I have two 0-10k trimmer pots from radio shack. Last night I set them to 1000 ohms each, while I was sitting in my air conditioned house. After installing them in my car as pull up resistors for sensors, they now read about 800 ohms each. Do they loose their stability based on ambient temperatures? Should I use precision resistors instead?

 

Yes, they change. but not so much as in this case.
It appears the mistake is in using a 10K pot setting it to 1000 nearly 10%. you may use 2K pot and set it to 1K it might behave better.

How ever there will be change wrt temperature. It is called temperature coefficient of resistance and it changes from material to matrial.

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Regards,
Sarma.

 

Why would you use trim pots as pull up resistors? Why not use a simple fixed resistor?

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Pay it forward.

 

Agreed using potentiometers for pull-ups does sound silly.

Pull-up resistors aren't critical, if the schematic specifies 1k but you don't have that value, use the nearest value you have. It can probably be between 820R to 1k2 without any problems.

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I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong.

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Ok, I'll swap in regular resistors. I have no electrical training, so was just trying to make the circuit as precise as possible. Perhaps pull-up resistor was the wrong terminology. I have using it as part of a circuit to measure the output of a thermistor (to measure air temps in my motor). I have a dataq data logger that measures anaolog DC voltages. In order to log the thermistor, I grounded one lead of the thermistor and sent the other lead to the datalogging input. I also connected this lead to a regulated 5vdc through the "pull-up" resistor. My theory was that the more precise the resistance on that resistor, the more precisely I could calculate the resistance/temp at the thermistor. In the temp range I am measuring, the thermistor has about 50 to 1000 ohms, so I opted for a 1K pull up.

 

By the way, will regular fixed resistors also change significantly in resistance based on temperatures? If a 1K resistor measures 1000 ohms at 70F, what might it measure in a 160F environment?

 

When RadioShack was still selling (they are gone now) over-priced poor quality stuff in Canada, I remember the no-name-brand trimmer pots. I never tried them.
Instead I bought top quality name-brand (Bourns) trimpots from a real electronic parts distributor (Digikey or Newark) for less cost.

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Uncle $crooge

 

That's an entirely different matter.

The correct terminology is "I'm using the resistor as part of a potential divider". In that case it does make a difference and pots have the advantage of being adjustable.

It's not normally significant.

How accurate does it need to be?


Use 1% a metal film resistor rather than a 5% carbon film resistor.

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I do not answer private messages asking for help because no one else can: benefit from advice I may give or correct me if I'm wrong.

Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

 

Radio shack gives you instant gratification. Once upon a time, there were many independent neighborhood distributors to choose from, but as fewer and fewer people do things for theirselves, the competitors have disappeared, sadly.

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You don't need a quadraphonic Blaupunkt -- you need a curve
ball.

 

A typical 1% metal film resistor has a temp coefficient of 100ppm/C or 0.01%/C.

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Carl
Curmudgeon Elektroniker

 

thanks folks. Switched to fixed resistors and all is fine.