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Voltage reference not working properly #SOLVED#
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not1xor1:

--- Quote from: FotatoPotato on February 26, 2019, 11:55:59 pm ---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 chop  :palm: I 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!  :)

--- End quote ---

TL431 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 of current greedy out-of-spec TL431.
iMo:

--- Quote from: FotatoPotato on February 26, 2019, 11:55:59 pm ---Again, 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...  ???

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The 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).
StillTrying:

--- Quote from: FotatoPotato on February 26, 2019, 11:55:59 pm ---Again, 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
--- End quote ---

Yes, 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.
FotatoPotato:
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 |O, 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:
iMo:
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.
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