
content: Hey presto, there you goEverything works now. I can't believe this was the issue... You guys saw the schematics, they were correct, but for some reason, the default Altium library switches pin 3 and the wiper.....For everyone who has pitched in on this post I want to say thank you so much, I really appreciate the help and it feels awesome to get so much help on this.Again, I can't thank you guys enough

author: StillTrying
author_rank: Super Contributor
timestamp: March 03, 2019, 12:25:23 am
content: I said it was the VR29 wiper on the previous page,although I didn't think it was going to be VR29 at all until you'd seen the -5V and +4.096V stable but the -2.5V still not.As I'm closing down all these data sheets, - the max. allowed -0.3V current in to the AD8660's inputs is 5mA.

author: iMo
author_rank: Super Contributor
timestamp: March 03, 2019, 07:49:34 am
content: Therefore I would go with 33k resistor between OP277 output and AD8360 input