Most coffee mug warmers are under-powered. One reviewer on Amazon said that they were restricted to about 24 watts by law.
This one
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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?
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If not, is there a way to ramp the the power down as an external thermostat?
Yes. It's a resistive load, just like a light bulb. I find it difficult to believe that 15 amps is correct, that would be 1800 watts which will do far more than warm a coffee pot.
If you use the Lutron dimmer, make sure that the warmer is under 300 watts, or get one a bit more robust.
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.
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.
An incandescent light bulb is inefficient at producing light and it gets much worse as the voltage is reduced, however, it is great at producing heat. If you reduce the voltage on a heating element, it will produce less heat but is still nearly 100% efficient at producing the heat. It does not become less efficient if that is what you were trying to say.
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.
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.
I'm not too worried about the efficiency curve or achieving 10 decimal places of accuracy on the temperature. I'm mainly curious about (a) if it will work at all and (b) it if will damage the unit.
It will work but if the warmer pulls more than the dimmer spec it will burn out the dimmer, 15a is an expensive dimmer,Ive never seen more than 1000w for 50$
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
I seriously doubt the veracity of that "120V/15 amp" rating, which I did see listed as a "spec" on a couple of other seller's websites.
I have a Bunn coffeemaker and I just measured the current draw of the warming plate on it at 1.05 amps. (I made sure that the main water heating element's thermostat was OFF at that time of course.)
So, if approximately 120 watts will keep a full caraffe of coffee at proper drinking temperature on my Bunn coffeemaker, the stand alone Bunn warmers probably draw no more than the "built in" ones.
Thus, that 300 watt dimmer ought to do just fine after you find the right slider setting to keep a single cup hot.
My gosh how we manage to make something so complicated.
1) Most dimmers are designed to reduce the brilliance of incandescent lamps. They do this either by reducing the voltage, whereupon the Voltage Squared divided by the Resistance of the load applies. (It's not a theory btw it is "Ohm's Law" etc. And as old as the hills!) And as noted if the voltage is reduced to half one will get one quarter the amount of energy (heat/light or a bit of both) at the load.
BTW: Some very old style variable resistance dimmers themselves would get hot in order to do the voltage reduction and this could be classified as 'wasted' heat!
OR, the amount of electric power applied to the load (lamp/coffee warmer etc.) is altered by changing the wave shape of the inputted sine wave so that only part of it (or a reshaped wave) gets through to the load which is then not as bright, or in this case not kept as warm. .
2) Since the coffee/tea warmer is most likely just some sort of resistance heater, yes, most likely the dimmer will work fine. Just find the right setting of the dimmer.
Also agree that any appliance that really does consume 115 volts at 15 amps (That's 1725 watts!) sounds a lot more powerful than a 'warmer'. That's sounds more like a kettle, to 'make/brew' tea or coffee! :-) Perahps it's designed to be used to heat up the water, drop in the tea or coffee and then be switched off? In other words it's more of a coffee/tea maker than a 'warmer'!
We used to make tea that way, at a work bench, by dropping a wire wound resistor connected to 48 volts DC directly into a mug of water. When it boiled we'd drop in the tea and after a moment or two disconnect and remove the heater. Can't remember the wattage but it was probably around one amp or less at 48 volts; hence V x I =3D 48 x 1 =3D 48 watts. The colour of the heater resistance was dark brown; deepening with use. But it was quite easy to make another one if it looked 'too unsanitary'.
For example many small floor heaters are no more than around 1500 to
1800 watts for the very reason they can then be plugged into a typical domestic outlet! And hair dryers!
And forget all this stuff about the resistance (of the lamp or the coffee warmer) varying with temperature; yes it does. Usually increasing but not necessarily in a direct linear relationship with temperature. But for practical purposes forget it. Just tiddle with the dimmer adjustment until one finds a setting that works. After all the amount of drink fluid will, presumably, be decreasing as it is consumed, so we don't want to get off on the subject of a fixed amount of kilo-calories of heat energy being applied to decreasing amount of fluid of specific gravity X; minus the thermal mass of the container and the rate of heat loss from that container taking into account the ambient temp. of the room! Just sip and enjoy.
Also btw if the warmer only takes very small number of watts the dimmer which has probably been designed and produced for a typical incandescent desk lamp bulb of say 40 to 100 watts (maximum 300) may be a somewhat finicky to adjust; also taking into account, as above, the fluid will be decreasing!
Which reminds; right now have an almost full pot of Sri Lankan tea 'on the hob'. Must go and refill this cuppa. Cheers.
I don't know where that 120V/15A spec came from. Bunn says nothing about the requirements of the BCW. Two other heavier duty warmers on the product page have 100W elements.
Well the warmer on my coffee pot is definitely on one. We had to run things off a generator last year because we lost power and I could hear the load on the generator cycle up and down at regular intervals when we used the coffee pot.
I would think all such things would have one of those simple bi-metal switch thermostats on them. Otherwise they could overheat as the liquid in the pot is reduced (or if the pot is removed altogether).
Don't forget the Variac type light dimmers used in some homes before the development of the present wave chopping solid state dimmers. They wasted far less power than those rheostat types you mention.
Back in college in the '50s we used to cook hot dogs in our dorm rooms by jabbing two table forks into their ends and connecting the fork handles to line voltage with an old lamp cord.
For a while they made hot dog cooking kitchen appliances which worked on the same principle. They held four (or maybe six) hot dogs jabbed onto pairs of spikes which got line voltage switched onto them when a safety cover was closed over the dogs. I haven't noticed any of that kind advertised lately though.
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