How to test a potentiometer

To Test a Potentiometer (three pin connector)

Note: Manufacturers resistance readings will vary and must be checked for correct operation

  • Electrically isolate boiler / system controls
  • Remove potentiometer wires (at the p.c.b end) to prevent stray resistances from the p.c.b itself (Some potentiometers are built into the p.c.b and are sealed units, so not possible)
  • Set multimeter on ohms scale for resistance readings
  • Turn potentiometer to fully OFF position
  • Connect multimeter leads to bottom connection and to wiper (middle) connection on three pin connector of potentiometer 1 ohm = ok

Switch to ON position but no more further 1 ohm = ok

Slowly turn potentiometer to FULL ON position where the resistance reading decreases proportionally to 0 ohms = ok

No variation when rotated = faulty

  • Connect multimeter leads to top connection and to wiper (middle) connection on three pin connector of potentiometer

Turn potentiometer to fully OFF position 0 ohms = ok

Slowly turn potentiometer to FULL ON position where the resistance reading increases proportionally to 1 ohm = ok

Off position again should read 0 ohms = ok

To Test a Potentiometer (two pin connector)

  • Electrically isolate boiler / system controls
  • Remove potentiometer wires (at the p.c.b end) to prevent stray resistances from the p.c.b itself

(Some potentiometers are built into the p.c.b and are sealed units, so not possible)

  • Set multimeter on continuity buzzer mode
  • Turn potentiometer to fully OFF position
  • Connect both multimeter leads to both connections on potentiometer

Turn potentiometer to ON position continuity = ok

No continuity = faulty

Alternative test (where manufacturers resistance readings are known – example):

Potentiometer off (fully ant-clockwise)

Connections to test resistance (K Ohms)
Top / bottom 9.6
Top / wiper 0
Bottom / wiper 9.6

Potentiometer on (fully clockwise)

Connections to test resistance (K Ohms)
Top / bottom 9.6
Top / wiper 9.6
Bottom / wiper 0

If the readings are comparable with the manufacturers readings, the potentiometer can be assumed as working. A non-functioning potentiometer will normally give no variation between connections when the shaft is rotated.

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