Re: Can I put a coffee warmer on a dimmer switch?

Square Peg wrote:

On Fri, 19 Sep 2008 06:11:22 -0400, "John Grabowski"
<jgrabows1@xxxxxxxxxxxxx> wrote:


"Square Peg" <SquarePeg@xxxxxxxxxx> wrote in message news:lqb6d4hsbhai3h45j5ko5uk7r5a4t1rujb@xxxxxxxxxx

Most coffee mug warmers are under-powered. One reviewer on Amazon said
that they were restricted to about 24 watts by law.

This one

http://www.amazon.com/Bunn-BCW-Warmer-Home-Decanters/dp/B00023WAS0/ref=pd_bbs_3?ie=UTF8&s=home-garden&qid=1221796737&sr=8-3

is made for warming the whole coffee pot. It says it's 120V/15amp.
Several of the reviews say that they use it for a single cup, but it
keeps it too hot. It only has on/off -- no temperature control.

Can I put it on a 300 watt dimmer switch like this?

http://www.amazon.com/Lutron-Electronics-Plug-Dimmer-TT-300H-WH/dp/B0000BYEF6/ref=pd_bbs_sr_2?ie=UTF8&s=hi&qid=1221799800&sr=8-2

If not, is there a way to ramp the the power down as an external
thermostat?


You will probably be disappointed in the results and waste electricity at the same time. The heating elements are designed to perform at a certain voltage. As that decreases so will the heat level, but it will not be on a straight line but rather a drastic curve due to the resistive nature of the elements.


It's been a long time since college physics, so I did a little
research. I'm sure the real world is more complicated than this, but I
think the theory is that power = voltage x current

P = IV

Since voltage = current x resistance,

V = IR

power is

P = (V**2)/R

Heat is just power over time

H = Pt

or

H = (V**2)t/R

It looks like you are partially correct. (Right conclusion, wrong
reason?)

If heat varies with the square of the voltage, then one half the
voltage will produce one quarter the heat.

I already have one of those Lutron cord dimmers. In fact I have one
sitting right here on my desk attached to the desk lamp. The little
slider has about one inch of travel, so I would be able to just nudge
it a bit from full on.

If I can cut the voltage by 10% (x .9), the heat should be cut by 19%
(x .81). If the warmer, which is designed to heat a 10-cup coffee pot,
is heating my 10 oz cup of tea too much, I ought to be able to find a
good reduction.


Take a look at what happens when a light bulb is dimmed. Not only is the light output diminished, but the color of the light changes as well because they were designed for certain voltage.


I don't notice the color of my desk lamp changing that much, if at
all. Color is just frequency, right? Don't those little photons come
off a particular material as some frequency that depends on the
material? It seems to me that with less voltage, fewer photons will
come off, but at the same frequency. I could be wrong.

Your analysis is correct as far as it goes, but you need to recognize that the resistance is not a constant. It is a function of temperature. Usually the resistance of heating elements increase as they get hot.

The frequency (1/wavelength) is a function of the energy of those photons. Hotter materials emit more energetic photons that have shorter wavelenths. Put a piece of steel in a flame and watch the color go from brown to purple to blue to red to yellow, etc as it moves from about 500F to 2300F.

Boden
.

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