Are dimmer switches fixable?

[Jim S]

A bulb on a 3-bulb chandelier blew the other day and tripped the RCD. On replacing the bulb, the dimmer switch is either on or off. Without looking, is this easily fixed - as they are cheap enough?

[Frank Erskine]

They're dirt cheap, and a lot easier than sourcing replacement bits after faulting the switch.

[Brian Gaff]

Depends on the type and your resolve really. If your time and expense is not the issue then the ones that use old school diac and triac circuits can be fixed by simply changing ehe bit that died, but ehe more recent dimmer on a chip type, unless you have the chip are just as well thrown away. I must say that today I'm rather surprised that one can blow them up like this, I tought they had protection from a shorted load. maybe its a very old one?

Brian

[The Natural Philosopher]

Triac blown. I've got a few like that here :-)

If you are handy with a soldering iron fitting a new fatter one is no big deal. but not much cheaper than replacing.

[Graham.]

Triacs costs less than £2, so it's often worthwhile repairing dimmers, especially if they have a nice wallplate that has to match others, or if it's a multi gang plate.

[Andy Wade]

This one that I was asked to look at wasn't...

[Andrew Gabriel]

It's difficult to protect from fault currents (effectively, short-circuits). You can use a high current triac which can handle a higher non-repetitive current pulse without the junction melting, but then the triac's holding current will be higher too, which means when used in a simple pulse-fired circuit which most domestic dimmers with no neutral connection use, the minimum power load for the dimmer to be able to dim down properly without flickering or suddenly cutting out is also going to be higher, possibly higher than many common dimming loads.

More expensive dimmers do use higher current triacs than are needed for the normal load so they can survive short-circuits, but they provide continuous trigger current, not a pulse, so the triac's larger holding current doesn't come into it anymore and they have no minimum load.

There are also designs using MOSFETs which can switch off fast when the control detects a short-circuit, but I don't know if they are actually used in commercial dimmers. I have use MOSFETs for phase control of mains loads, but not for filamant lamps, so I didn't have to worry about them surviving short circuits.