Electronics > Beginners
Voltage reference not working properly #SOLVED#
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FotatoPotato:
@StillTrying So I found out I had the LM431 wired wrong  :palm: on 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
StillTrying:
"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.
FotatoPotato:
@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 wrong  :palm: I 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.
StillTrying:
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.
FotatoPotato:
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 0.9v

I 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.
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