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potentiometer not returning 0 to 5V
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author potentiometer not returning 0 to 5V
ello
i have an issue with my potentimeters. on one side i have connected ground, the other to 5v but the middle gives me a range of 0.2 to 4.68 volts. it is no wonder why my cv sequencer gives me strange offsets even with all pots to theoretical zero.

is this a normal behaviour? what can i do to get a proper range?
guest
measure the resistance from the wiper to ground with the pot turned all the way to ground. sometimes there is extra resistance in the contacts to the pot pins, which makes it not go all the way. using larger value resistors sometimes helps in this case, as that extra resistance will be a smaller percentage of the total resistance. another thing you can test, is measure the voltage at the ground and 5V pins on the pot. its possible there is some drop in the pcb traces that feed the pots (use a ground reference away from the pots).

if its not the pcb traces, then its the pot, and youll have to either get a new pot (in general they get should get closer to the rails than that), or put an amplifier after the pot that increases the range. perhaps there already is an amplifier down stream in the circuit that could be turned up.
ello
i am using 10k linear potentiometers. it is a bunch of new ones i just bought. and it is on every pot with slight differences. and since i was testing it directly with no further circuit around it, i guess they are just of bad quality than.

maybe i need to buy some bigger ones to test if that works better as you suggested.

as i am putting an amplifier lateron i guess the positiv rail would not be the main issue, but the issue of not getting zero volt from ground is a real problem
guest
you can bias the final amplifier down a little bit. will the final output amp be able to hit the rails (some rail-to-rail amps only get within 50mV or so)?
ello
i have a tl074 which will be powered by -12 and +12 volt. and i was thinking to use 100k as input resistor and maybe 150 as feedback resistor to get a bit of amplification there.

so to bias it down i guess i need a trimpot to the negative rail and adjust it when i put everything together to get a zero for all pots?
guest
to bias it down, you can take a high value resistor (1M maybe) and tie it from the inverting pin of the opamp to the +12V rail. and you can porbably get away with less amplification, 100k and 110k would give 10% (for inverting configuration), which would more than make up for the ~300mV of error.
ello
thank you very much smile i believe this will save me a lot of efford
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