Lights keep blowing the dimmer

In article , KS writes

It happens because there can be a big current surge when a bulb blows. Wall dimmers use cheap triacs (the bit that does the power switching) which can't cope with a big surge and fail in the "full on" state.

The dimmer should really have (a) a triac which is rated well above the operating current (stage lighting dimmers use 4x the max load) and (b) a fast acting fuse to protect the triac. However few of them have these. Maybe some of the more expensive ones might.

I guess the broken dimmer had some sort of electronics in it, when the lamp blew there was a short period of high current which damaged the dimmer.There are plenty of posts on the theme 'light circuit mcb trips when a lamp blows' -again because of the momentary high current at the instant the lamp goes. Will it happen again? Cann't say for sure, you may have had a dimmer not quite up to the job before.How many transformers does the dimmer control? What kind of current is it passing normally?

Tim Mitchell wrote in news: snipped-for-privacy@tega.co.uk:

In my lynge I've got 4 100w halogens, and two dimmers have (sort of ) failed quite quickly by not dimming as dim as they did when new.

Do you have any theories or advice about this?

mike

I blew two dommers really quickly on candle bulb loads, complained, and got replacement guts with BIG triacs in them.

Triacs blow short circuit, hence lack of dimming at all when buggered.

All I can suggest is take em back and get em swapped out for something beefier.

Try an IGBT based one. Different failure modes but they do protect themselves a bit better than triac based ones.

The Natural Philosopher wrote in news: snipped-for-privacy@demeter.uk.clara.net:

They still dim, but the dimmest is quite a bit brighter than when new

I guess I'll try that (they *were* cheapo)

mike

"Mike" wrote in news:d0800l$179$ snipped-for-privacy@newsg2.svr.pol.co.uk:

I had to look it up

Are these available in a domestic lightswitch package?

mike

Who makes those?

Regards Capitol

Thats extremely odd. Triac must have changed its characteristics.

Dr Evil Enterprises inc.

Guys

Thanks for all of the responses. Had a look at the blown dimmer yesterday and no surprises it blew. It was only rated to 250w. The 6 lights at 40w each are 240 so not a lot of margin.

I've seen dimmers for sale up to 450w which I was gonna buy and use the actual triac/switch bit, keeping my nice stainless wall plate. Or, is there a source i can just buy the dimmer *innards" from.

Thanks

The Natural Philosopher wrote in news:1109905360.41486.2 @despina.uk.clara.net:

Two have behaved exactly the same - B&Q's finest

mike

My 400W dimmers were blowing on 40W candle bulbs no sweat.

But the replacements they sent have not.

You can solder in a new TRIAC if you feel up to it.

The other possibility is to buy a cheap plastic unit and swap the guts.

In article , KS writes

Don't think so, though Velleman do some kits you could maybe fit (from maplin.co.uk)

If you swap the "innards" make sure the face plate is still earthed!

Well though they now dominate stage use it does appear domestic versions are as rare as hen's teeth compared with triac ones. I think currently quite a few remote control ones are IGBT but I can't find a 'normal' one though I'm pretty sure they do exist as they are sometimes referred to in 12volt electronic transformers instruction leaflets.

Sounds almost like it's actually back to back thyristors and one is short-circuit.

No, the trigger diode is a transistor(three layer device--npn say) operating in avalanche mode breakdown. This is infamously unreliable from a breakdown voltage specification viewpoint. Replacing the diac is the solution, if it is not integrated within the triac. Generally speaking, throw it away and buy a new one, it's not worth the effort of repair!

Regards Capitol

And if you look at the specs for bog-standard dimmers (where the manufacturer bothers to provide one) you'll see that they are only supposed to be used with GLS lamps, not halogen (not even mains-voltage halogen).

What you need is an MK 'intelligent dimmer' - which contains various sorts of inbuilt protection and is spec'd for use with halogen lamps:

Unfortunately many lamps short as they blow - it's common for a 6 amp MCB to trip too. And domestic dimmers can't handle this sort of fault current.

It might be possible to repair the dimmer with a new triac and diac. Or as you've done, find a source of cheap dimmers while keeping the original expensive plate. Wholesale Fittings own brand used to be good for this. I've used those to replace grid switch dimmers - there were an identical unit from the same maker (Home Automation) but at a quarter of the price.