### Author Topic: Potentiometer Value Question (resistor across pins 1-3)  (Read 492 times)

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#### belzrebuth

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##### Potentiometer Value Question (resistor across pins 1-3)
« on: October 15, 2018, 11:35:49 am »
Hi,
I've got a project that requires a 250kA 9mm potentiometer but I haven't got any in stock and from what I've seen it's not an easy value to get anyways..
Is it okay to use a 500kA with a 500k resistor across legs 1-3?
Is the logarithmic curve going to be affected somehow?
I guess not but thought I'd ask...

#### tooki

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##### Re: Potentiometer Value Question (resistor across pins 1-3)
« Reply #1 on: October 15, 2018, 11:57:00 am »
Putting a resistor in parallel across a linear pot is a common way of approximating a logarithmic pot. So yes, I would expect the curve to be further affected, though this may or may not be bad in practice.

#### belzrebuth

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##### Re: Potentiometer Value Question (resistor across pins 1-3)
« Reply #2 on: October 15, 2018, 01:20:35 pm »
So the best way to approximate a 250kA pot would be putting a 500k resistor in parallel to a 500kB pot?!

#### drussell

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##### Re: Potentiometer Value Question (resistor across pins 1-3)
« Reply #3 on: October 15, 2018, 02:29:03 pm »
Approximating a log pot would be putting an additional resistor from the wiper to one of the ends, not across the whole pot.

Whether going across the entire pot to lower the effective total resistance may work well or may work poorly depends on how it is being used in the circuit, the reason why it "needs" to be a certain value and the power level involved.  If it just a simple voltage divider to a high impedance input, for example, any value from about 1K to 1M will essentially produce the same result, even without any additional resistors, though the power dissipated by the divider itself will obviously increase as the resistance is lowered.  This could easily be an issue in a battery powered circuit, for example.  Pitfalls abound!

If you shared more specific details about your project, perhaps with a schematic, we can provide more specific answers.

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#### tooki

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##### Re: Potentiometer Value Question (resistor across pins 1-3)
« Reply #4 on: October 15, 2018, 02:33:00 pm »
Approximating a log pot would be putting an additional resistor from the wiper to one of the ends, not across the whole pot.
Oh, thanks for the clarification! I remembered wrong, I just read about it a while ago and saved a link!

#### belzrebuth

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##### Re: Potentiometer Value Question (resistor across pins 1-3)
« Reply #5 on: October 15, 2018, 04:31:01 pm »
Here is the schematic of the project: