Weird Problem

[isaac]

Hi All
i have being experiencing this weird problem with my circuit and for the life of me i just cant get my head round it.
i have a simple potential divider circuit like this

5V|---------/\/\/\/\------/\/\/\/\------GND
10K X 20K pot

i am trying to get 2.53V so when i power up the circuit and vary the pot i get the
2.51v when i check it with a voltmeter
Now here comes the problem , if i take the probe of the volmeter off from point X and put it back to the same point x the voltage drops to 2.34v
and i have to adjust it up again with the pot .
but again if i disconnect and reconnect the voltage drops again.
i havedone this about 20 times is there something weird going on here ?

Isaac

[isaac]

5V|---------/\/\/\/\------/\/\/\/\------GND
..................10K.....X..... 20K pot

[mackrackit]

What are you using for a power supply?

[isaac]

Thanks for your response its a Variable bench power supply 0-30V @ 1A

Isaac

[mackrackit]

If the power supply is stable, thought maybe you were using batteries, I will have to ask if there is a load on the divider.

Other than that, I will have to guess that the pot becomes unstable with a little heat.

Why not use fixed resistors?

[skimask]

Other than that, I will have to guess that the pot becomes unstable with a little heat.
And it's a 20K pot! I'd bet a bump from the other end of the table would knock it out of whack.
Try a smaller value pot.

[Dave]

isaac, The maximum recomended input impedance for all Microchip Processors is 10K....

Dave Purola,
N8NTA

[isaac]

isaac, The maximum recomended input impedance for all Microchip Processors is 10K....

Dave Purola,
N8NTA

its not for the input the pic
but i suspect its got to be the pot would try a lower vaue like 12k

[skimask]

its not for the input the pic
but i suspect its got to be the pot would try a lower vaue like 12k

Same thing most likely applies. Lower the values of the resistors.

[Bruce]

Daves' suggestion of using a fixed resistor is good too. I would replace at least one pot
with a fixed 1%, and if using an A/D input to read this I would drop the pot to around 5K
with the fixed value set to limit total current.