Failed dimmer switch.

[Ret.]

I have two contemporary wall 'uplighters' on my lounge wall. They are fed from the ringmain via a switched fused spur and a dimmer switch. The lights each use a GU10 50w spot bulb.

We tend to leave the dimmer switch on about a 3/4 setting and just switch the lights on and off via the switched spur.

Yesterday, when the lights were switched on, there was a brief flash of light and both uplighters went off.

The fuse in the spur had blown and, on replacing it, I found that one of the GU10s had failed. I replaced it and both lights are now working again - but the dimmer isn't...

The dimmer is a rotating knob of course. Turning it clockwise first switches the lights on - but at the lowest dimming level. Further clockwise rotation raises the brightness of the lights. There is now no dimming effect at all, however. Turning the knob clockwise just switches the lights on at their brightest level and further rotation has no effect at all.

So, after that lengthy description, is the dimmmer switch completely stuffed?

[fred]

In article , Ret. writes

'fraid so (or at least is for those not handy with a soldering iron).

Lamp blew short circuit which took out the fuse but also took out the triac (switching device) in the dimmer. An electronics handy person could identify the part and replace it with a pin-compatible replacement but full replacement is prob the easiest option. Replacing the dimmer with one having a higher rating may reduce the likelihood of it happening again.

[charles]

yes - but they aren't very expensve to replace.

[charles]

Modern dimmer switches are totally encapsulated with no chance of a user replacing a part. Rememeber that one definition of a fuse is: " A device that always blows after the expensive semiconductor it is designed to protect." But what was the fuse in the spur? A 1 amp one might blow first - a 13 amp one certainly wouldn't.

[The Natural Philosopher]

triac has gone. You may be able to replace it, or get a new board to go in the switch from the manufacturer of the dimmer.

[Ret.]

It was 5 amp. I've replaced it with a 3 amp.

[Ret.]

Thanks everyone for your responses. I'll get myself a replacement dimmer!

[Brian Gaff]

Is it really worth fiddling with a new Triac when dimmers are relatively cheap? Brian

[Brian Gaff]

Or adding a quick blow fuse on the other side of the dimmer of course. I often wonder why this is not done, as I've had dimmers go regularly with this problem in the past. seems the tungsten bulb builders are secretly supporting the dimmer replacement industry.. grin. Brian

[Andy Champ]

Having read many other replies - yes, unless it's one of those very rare ones with a built-in fuse.

Andy

[Hugh - Was Invisible]

We have 3 x 39mm filament 30w spotlights in a display cabinet. They are downward facing and blow regularly presumably because of overheating. Put in a plug in 300w dimmer and they took that out when one or 2 blew.

Off down to the garage tomorrow to see if I have any old wall light dimmers I can put in a housing protected by a suitable fast blow fuse.

Anyone know what the start up current on 3 x 30w filament bulbs is likely to be? I am guessing it will be well over 90/240 amps starting from cold. Need a fuse that might protect the triac but will not blow on start up.

[NT]

Now you know the cause, the next question is how to prevent recurrence. Switching from mains halogen to LV solves it. Use a dimmable ballast

NT

[Hugh - Was Invisible]

Lol

[Charles Fearnley]

You could try IQ microprocessor based dimmers - search TLC for them, or at

much more expensive than standard dimmers. In theory these shut down safely under gross overload conditions. I've recently replaced three standard dimmers with these, mainly because we are now using mostly halogen based lamps where dimming is wanted, and allegedly the soft start gives lamp better life. I've yet to have a bulb fail, so can't comment on whether the auto shutdown works or not.... The soft start is nice though - comes up evenly over about two seconds. Final thought - you need fairly deep back boxes for these, and the spare tabs need to be cut off or knocked back.

Charles F

[Andrew Gabriel]

There is probably some truth in that.

It's possible to make a dimmer with a rotary knob from a handful of very cheap components, so that's what most are. These dimmers turn the triac on with a short pulse into the gate, and then it's held on by the current flow throught the lamp, but this requires a minimum current flow through the lamp (related to the triac's holding current) for the circuit to work, particularly at low set levels. The holding current of a triac is roughly proportional to the triac's current rating. This means that if you fit a higher rated triac with a view to surviving the flash-over as a lamp dies, then the triac will need a high minimum load to work. So whilst it would be quite easy to use a high current triac which would survive long enough for a fuse to protect it, it would probably have a minimum load of 500W.

The other way to fire (turn on) a triac is to drive the gate continuously during the on-time. However, this requires more complex circuitry. You are more likely to find this in dimmers which need the complex circuitry for other things anyway, such as touch control or soft-start or designed for inductive loads. Where the gate is driven continuously during the on-time, a much higher current rated triac can be used without requiring a high minimum load. Such a triac is more likely to survive the occasional flash-over as a lamp dies.