Level shifter /opamp voltage booster?

Is there a noise reason you can't do PWM? On your top design, I wouldn't worry about the resistor, how about the transistor? It could be dissipating MORE than the heater under some conditions.

The bottom circuit might be pretty unstable, it will be running the first transistor in a region where tiny voltage changes at the base will cause large swings at the collector. The circuit can put 50 V on the gate of the FET. Some power FETs do not behave well in the linear region.

There's got to be a better way to do this.

Jon

Use less than 1ohm? You can always shift the input with 2 Rs, or D & R.

NT

Den mandag den 14. november 2016 kl. 22.26.41 UTC+1 skrev George Herold:

is 1W really an issue when you can dump 12W in the transistor?

anyway, the first one is constant current, the second basically a voltage regulator, if constant voltage will work I think you can just flip it around like a negative regulator sinking current

i.e. similar to the first drawing but regulating the voltage across the transistor instead of the current

-Lasse

I sure hope a professional electronics designer isn't walking around in his sox.

(One of my guys walks around in sox, and they don't even match. No shirt, no shoes, no salary!)

Is that lower fet a p-channel? If so, 50 volts is a lot of gate drive. Loop stability will be interesting, too.

If it's n-channel, it doesn't have enough gate drive.

Is the fet going to dissipate 12 watts?

You could use a 0.1 ohm resistor.

We had a guy who would wear Birkenstocks regardless of the time of year. He took public transit and seemed to get sick a lot in the dead of winter. He was (and is) a legend though so nobody was going to say anything.

--sp

Knock the current sense resistor down to 1/10 ohm, then it'll only dissipate 1/10 watt at 1 amp, and still generate 100mV for your op-amp. You'll need to make sure that your requirements are still met by your op- amp, and maybe get a nicer one.

And read what Jon Elson said about more heat in the transistor than the heater -- he's got a point.

This is why PWM-ing heaters is a good idea.

You can level translate for high-side switching easily enough (and put sense resistor at the ground connection) like this

I've added an idle drive R3 resistor, and was thinking of linear operation, actually. That's kinder to the power supplies than an abrupt 1A draw would be, and heaters don't usually want HF transients anyhow. If the heater operates on 15W at idle, choose R3 to deliver 12 W, and the transistor dissipates circa 0W at full ON, peaks at circa 6W.

R3 will get warm, too, of course.

Or use the transistor as the heater. ...Jim Thompson

If things aren't too hot, yes indeedy. If you're going up to red heat then a suitable transistor may be difficult to source.

My floor is much too dirty to walk in sox, solder, spills, pieces/ parts of things. You could step on a dip loaded with solder.

I'm sorry for the second circuit, I wanted some nice way to run my opamp high side... or something. I had this box between my opamp and N-channel Fet. (we should agree on some "right way" to put arrows on fets.)... I filled it in with my first guess... perhaps N channel is not so good here?

No.. but I did want to round off the corners, (of a PWM) and have a nice gentle turn on and off.

Yeah only 100 mV of control voltage, so 0.5 ohms then... OK I'll stick with the first circuit then, (It's an old friend.)

George H.

Yeah OK... I was just thinking about high side... I mean there is this big fat resistor sitting there. Isn't there's an easy way to use it as the current sense element?

(I'm going to do the first circuit.)

George H.

I'll have to check that out tomorrow... at home.. no BW. George H.

Yeah that was my first suggestion, It's why I'm just going to do the first circuit.

George H.

If you want to use the heating resistor as the sense element then use identical voltage dividers on it and your HV supply, and look at the difference between those two voltages.

Or, if you're going to use the transistor as part of the heater, use the source resistor as part of the heater, too, and nothing is lost.

Just use the dumbest fet switch with a biggish series gate resistor! Let the Miller capacitance round things off.

I don't know the temperature of your heater, but, if you use a

...Jim Thompson

Ahh voltage dividers.. let me see if I can draw that.

Thanks George H.

I'm fairly sure one could get a transistor red hot. HFE might fall though :)

NT

HFE has a positive tempco.