FET as linear mode heater

George Herold · 2015-05-01T20:05:42+00:00

So I'd like to make a linear heater. Power 50W, supply 50V, 1 amp max current. Temperature from 77 to 373 K (100 C). I was thinking of a FET and 25 ohm power resistor in series, with feedback from the resistor to control the current. (Yeah an extra wire.. I could also have a smaller monitor resistor in the electronics box.) The whole thing will be inside a control loop. My question is should I choose a FET with small on resistance or something closer to 1 ohm. (Looking at the nice table in AoE3 pgs 189-191) Say either an IRF830, IRF840. or the lower Ron IRF530, IRF540. My (perhaps muddled) thinking is that the larger Ron devices will be easier to control. Or is this just silly... once it's in a control loop it won't really matter. Thanks, George H.

May 01, 2015 65 Replies

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George Herold 11 years ago

On Monday, May 4, 2015 at 4:09:46 PM UTC-4, Lasse Langwadt Christensen wrot e:

ax current.

5 ohm power resistor in series, with feedback from the resistor to control the current.

n the

ething closer

easier to control.

matter.

er,

Yeah sure, (I think the short answer is because I've mostly only read about SMPS.) So to get power control I would change the duty cycle of the buck? What sort of dynamic range can you get?

I was thinking I should make a PWM heater too, with just resistors.

Oh that brings to mind a question. (perhaps simple?) What's the easiest voltage to duty cycle converter. (OK, uC's or some other controller... but after that.)

George H.

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George Herold 11 years ago

Voltage to duty cycle converter.

George H.

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Phil Hobbs 11 years ago

Sure thing. If the pickup doesn't destroy your measurement, PWM is the way to go.

Cheers

Phil Hobbs

Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net

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Bill Sloman 11 years ago

ore

ive

ime

Jim's solution is easy - he understands it the way he wants to understand i t.

He objected to my "emitter-pushing" comment for quite the wrong reason. I'd directed Sylvia Else's attention to the right part of the LM311 integrated circuit, and made a standard get-it-180-degrees-wrong "error-of-action" mi stake in my discussion of what was going to go wrong.

Jim ought to have much clearer insight into what happens inside an LM311 wh en the inputs go outside the design range, but he didn't bother to spell ou t what was actually going on until I'd made a bad attempt at it and he was prompted to correct me.

One of the virtues of sci.electronics.design is that people can post stuff that's not entirely right - Linus Pauling style - and get corrections that that take the discussion to the right conclusion.

Bill Sloman, Sydney

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Bill Sloman 11 years ago

Worrying about the image you present isn't a useful form of intellectual ac tivity. Phil Hobbs does rather more of it than he should.

Winfield Hill long ago perfected the "I'm academically well-informed and th is is how it looks to me" style, but writing and correcting three successiv e editions of his (with Paul Horowitz) text-book has given him a lot of pra ctice on abstracting only the technical points and laying them out clearly.

Bill Sloman, Sydney

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krw 11 years ago

If you allow pulse skipping and aren't constrained by frequency, quite a lot.

Resistor, capacitor, and schmidt trigger? Single shot?

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