I have a household, dual module dimmer switch. One module is working and dims a 3A sidelight loop, the downlight loop with 4 drivers doesn't work and I'm fault finding.

I noticed on the working module that the voltage across the terminals is 237V when off and then reduces when on - it's a push on/off and turn dim. The voltage changes when turned as expected.

But I was expecting to see 0V on my tester when off, what am I misunderstanding?

Is the tester and the way the dimmer work showing a potential difference even though no current is flowing?


There is always voltage across the circuit, supplied by the power company. By Kirchhoffs's Voltage Law, the sum of the voltages across the individual components must always equal the supply voltage. When the light is on, there is little or no voltage across the switch and all of the voltage appears across the light. But when the light is off, all of the voltage appears across the switch and there is no voltage across the light (and no current through it).

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  • Having read some other QAs, I think the answer is more simply explained by pointing out my stupidity! The meter itself bridges the circuit and shows the PD (supplied by the power co.). I don't know the science behind the value showing when the dimmer is on but I assume its based on the two available paths, via my tester vs. via the TRIAC. – Luke Puplett Jan 2 '14 at 15:39

In order to properly measure the voltage of devices like dimmers, you need a true RMS (root mean square) multimeter. Some dimmers work by switching the power on and off many times per second and cheap non-RMS multimeters don't properly read these types of circuits.

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