# Best configuration for 400W resistive heating device

Suppose I wanted to heat a large aluminum disk with joule heating. I would use one of its flat faces, which has a radius of about a foot (30cm, SA = 0.28 sq m) and enough resistors to dissipate 400W of energy.

If I use 12V across a circuit:

P = V^2/R

P = 320W

V = 12V

R = 0.45 ohms

and the current for such a circuit would be I = V/R = 12/0.45 =~ 27 A (which seems very high)

Is this correct? Is this kind of set up "safe"?

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And if you are interested:

If you were to build this circuit, how would you arrange resistors of any R value while:

1. Distributing the heat across a large (1ft radius) surface
2. Remaining under 100W per resistor
3. Maintaining R=.45 ohm total circuit resistance as closely as possible

Here is one attempt of mine, using 10 parallel 4ohm resistors (.4 ohm circuit resistance):

simulate this circuit – Schematic created using CircuitLab

If you think it is safe/worthwhile, I am open to setups with higher voltage, but for some reason 12v seems to be the standard.

Also if I wanted to use PWM (DC) to control the output of this circuit, what would be the cheapest and most efficient way to set that up with Arduino?

My disk is about 10.8 kg with specific heat capacity (Al) = 0.9 J/gC

So I chose 320W because this would allow me to heat by 10 deg C every 5 min:

0.9*10800*10/320 =~ 300 sec = 5 min

• It'll work. Main safety issues come from the heat and the high current. If you're paranoid about safety or have difficulty finding 30A rated fuses, switches, etc, you could wire pairs of resistors as separate 6A circuits. Aug 7 '15 at 11:00

Personally, I would (if possible) have a groove milled into the disk and use a custom tubular heater that runs from the mains. I'd specify the heater at about 1.5x the power I think is required.

Tubular heaters are like the elements in a domestic oven- swaged with powdered ceramic inside- and are very long-lived and robust.

Easily controlled using an SSR- 500W would only be a few amperes.

• This would be a nice solution, but probably more expensive than I have funding for. They wouldn't happen to fabricate these for $100 or less would they? – Ryan Aug 7 '15 at 4:32 • I've been looking around for hours and the resistors are one of the cheapest and easiest to assemble, but at the cost of safety I suppose. Another solution I'm looking at is this: protovision.com/2011/09/25/milled-reprap-heated-bed-pcb But I don't have access to a CNC mill at the moment. – Ryan Aug 7 '15 at 4:40 • What country are you in? Electric skillet looks like a goodish fit. 10 Watt ceramic resistors would also be easy enough and no great safety hazard and moderate cost. | Also possible with some insulating head scratching needed is NiChrome wire of suitable rating. This can be obtained in a range of thicknesses to suit current and power needed. Aug 7 '15 at 11:54 • Custom calrod elements were not expensive last time I bought them, should be well under$100 for a couple. Aug 7 '15 at 12:23
• Ok thanks for the help everyone. I will try out a prototype with just a couple resistors and see how it goes, but since you mention the custom heating element, I may switch plans if I don't like the resistors. (btw I am in the US)
– Ryan
Aug 7 '15 at 21:17

What Spehro says re mains encapsulated element BUT it's called an electric skillet (electric frypan in NZ) - obtained at garage sales / yard sales (what country are you in) for a few dollars or somebody's attic or dumpster for free. Or Amazon / Walmart new from about $30. Typically 1000 Watt. Some are 1500 Watt, maybe more. PWM to suit. Heat distribution evenness may be an issue but a copper disk on pan base should help. Get friendly machine shop to remove sides OR use a hacksaw, file and effort if necessary. This gives you a metal heated base with sealed embedded element able to meet UL & other consumer safety specs. Amazon selection Amazon cheapie -$12.99!!! but at 7" square may not easily spread heat evenly enough.

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Walmart $low to$ high

• @Ryan But it still gets zero votes :-). Low or mo cost, robust, waterproof, available, minimal effort, ... :-). Aug 8 '15 at 14:57