Mysterious repeating dimmer failure

[jon1270]

This is my first post here. I hope the topic is appropriate to the forum.

When I remodeled my kitchen about a year and a half ago, I installed nine line-voltage 4" recessed lights which are controlled by two Leviton Illlumatech preset dimmers, with three cans/150W on one dimmer and six cans/300W on the other (these are 600W dimmers). Everything works fine, except that when a bulb burns out there's a better-than-even chance it will take the dimmer with it. A dimmer has failed three of the four times that a bulb has burned out since the initial installation. Only the slide dimmer fails -- the on/off button continues to work normally.

Leviton has been replacing the dimmers as I mail them in, but their tech support people are, so far, unable to imagine how a bulb's burning out could cause the dimmer to fail. They've speculated that it could be a problem with a fixture or a loose connection, neither of which ideas holds water; the failures have all been simultaneous with burn-outs in different fixtures, and there are no symptoms of loose connections anywhere.

I'm hoping someone here has a brilliant insight to offer. The bulbs, if it matters, are Sylvania 50W PAR20s.
_________________
--Jon

[vtech]

It would be hard to tell without more data....from what is available in today's cheap market, I'd probably call it a cheap/limited design on the part of the dimmer.
Ofcourse the sure way to tell is to examine the failed device which is not possible for the average consumer & no doubt is a futile attempt to get that type of info from Leviton.

Have you tried going to a beefier wattage dimmer or maybe reducing the number of bulbs(loads) --just to see if it makes any difference?
better yet, if possible, try getting use to bringing up the lights slow while avoiding full tilt as a habit. you will be suprised how much longer a light bulb will last.

By dimmer failure, do you mean it stops controlling or simply won't turn back on? Again I can't be sure but it sounds like as the bulb burns out, it may be shooting back a spike to the SCR or control ckt within the dimmer?

[Guest]

The failure is such that the on/off button still works fine but the slide dimmer has no effect; the lights are either on full-brightness or off.

No, I haven't tried reducing the load, but each dimmer's normal load is half (or less) of its rated capacity.

[vtech]

[minnie]

Leviton has no excuse. The switch works but the variable portion doesn't cut it. The 'switch' isn't really bad. It turns it on and of. But you can't dim. 3 in a row? E=IXR. Ohms law. Seems the bulb burns out and fries the slider. But yet you can still turn on and off. Call it a rheostat. The slider decreases the voltage through resistance to the source/lamp. It fries when the lamp goes.??? vtech???

[vtech]

[torbjorn]

It is very common that when a light bulb burns out, it causes a temporary short circuit due to arcing. This short circuit current would certainly cause the SCR of a cheap dimmer to fail unless it is limited in some way.
At least here in Europe, with 230 V mains voltage, all light dimmers are equipped with a fast-blow fuse. But i don't know what the practice is like in 120 V countries like the USA.
Usually, the fuse will protect the SCR, but it should NOT be replaced by a slow-blow fuse.

Another possibility is that inductive kickback from a RF suppression coil in the dimmer causes the SCR to fail due to overvoltage or due to a fast voltage transient when the short circuit current is interrupted. In this case, it would be possible to protect the SCR by a varistor and/or a RC snubber network in parallel to it.

Another possible way of protection would be to connect a current limiting resistor in series with each bulb, select the resistor to give, for example 5 % voltage drop at rated current of the bulb, so will the short circuit current be limited to 20 x each bulb's rated current. Make sure to have a fuse in series with each bulb + resistor, to avoid overheating the resistor and causing fire in case of a permanent short circuit.

[minnie]