electrical question..dimmer getting hot

[Rob Bourgeois]

The dimmer on my light gets hot to the touch when its on for a period of time 2 hours or so tonight. Is this normal or should I replace it. The hot part is the screw which come through the plastic plate. The plastic plate is warm but not overly hot.

[Don Baer]

Most dimmer use the case of the Dimmer as a heat sink. They are designed so that when properly screwed into a metal box the box will also act as a heat sink. Make sure that the screw fastening the dimmer are tight to the outlet box and hopefully it is a metal box not the new plastic one that some are using. Some heat is normal.

[Randy Meijer]

Don: Your comment puzzles me?? I've installed a few dimmer switches......600 watt units for incandescent lights.....and do not remember ever seeing any warning about avoiding plactic boxes??

[Don Baer]

Don: Your comment puzzles me?? I've installed a few dimmer switches......600 watt units for incandescent lights.....and do not remember ever seeing any warning about avoiding plactic boxes??

I too have never seen any warning about plastic boxes. I do know however that the manufacturers do count on the box as being part of the heat sink.
I do remember reading this on one of the forum I hang out in I just can't remember where (The memory is the second thing to go, I don't remember what the first thing was). I'll try to find it and post a link.

[Dale Thompson]

Rob,
I had the same "hot" concerns over a series of dimmers that I installed in my daughter and SIL's basement. I called the manufacturer and was told that a temperature of 120 F. (too hot to hold your hand on for any period of time) was not unusual and even a temperature of 140 F. was acceptable. :eek:

That's what they told me but the "hot" problem still bugs me and is one of the reasons that I NEVER was a big proponent of dimmers. :( The energy has got to go somewhere - RIGHT?. :confused:

Dale T.

[Bob Johnson2]

Mine get warm, but not hot. And the boxes are plastic.

[Vaughn McMillan]

We've got three dimmers that get pretty hot, but there's between 500 and 600 watts of light on each, so I'm not surprised. I also remember reading that they're designed to get pretty warm, so I haven't been real worried about it.

- Vaughn

[Rob Bourgeois]

I will turn it on later and get a temperature reading.

Dale, I know it has to go somewheres and its just a big variable resistor but I guess I never thought that it would get that hot. I think I might be changing those out. I dont like them anyway. Why on Earth would I want to have the hall light dim..makes no sense.

I will let you guys know what I find out...adding this to my ever growing preholiday( Christmas and New Years) list of things to do. :rolleyes: :(

[Dale Thompson]

Dale, I know it has to go somewheres and its just a big variable resistor but I guess I never thought that it would get that hot. :(

Rob,
I haven't paid much attention to dimmers lately but I thought that by now they would have invented some sort of an electronic dimmer that siphons off a variable amount of voltage. I guess that's not the case. :( If you don't want to illuminate an area, why put a light there in the first place? :confused: If you want low light, put in a smaller wattage bulb. :)

I can understand the "romantic' theme in certain areas but, at MY age, that is just a few "yellowed" pages in a long-lost book. ;)

People may think that they are saving electricity. They are NOT! The energy is just dissipated in the dimmer instead of in the light bulb.

I think that contractors install those zillions of dimmers just because they are more expensive. :rolleyes: Well - that's capitalism - more power TO them!! :) :D

Dale T.

[Rob Bourgeois]

HEy Dale...maybe we can invent the item you speak of and say the others are a fire hazard and should be replaced....

The screws got up to 110-115 degrees and it is attached to a metal box. I am planning on changing it out , maybe my wife wont notice..:eek:

[Dale Thompson]

HEy Dale...maybe we can invent the item you speak of and say the others are a fire hazard and should be replaced....

The screws got up to 110-115 degrees and it is attached to a metal box. I am planning on changing it out , maybe my wife wont notice..:eek:

Rob,
I'm with you on the first sentence. I think that we would be not too far off in our advertising. It is a FACT that old dried wood that has been exposed to high temperatures for a period of years (such as a ceiling joist next to a furnace) can auto-ignite at a temperature as low as 150 F! :eek: :(

I used plastic boxes in the daughter and SIL's basement and I would guess that the temp of the screws was right about what you measured. :)

Be careful, Rob, what are you going to say when your wife says, "Honey, why don't you turn down the lights - purr - purr?". :D If you don't have a good explanation, you may be sleeping with a bunch of cold discarded dimmers in a Galaxy far, far away. :eek: :D :)

Dale T.

[Frank Chaffee]

It is a FACT that old dried wood that has been exposed to high temperatures for a period of years (such as a ceiling joist next to a furnace) can auto-ignite at a temperature as low as 150 F!

That is a quote from Dale's last post btw. I guess I don't have this posting thing down yet.

Read it over enough times till you don't trust building codes over your own judgment when you build your house.

Frank

[Shaun Wilson]

Rob,
Every dimmer I have seen does this. I think it is due to the way they are constructed. The front is usually aluminum, which conducts electricity and heat very efficiently. The heat is radiating outward towards the switch plate and ultimately into the room. This makes sense and they are likely designed this way for a reason - the heat has to go somewhere, better the room than building up inside and getting really hot and melting or catching on fire. I believe a dimmer works on the resistance principle, which would explain why they run hotter the more "dim" you have them set at. Actually, the wires inside the wall will get hotter than the dimmer since they carry the same or more current with no place for the heat to go. I would not worry about it. If you are really concerned, just email Lutron or Leviton or whomever the dimmer manufacturer is and see what they say :).
Shaun

[Gary McKown]

Couple misconceptions above about how dimmers work - they are NOT simply variable resistors, and the "gadget" (called a silicon controlled rectifier, or later triac) was invented somewhere around the '50s or '60s. Same thing as in a VS drill, etc.

A 600 watt load means about 5 amps of current at 120 volts. If a variable resistor were used to lower the output by, say half, then the resistor would need to dissipate around 300 watts as heat. That would be a disaster if contained in a wall box, drill body, etc., whether plastic or metal.

The SCR/triac, on the other hand, reduces the power output by switching OFF during part of the AC duty cycle (or above some DC voltage), the OFF portion being controlled by a low-power variable resistor connected to a slider, knob, or trigger. The only heat dissipated is due to a slight voltage drop across the silicon chip and electronics, maybe 10-20 watts. In a dimmer, the SCR is electrically isolated from, but thermally connected to the metal case, which is sized to dissipate the heat without a large temperature increase. Look at the instructions for a dimmer - they will say, for example, that if you remove the mounting "ears" on the case, then the maximum load should be reduced.

Plastic box should be O.K. in normal use, metal is better. Big wad of ground wires pigtailed together helps, too, as do metal cover plates.

BTW, if the load shorts out, odds are that the SCR (now passing very high current) will blow (long before the house breaker) and the dimmer is history. This happened to me recently when a chandelier socket shorted while installing a new bulb. Flipped the switch and then headed for the hardware store for a new dimmer (and new socket).

[Ralph Barhorst]

What Gary said is correct. Dimmers are not variable resistors. Very little power is dissipated in the dimmer, but there is enough to warm up the switch plate. This should not cause a problem.