Hello,
happy new year everyone
In my circuit attached, the resistor R2 (20 ohm and rated 20 watt). from calculation I get 7.2 watt. but this resistor is getting so hot I cant even touch it. Is it because of the switching ? the resistor it rated more then twice its power dissipation. It's out on the open with a small fan blowing on it.
The output is not connected, just to my scope to view the form wave.
thanks
Ken

 

A resistor dissipating 7.2W is going to get hot. If it is rated at 20W, it is probably fine.

 

Your simulation has a 1000V power supply and a 2000 ohms resistor.
The Simulation does not show the 60V zener diode in the IRFZ44 conducting then allows the drain to have a flyback voltage of 1000V.

In actual use with a 12V supply and a 20 ohms resistor, the resistor has 48V pulses in it in addition to the 12V pulses in it which will make it hot. Editted

 

The total power can be no more than

12^2 / 20

or 7.2W. The fact that some energy is stored in the inductor does not increase that.

 

What is the pulse frequency and duty cycle?

 

MY bad, the datasheet requires a heatsink above a certain power. and I am above that required spec. So i will adding a heatsink.
thanks for your quick reply

 

In my circuit attached, the resistor R2 (20 ohm and rated 20 watt). from calculation I get 7.2 watt. but this resistor is getting so hot I cant even touch it. Is it because of the switching ? the resistor it rated more then twice its power dissipation.

Your discomfort is meaningless to a resistor. A resistor will easily tolerate 70C, but you'd probably find that a bit warm.

 

Is it possible that this circuit is being used to produce short positive pulses, and that the transistor is normally conducting, and only momentarily switched off? That would result in a lot more heat.

 

Is it possible that this circuit is being used to produce short positive pulses, and that the transistor is normally conducting, and only momentarily switched off? That would result in a lot more heat.

But still limited to 7.2W.

 

dl324 summed up the situation succinctly in post #7. Can measure the resistor's temperature and compare it with manufacturer's rating for it?

If its maximum temperature is not exceeded then things should be ok. If you have the resistor mount on a circuitboard you might want to mount it above the board (using spacers would be ideal) to avoid damaging or discoloring the board with the heat.

 

7.2 watts is able to produce a lot of heat in a small volume.