Tacho - 23-6-2004 at 03:42
Botomline:
Common resistors of 2 or 5 watts (widely available) can be used as heating elements for low to medium temperature DIY heat sources.
Here is a picture of the building of my warmplate. I’m pouring phosphate cement in it’s back:
Making a short story long:
I needed a low temperature plate to dry things like TLC plates and keep solutions warm for long periods.
I bough sixteen 33ohms 2W resistors and arranged them in series. This, under our 127V mains should give me about 32W of dissipation.
I Put them in a aluminum plate, making sure only the ceramic-like body of the resistors touched the metal (none of the wires). I Insulated the exiting
connections with fiberglass insulating tube and filled the thing with phosphate cement. Notes:
1- I can’t see why gypsum or common cement could not be used instead of phosphate cement, just make sure is very dry when you turn on the power.
2- Make some dents on the aluminum, so your cement won’t come out, nicely molded by the aluminum pan.
3- Sorry, have not tested yet, must wait until the cement hardens and dries.
Mr. Wizard - 23-6-2004 at 06:42
Don't be too surprised if the hot plate works for a few minutes and then quits. Your math on the wattage and voltage is right, but the wattage
ratings on the resistors is figured for free air flow. Without the air flow even a two watt resistor could be incandescent in a few seconds. Think of
it as a two watt light bulb. Covering the resistor heating elements with phosphate cement might slow down the heat flow, causing them to operate over
their rated temperatures. Keeping the resistors close to the metal is a good idea, as the heat can flow more easily, but if a resistor fails, and
arcs, the metal won't be insulated. Testing it will give you, and us, the answer. I hope it works, because it looks useful.
It works. Resistors ARE a good heat source.
Tacho - 23-6-2004 at 16:36
After 24h, I dried the plate under 90C for 1 hour, and connected it to the mains. On top, I put a 250ml beaker with 150ml of water. After 1 hour, the
temperature of the plate was between 70 and 100 degrees celsius (melted stearic acid, but didn’t boil water) and the water was at a steady 55
degrees celsius.
Mr. Wizard,
Your reasoning is right, and one should be careful about that. Probably gypsum is not a good idea. In the case of my warmplate, however, I doubt that
the temperature gradient could be more than a few degrees/mm, so the resistors were not much above 100 degrees. I’m sure they can withstand that.
chemoleo - 23-6-2004 at 17:03
Just to add - gypsum, plaster of paris, does lose a lot of water above 150 deg C. It changes colour too (slightly yellowish), and it becomes more
brittle.
I'd probably stick to fire cements (I dont know the characteristics of phosphate bonded cement). In the 'Exotic thermites' thread, I
believe I mentioned how much weight a normal plaster of Paris cast would lose. But then, this may not be problematic to your goal, which is to operate
a mild heating plate ....
You must know, though, that coffee machines heating
plates produce the heat necessary for slow evaporation? At least that's waht I used to dry my various 'projects' 
Still, I think it's a great idea.
froot - 24-6-2004 at 08:39
If you could place the 'heating element' in a sealed vessel of the same shape full of transformer oil. That would aid heat distribution and
prevent any hotspots, prolonging the life of the resistors. Perhaps using a metal screwcap pillbox, or a photographic lens filter container. Sealing
it up with automotive heat resistant gasket silicone. Place the resistors inside, positioned away from the sides with strips of pc-board. Seal the
wire entry. When that's dry line the thread with silicone, fill with oil and close it up.
Great idea anyhow.
[Edited on 24-6-2004 by froot]
janger - 21-8-2004 at 05:48
Hey you stole my idea. But you get the prize for finishing first
Although I hadn't thought of a good sealant as yet.
Metal film resistors are known to hold up to heat better than the usual carbon ones. We tested some by applying enough power to make them glow red for
a while, cooling them, and the resistance had hardly changed.
Why not have a higher wattage of resistors, and connect to the mains through a light dimmer?
Dave
axehandle - 21-8-2004 at 06:14
Well, if going up in power it probably would make more sense to use a NiCr coil. I really like the idea, it seems like the perfect solution for low
heat (handling coils spun from extremely thin (0.2mm) NiCr wire is very troublesome and using resistors seems much easier). Nice work!