# Silicone heater double check my maths please

Posted by deckingman
 Silicone heater double check my maths please July 02, 2016 06:52AM Registered: 6 years ago Posts: 617
My new silicone heat pad has just arrived from China. I asked for 240V 800W. I think I have got what I asked for but just wanted to check. I measured the resistance at 72.5Ohms. From what I remember of Ohm's law V=IR therefore I=V/R. So 240v / 72.5Ohms gives me 3.31 Amps. Wattage = voltage x current = 240 x 3.31 = 794 Watts which is close enough to 800. Does that all seem reasonable?
 Re: Silicone heater double check my maths please July 02, 2016 12:38PM Registered: 9 years ago Posts: 293
Yes, you did the math right.
 Re: Silicone heater double check my maths please July 04, 2016 02:56PM Registered: 6 years ago Posts: 44
V^2 / R can be faster next time, but you've got it right!

800W... either it must be pretty big, or you want it to heat super quickly!

DYZE DESIGN
Hotends, Extruders, Liquid Cooling and Accessories.
 Re: Silicone heater double check my maths please July 04, 2016 03:43PM Registered: 6 years ago Posts: 617
Quote
Dyze_Design
V^2 / R can be faster next time, but you've got it right!

800W... either it must be pretty big, or you want it to heat super quickly!

Thanks for the tip. Bed is 400mm x 400mm and I'm assured by others far more knowledgeable than I, that 800W is about right for that size bed and a reasonably short warm up time.
 Re: Silicone heater double check my maths please July 04, 2016 04:07PM Registered: 6 years ago Posts: 68
When you measure the resistance when the heater is cold, the resistance is usually different. I have 300W silicone heater and it calculates to 310W. However energy meter before my printer and I get 320W in the very beginning of heating and it drops down to 260W when the bed reaches 70C. And this is 100% on situation. I drove my SSR with DC from lab power to determine how hot and how much power the bed heater would take. It reaches equilibrium in 130C and draws 230W. Another theory is that SSR heats up and losses increase, but it has good active cooling and still the energy meter measures those losses too. I derive the power from 1Wh pulses by measuring the time between them.

Anyway 0.8kW sounds good.
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