# Switching AC mains(2kw) with MOSFET

Discussion in 'General Electronics Chat' started by arthur92710, Jan 23, 2011.

1. ### arthur92710 Thread Starter Active Member

Jun 25, 2007
307
1
Hi, guys im building a digital thermostat to replace a mechanical thermostat in a portable heater. I have the digital part built, it output a +5v to turn on the heat and 0v to turn it off.

The heater is 1500w, lets say 2kw.
I thought I could use a IRF640N (N - CHANNEL 200v 18A .15RDS)
But If my calculation is correct the MOSFET will need to dissipate 41w.
$I^{2}$ * $R_{ds}$
$16.7^{2}$ * 15 ≈ 42w

Is there a way I could use this MOSFET or do I need something bigger?

*EDIT*
I do have a E45NK80ZD N-channel 800V - 0.11Ω - 45A but that still calculates to 30w

Last edited: Jan 23, 2011
2. ### eblc1388 Senior Member

Nov 28, 2008
1,542
102
I don't know why you have the idea of using MOSFET to control AC power, instead of using the more conventional method of TRIAC or Solid State Relay.

Bad news is, most MOSFET has an internal intrinsic body diode and can only block current in one direction under zero gate drive. Therefore, two MOSFETs connected back-to-back in series are used for AC current control.

This means doubling of the power dissipation in your calculation.

Or alternatively you can place a single MOSFET after a diode bridge and let the bridge does the current commutation.

3. ### arthur92710 Thread Starter Active Member

Jun 25, 2007
307
1
Mainly the 200v 18A rating, and it is the only component that I have that can handle so much power.

I will look into using a TRIAC, but a relay would need more the 5v. I have a limited amount of space, so the extra circuitry to use a relay might not fit.

4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Too bad you limited the supply to 5 volts. With 12 volts and a P & B T92 relay, you would have contacts good for 40 amps quite inexpensively.

5. ### arthur92710 Thread Starter Active Member

Jun 25, 2007
307
1
The PS is not finalized. I found a small 12v transformer that I can use. I will probably use a relay.