# Most efficient use of resistors to heat an aluminum plate?

I'm building a reprap printer. I have a sheet of aluminum about 8x8" to use as the print surface, which must be heated to prevent problems while printing. </background-info>

My controller board can handle a maximum of 60V @ 11A, before either the PTC fuse trips or MOSFET overvolts. I'm not changing these components, so whatever solution I find has to fit within these constraints. I have 12x 1 ohm, 10W aluminum chassis-mount resistors that I intend to abuse as heater elements. I'm trying to find the most efficient way to parallel/series the resistors and what supply voltage to use to draw the most power. Existing power supplies I have are either 12V @ 18A, or 24V @ 6.5A, and I would prefer to not purchase another one if possible.

Exceeding the wattage of the resistors temporarily is perfectly acceptable, because I'm only shooting for a target temperature of 65-110C (depending on the plastic used). Any solution I'll be happy with should hit that temperature within 10 minutes or so, so pushing 25W through a 10W resistor in that time won't really cause them damage. And if it does, meh, they're a buck each. After the platform is at temp, a microcontroller will cycle the power of the resistors to control the heat.

• This question is actually quite specific and i have the same question title with different info needed as the answer, such as, how reliable are chinese 100w resistors for extended periods etc. – DeltaEnfieldWaid Mar 19 '15 at 10:45

Seems like you want to get the max power from the resistors and power supplies that you've got. 3D printing is a long process. I don't see how temporarily exceeding the rated power dissipation can help. The following is for the steady state.

12x 1Ω resistors, each rated for 10W. The max steady state overall power you can dissipate is 120W. Each resistor dissipates $P = I^2R$. The max rated power is reached when

$I=\sqrt{\dfrac {P_{rated}} R}=3.16A \approx 3A$

Pick a series/parallel combination of resistors such that

$\dfrac {V_{supply}} {R_{series}} \approx 3A$ (just under 3A)

This can be achieved by using the 12V power supply and resistors in 3x parallel strings each with 4x resistors in series. Each string will pull 3A. Together, all 3x strings will pull 9A. The 12V power supply in the O.P. is rated for 18A. It should be able to supply the current.

## Construction tips

• Use thermal grease
• Thermally insulate the bottom of the plate, if possible.
• Temporarily exceeding the rating would be just to allow the temperature of the plate to reach its target area, between 65-110C depending on the plastic used. After that, the uC monitors the temperature and uses bang-bang control to control the heating. I'll update the OP. – Bryan Boettcher Oct 26 '12 at 14:51