Boiler Fault Finder

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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