Power supply and heating pad

Help & Support
1

Hi!

I require your help regarding a problem I’m encountering. I think this is basic electronics 101… But I must’ve slept through that class.

Just for background… I have been struggling with a 3d printer for ages. Finally making some progress. Found that prints stick much better if I pre-warm the build plate. Works pretty well with a hair dryer. I thought I’d get clever and so I found an old PC power supply, and bought a 12v, 120w heating pad. But it gets too hot, and I’m afraid it will melt parts of the printer.

So I tought I would use the 5v line of the power supply. But (and here’s where my limited knowledge comes in), if the wattage is fixed and I drop the voltage, doesn’t that increase the amperage? Then it would draw something like 20amps… and the power supply 5v line is only rated for 15A. Using a multimeter and the 12V supply, it drew 8.6 amps, so maybe it’s not really 120W, but 90-something… But that’ll still be more than the 15A the 5V line is rated for…

Will this:

  1. work fine, lowering the temperature as I hoped?
  2. destroy the power supply?
  3. catch something on fire?
  4. blow the circuit breaker in the basement?
  5. something else?

Thank you so much in advance for your help!

2

Ohm’s Law
E = I * R --> Voltage (E) = Current (I) * Resistance ®
Rearrange it:
R = E/I
The heating pad is a resistor. You can find the resistance two ways: measure it directly or calculate it from voltage and current. YOu stated it drew 8.6 Amps at 12 V (That’s 103 Watts) So the resistance is R = E/I = 12/8.6 = 1.4 Ohms. Now you can find the current if 5V is applied:
I = E/R = 5/1.4 = 3.6 Amps
YOu can find the power by either multiplying P = E * I = 5 * 3.6 = 18 Watts
or directly by P = E^2/R = (5 * 5) / 1.4 = 25/1.4 = 18 Watts
So assuming the numbers you gave are correct it would draw 3.6 Amps from the 5V supply and provide 18 Watts of heat.