hot plate

[rrch123]

Hi Guys! Once again I come back here, because I cannot handle a problem myself, not even a simple one. Ok, here is the story. I got one of those cheap little coffee mug warmers from Walmart to keep my coffee warm in the moring, but warm is a joke, more like lukewarm (yuk). So I went to the troble of taking it apart and looking at it to draw a schematic. I hope the one I made was accurate. (see attachment)


Any point, does anyone have any suggestions on how to make it hotter? I know that greater the resistance, greater the heat, but what is the best way to go about this. The wire that is heating the plate says 300v, 125C on itself (I am assuming maxium values?). Should I get a different wire? Boost the power somehow? One of the things that has me so confused is that since it draws it power from an AC plug, why is it only drawing less than a Amp? As you can see, I am not very bright at this yet. I would love to hear any suggestions.

 

[Sceadwian]

Altering the circuit to increase power output will likley blow the fuse, altering the fuse will likley cause a fire or melt the entire assembly into a mass of plastic, start smaller, don't start off with something that uses mains voltage directly, especially if you can't figure out how the attached circuit works.

 

[rrch123]

That is part of what is confusing me. There is no step-down transformer, nor a resistor (execpt for the LED). What is keeping that fuse from not blowing right now?

 

[Sceadwian]

The resistance of the heating coil, possibly the inductance of the heating coil if it's wire wound. The resistor for the lamp is only to keep it from burning out from over current it may not even be an LED (unless it flickers), it has nothing to do with the heating element itself. The mains AC is only passed through the diode when it's on the 'low' setting which effectively cuts the current going through the entire circuit in half.

 

[Sceadwian]

The 300V 115C wire you're talking about is only the rating for the wire going to the heating coil, not the heating coil. If you have a multi meter put it on DC resistance and check what happens when you place the leads across the two wires leading to the heating pad itself. (While it's not plugged in) Check it both ways (Switching the multi-meters leads) just in case the LED causes a false reading in one direction.

 

[s.balasubramanian]

The heating is most probably done using a wire wound on something. Its like an inductor. The inductor stores energy in a magnetic field in its coils, when the AC supply cycle alternates, the magnetic energy discharges, but this process is not efficient, a lot of stored energy is dissipated, as heat. This heat is what is used to keep your coffee warm, well, lukewarm. This also provides some resistance to the AC supply and prevents the fuse from blowing.
As Sceadwian has warned, dont try to alter the wiring or fuse. but try to see if u can alter the core on which the wire has been wound. The core is where the energy is dissipated. If its a low loss material, then it wont generate much heat, but if you can replace it with a lossy material, then it will dissipate a lot more heat, keeping ur coffee warmer.

Good Luck to you! and in case ur experimenting with the fuse, happy 4th of july!

 

[evandude]

You probably also ought to look at other, possibly more important issues. The bottom of most coffee mugs has a raised edge which leaves an air gap between most of the bottom and the surface it's sitting on. An air gap like that is a terrible way to transfer heat. if you found a mug with a nice flat bottom, or modified your mug, you'd probably get better performance.

Making the plate get hotter, when the real problem is heat transfer, would just be unnecessarily dangerous.

 

[Russlk]

"I know that greater the resistance, greater the heat"
That is not correct. On a fixed voltage, the lower the resistance, the greater the heat (P =E^2/R). If you shorten the heater wire (about 10%), you will get more heat.

 

[rrch123]

Ok, man, a lot of replies. Um, Sceadwian, I tried the test you talked about and I got nothing. (Now s.balasubramanian as well) I think this is because when I take it completely apart, I find that the wire is insulated (I do not think I have to do much to figure that one out, I prefer a nice warm drink, not a deadly shock in the moring), and it is a single wire that runs inbetween two middle plates that do not seem to conduct any electricty.
And no I am not planning on messing with the fuse.
evandude, you idea is a very simple one, I have always been a fan of KISS, but the plate itself is not just hot enough. I can keep my hand on it and not get burnt. (and no I am not trying to make a past time holding my hand on hot plates)

All of you have suggested several things, what would be the best thing to do?

 

[Roff]

Ok, man, a lot of replies. Um, Sceadwian, I tried the test you talked about and I got nothing. (Now s.balasubramanian as well) I think this is because when I take it completely apart, I find that the wire is insulated (I do not think I have to do much to figure that one out, I prefer a nice warm drink, not a deadly shock in the moring), and it is a single wire that runs inbetween two middle plates that do not seem to conduct any electricty.
And no I am not planning on messing with the fuse.
evandude, you idea is a very simple one, I have always been a fan of KISS, but the plate itself is not just hot enough. I can keep my hand on it and not get burnt. (and no I am not trying to make a past time holding my hand on hot plates)

All of you have suggested several things, what would be the best thing to do?

If you have your receipt, take the POS back. Walmart will take almost anything back.

 

[HarveyH42]

Never used a coffee warmer, usually just add more coffee from the pot. Won't the heating pad make the mug hotter than the coffee inside? If the heater will keep the coffee at a just-drinkable temperature, wouldn't the mug be a little too hot to handle comfortably? Just curious. I've seen mug warmers for sale, even one that plugs into a USB port, but have never once seen one in use.

 

[Nigel Goodwin]

Just pop the coffee cup in the micowave for a few seconds if it gets a little cold!.

 

[rrch123]

A simple solution that I employ all the time, but I am mostly doing this so I can learn more.

 

[Sceadwian]

Learning is good, learning on mains AC is not so good =)

 

[AllVol]

If you have your receipt, take the POS back. Walmart will take almost anything back.

Now I know why I like you.

 

[rrch123]

Learning is good, learning on mains AC is not so good =)

Ok, radical question. Is there possibly some sort of DC curcuit that would work for a hotplate?

 

[Sceadwian]

Heating coils (typically) don't care what kind of voltage you send them, you could pass 120volt DC through the hotplate and it'd function the same, though the hi/lo button would turn into a secondary power switch because the current direction never reversed. The problem is you can't adjust the hot plate to use a lower voltage. Applying a lower DC voltage won't do much of anything, most power supplies would heat up more than the coil.

 

[Roff]

Heating coils (typically) don't care what kind of voltage you send them, you could pass 120volt DC through the hotplate and it'd function the same, though the hi/lo button would turn into a secondary power switch because the current direction never reversed. The problem is you can't adjust the hot plate to use a lower voltage. Applying a lower DC voltage won't do much of anything, most power supplies would heat up more than the coil.

He could heat his coffee on the power supply.

 

[rrch123]

He could heat his coffee on the power supply.

lol! oh the irony.

 

[Brevor]

Check out this link.

**broken link removed**