Fet as heater (one mo' time)

George Herold · 2017-04-05T19:19:05+00:00

So a fet heater circuit. Vin = 63 V (dang moving specs, it started at 50V) Then a series of three ~20W resistors. TO-220 (At the moment these are 15 ohms, 45 total though writing I think my solution is raise this to 22 ohms ea.) Then a FET and feedback/ control loop. The max. current at 1A. This worked OK on the lab bench, but my boss blew up the fet running it in vacuum. The fet was running close to the edge on the bench. (obviously) Anyway I started looking at TO-247 fets on DK. This got me again thinking about fets as heaters. And it seems to me I want to pick a fet with an on resistance that is relatively large 0.1 to 1 ohm. That way I need a larger change in G-S voltage to change the current. On the other hand I built this with a low Ron fet and it works just fine. What's wrong with my thinking? George H.

Apr 05, 2017 42 Replies

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George Herold 9 years ago

Right RL, I thought about that back at the beginning. There are two issues with a remote pass element. One, as you say I've got to get rid of the heat. ~15W or so. Two, The gain of the loop is no longer linear in power.

George H.

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legg 9 years ago

Is power is what you want to control, or is it temperature?

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George Herold 9 years ago

Is that a trick question? :^) Of course I want to control the temperature. (I made this 'silly' sqrt(V) circuit, but now don't need to use it.)

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