capacitive load on potentiometers

Thread Starter

DaniKowa

Joined Sep 23, 2020
175
Happy New Year everyone!

today I was playing with an LCR tester, I often use it to check inductors and capacitors. This morning I had a "blue" 3590s multiturn potentiometer (wire wound) in my hand. I was testing the value in ohms, then curiosity made me set to measure C too, that is the capacity. I noticed that by rotating the potentiometer the capacitive value is inversely proportional to the value of R. That is, the more the resistance increases, the capacitance decreases. The question is: do all potentiometers have this variable capacity? Thank you.
 

Thread Starter

DaniKowa

Joined Sep 23, 2020
175
Sorry for the late reply, so I understand correctly if I say that:

1) Is it normal to read a capacitive value on a resistive potentiometer?
2) Then by rotating the potentiometer, this capacitive value decreases as the resistive value increases and vice versa.

Thank you
 

drjohsmith

Joined Dec 13, 2021
383
Sorry for the late reply, so I understand correctly if I say that:

1) Is it normal to read a capacitive value on a resistive potentiometer?
2) Then by rotating the potentiometer, this capacitive value decreases as the resistive value increases and vice versa.

Thank you
There are many types of variable resistor,
you say you have a wire wound one, "most" are a metal film and a wiper.

Any two conductors, separated by an insulator, form a capacitance,

There is no inherent relationship between a variable resistors resistance and its capacitance,
I'd imagine if you change how your holding or not the parts, the capacitance will change.

You say this is a wire wound resistor type,
this is effectively coil, a coil by definition has resistance, capacitance and inductance.
a wire wound resistor is designed to be used "near DC "
at "near DC" the inductance and capacitance have negligible effect,
Your meter is using a wave form to measure the phase capacitance
QED, its not DC,

An interesting effect,
but not un expected side effect of this type of component and not repeatable to any degree
 

MisterBill2

Joined Jan 23, 2018
11,603
Happy New Year everyone!

today I was playing with an LCR tester, I often use it to check inductors and capacitors. This morning I had a "blue" 3590s multiturn potentiometer (wire wound) in my hand. I was testing the value in ohms, then curiosity made me set to measure C too, that is the capacity. I noticed that by rotating the potentiometer the capacitive value is inversely proportional to the value of R. That is, the more the resistance increases, the capacitance decreases. The question is: do all potentiometers have this variable capacity? Thank you.
One other consideration is that a meter set to read capacitance may not deliver a useful reading when connected to a resistance. An ohm meter will not often provide the inductance measurement when it is connected to an inductor. It will display the resistive part of the reactance.
In short, readings obtained with the "wrong" settings may not be right.
 

drjohsmith

Joined Dec 13, 2021
383
One other consideration is that a meter set to read capacitance may not deliver a useful reading when connected to a resistance. An ohm meter will not often provide the inductance measurement when it is connected to an inductor. It will display the resistive part of the reactance.
In short, readings obtained with the "wrong" settings may not be right.
Just out of interest,
wonder what a "LCR" bridge would give you

for info
https://www.hioki.com/global/learning/usage/lcr-meters_1.html
 

Thread Starter

DaniKowa

Joined Sep 23, 2020
175
There are many types of variable resistor,
you say you have a wire wound one, "most" are a metal film and a wiper.

Any two conductors, separated by an insulator, form a capacitance,

There is no inherent relationship between a variable resistors resistance and its capacitance,
I'd imagine if you change how your holding or not the parts, the capacitance will change.

You say this is a wire wound resistor type,
this is effectively coil, a coil by definition has resistance, capacitance and inductance.
a wire wound resistor is designed to be used "near DC "
at "near DC" the inductance and capacitance have negligible effect,
Your meter is using a wave form to measure the phase capacitance
QED, its not DC,

An interesting effect,
but not un expected side effect of this type of component and not repeatable to any degree
Ok, the potentiometer I used is a 3590 multiturn model, so definitely wire wound. But for my purpose I want to use this vessel not in DC but in AC or RF. So in case of AC / RF purposes, wrapped wire may not be recommended. Correct? If so, what is the solution, can I always use multiturn?

Thank you guys!.
 

MrChips

Joined Oct 2, 2009
25,928
Ok, the potentiometer I used is a 3590 multiturn model, so definitely wire wound. But for my purpose I want to use this vessel not in DC but in AC or RF. So in case of AC / RF purposes, wrapped wire may not be recommended. Correct? If so, what is the solution, can I always use multiturn?

Thank you guys!.
It depends on the AC frequency. If it is 50 or 60Hz it should have little effect.
 

MisterBill2

Joined Jan 23, 2018
11,603
Using wirewound pots at higher frequencies often produces unanticipated results because of the inductance. That has been the reason for carbon film and cermet variable resistors. A wirewound deevice is not going to do what you want, probably. Thus an alternate scheme is required.
 
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