I have 3 dimmers in my house that are the same model dimmer. Have them
controlling ceiling lights. Two give of very minor warmth generally
during operation, while one gives off so much heat that the screws
holding in the face plate are almost too hot to touch.
Is this likely a bad switch? Just get a new one? Worrisome as it's
housed in a plastic box, not metal.
I don't know. I would be curious though whether it gives off any heat
when it's 1) almost off, very dim; and 2) fully on, max bright.
If you're going to replace it, the upscale Lutron (eg. Diva model and
similar) have the advantage of being at least somewhat radio quiet;
your neighbors can listen to their AM radios even if you can't for all
stations, then. Most dimmers are tiny angry radio stations.
A thyrister device, in this case, a triac, is a silicon device. There will
be a voltage drop across the device, thus heat generated proportional to the
current. Since the triac is operating as a switch, triggered to turn on at
different points in the AC waveform on each half cycle, depending on the
control dial position, the heat produced will be proportional to the dial
setting. However, if the triac has a slow rise time, it may not behave this
Maybe the screws are getting VERY hot, because the OP is using a plastic
coverplate. The plastic is insulating the entire dimmer faceplate, so the only
way the heat can now escape is via the 2 metal screws, effectively
concentrating a possible 6 sq. in. of heat sink into 2 tiny dots.
I'd change to a metal switchplate if at all possible.
The Triac -- the semiconductor device in the dimmer that does all the work
-- is fastened to the metal strap of the dimmer for heat sink function. It
is obvious that the dimmer is at its maximum with the lights on full bright.
If you turn the lights on the dimmer on dim, the heat should diminish.
Maybe you should consider getting a new dimmer with a higher wattage rating.
Isn't that backwards? When the lights are full bright, the dimmer is doing
nothing and the triac should be completely bypassed. At the very least, it's
doing no work whatsoever.
When dimmed, the dimmer the lights, and the higher the total wattage, the more
work the triac is doing and therefore, the hotter it gets.
full power dissipation occurs when the triac is full on (full brightness).
The triac has only 2 states (on and off) and dimming is controlled by where
in the AC cycle it turns on. Compare the wattages of the bulbs on the 3
dimmers, the warmer dimmer should have the higher wattage bulb.
Go back and read the OP. Only one of three dimmers is running hot.
The OP says that the SCREWS are too hot to touch. What do you suppose the
temperature of the plate that the device is using as a heat sink would be in
that case? The maximum dissipation of triac should never get that high. The
current flow may be at a maximum, but the "on" voltage drop should never get
very high. Typically it should only be around 2 volts.
If the dimmer has a switch position at the end of its range that bypasses
the triac it should not get hot at all.
At an rate, I would replace that dimmer as it is certainly not acting
Also see > http://www.americanmicrosemi.com/tutorials/triac.htm
Add up all the wattages from the lamps that are controlled by the dimmer
that is getting hot. Next, remove the face plate of the dimmer an see what
the maximum wattage is (typ. 600w). If you are exceeding the maximum
wattage, you should reduce the lamp wattage by replacing the bulbs with that
of a lower wattage.
Also, you may find the wattage below the capacity of the dimmer. It is
normal for them to be quite warm when there are several hundred watts of
It is a 600W max Lutron dimmer. This one, of the three I mention having
in the house, has by far the greatest load on it. There is another one
where the screws are slightly warm and it has 180W total on it. This
one has between 400 and 500W on it. It is also in a PLASTIC phone-style
box in the wall, not metal, oddly. Bad construction, clearly. So the
box itself is not helping as a heatsink. The screws are quite hot (not
too hot too touch, really, after more experimenting), but the plastic
faceplate on the dimmer switch just feels warm.
Can't decide if this is a problemed switch or if I should leave it as
It would have been easier if you just *accurately* posted the "problem", that
is, "dimmer switch - _WARM_ screws" instead of "dimmer switch - _hot_ screws".
There are enough real problems in homes without you going around exaggerating
your symptoms, and berating long accepted, standard construction techniques
which clearly you have not a clue about...
Not really, and certianly not "clearly bad construction." Most boxes installed
since the early 60's in homes are either composite or plastic. The dimmer is UL
listed and rated for your load, and there should be no danger whatsoever with
such a typical, standard arrangement.
And it shouldn't. Even if it were metal, the dimmer would not get a UL listing
if it required a metal box, or purposefully dissapated heat into the box
itself. That would eventually damage all the wires contained therein. The
dimmer is *supposed to* dissapate heat through the face, not into the box.
OK then, just warm, correct? Perfectly normal operating parameters.
It does not sound like a problem. 400-500W on a 600W dimmer would
result in warm operation. You could consider going for a higher rated
dimmer if you like, but you will find that it will also get warm. If I had
to replace the dimmer, I would go for a more heavy duty one.
Warm is normal. The screws on a 600W dimmer go right into the heat
sink of the dimmer, so warm screws would be normal. And if the dimmer
is running too warm, just wait a while and the triac will short out and
you will have only an on-off switch with NO dimming.
The next step up from the 600W in-box dimmers is a dimmer with higher
wattage ratings, but the 1000W dimmers have external heat sinks. If you
can manage with the 600W dimmer (for how many years?) with it not going
bad, just stick with it.
It should be noted that the higher wattage dimmers will dissapate the
same power, but with their better heat sinking, their temperature will
be some lower. But they are not as pretty, since the heat sinking is
Blake Patterson wrote:
Phil Munro Dept of Electrical & Computer Engin
mailto: email@example.com Youngstown State University
There's a lot of quasi technical discussion about triac/ thyristor theory
going on here that does not explain the excessive heat. The whole idea
behind using a solid state switch is that it is a low loss ( and so low
heat ) device.
Now then if the screws are too hot to touch, consider how hot the plate that
they are screwed into must be. And that is inside the wall!
Since you reported that it controls a ceiling light and I assume we are not
talking about a chandelier or other multi- bulb arrangement, I would
replace that dimmer in a heart beat.
You left out a few important facts. First, are they all handling the
same load (total wattage?) Are they all set to the same brightness? Are
they are rated the same and the same brand/model.
As suggested, I suggest you get a heavier duty unit to replace the one
that is getting hot.
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