Electronics > Beginners
Voltage reference not working properly #SOLVED#
FotatoPotato:
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!
Thanks :D
Here is a PDF of the schematic. The -5v generation and -2.5v schematics are in the top right corner of the schematic sheet.
Shock:
When in doubt test it out (of circuit).
BNElecEng:
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.
Arznei:
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?
iMo:
Try this:
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