Can you check potentiometer resistance while in circuit while powered?

I'm troubleshooting a circuit board. The potentiometer is a 1K rotary dial one.

With the circuit board not powered it seems to test fine with an ohms meter.

886 ohms the two outer prongs
Either of the two inner prongs range from 6 ohms to 886 ohms while I turn the knob.

When I apply power to the circuit board, I don't get stable readings from the prongs. The two outer prongs also sometimes read 0 or a really high negative number. The two inner ones sometimes don't change while turning with power and also have a odd number.

Is this normal to have differences when voltage is flowing through the potentiometer compared to when there is no voltage?

Can you check a potentiometer while it is powered?

Thanks.

An old potentiometer can acquire dirt on the carbon pad. Or excessive current might burn it. (I've done this more than once.) Often you get a burned smell when this happens. Or smoke, or even sparking.

The resistance is stable when measured out of circuit, except when the wiper contact hits a burned area, which causes sudden jumps in reading.

If the pot is in circuit, voltage is stable, except for sudden jumps when the wiper contact hits a burned area.

Mechanical damage can result in contacts which move slightly, so that they touch and separate. This causes unpredictable operation.

you should not check resistance values while it is powered.

depending on your meter type , the meter may get damaged.

Hi,

How does an Ohm-meter work?

There are different ways, but always it is introducing some current (I) to the "unknown" resistance and measures the voltage drop (U) across the resistor.
Then it shows the value with using the formula R = U / I.

Now in a powered circuit in almost any case there is current and voltage across the resistor (else it us not working).
But at least this current is unknown to the ohmmeter, therefore R = U / I gives wrong results.
If now in the circuit is some varying current causing varying voltage you will find fluctuating reading at the ohmmeter.

****
But even if the circuit is not powered the readings are not reliable. Usually there is additional circuitry connected to the resistor you want to measure. If now some current flows through the extra circuit, it is unknown by the ohmmeter, so the ohmmeter calculates with the wrong I.
Also the voltage reaading is wrong caused by the current flow through the extra circuit.
So the U/I calculation is not reliable.

Klaus

You can check voltage drops on the potentiometer with a voltmeter and may be able to infer what you need to know with that measurement.

KlausST said:

always it is introducing some current (I) to the "unknown" resistance and measures the voltage drop (U) across the resistor.
Then it shows the value with using the formula R = U / I.

Click to expand...

Most commonly, modern digital multimeters apply a small voltage and measure the current. This way, the LEDs will not light up, the diodes will not get turned on and so on and on.

For the diode test function, it injects a constant current and measures the voltage drop. This constant current can turn on the diode on the forward direction but the compliance voltage is not sufficient to turn on white LEDs.

You can get an idea of the value of the pot to be replaced but for best results remove it from the circuit before doing any resistance measurements.