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High voltage attenuation

I have a circuit that has around 500V on it. I want to reduce the voltage to some sub-circuits. Effectively there is a load that has around 500V and I wish to attenuate this with regulation. A simple emitter follower with degeneration should do the trick. The bjt needs to be on the high side because of the common ground between the different subcircuits.

The only problem is actually programming the base current. I want to avoid having high voltages on a pot that will be used to set the voltage attenuation for safety reasons(else I would just use the pot directly instead of the bjt).

I eventually want to program the base using a uC instead. The load current, which should be irrelevant to the problem, is around 50mA. I don't need full scale range but from about max Vcc(95%+) to around 1/4 Vcc. The main thing is proper attenuation and regulation(which can be done with caps afterwards). It does not have to work "fast" either.

Any ideas?

Reply to
Jon Slaughter
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How about using a P-channel FET for the main control: source to

+500V, drain to load; resistive divider from +500V to gate to collector of NPN with its emitter to ground. Drive NPN base as you see fit.
Reply to
Robert Baer

to

Well, a grounded-base NPN transistor, base =3D> +5V and emitter =3D>

resistor to a DAC, can translate a safely low DAC output to a higher voltage. You can add an op amp, change the NPN for NMOS, etc. if the emitter/source bias point is an issue. Be prepared for power-sequence issues, of course. The collector might want a fuse, the emitter a clamp.

To 'regulate', you need in addition some kind of high-voltage sensing, my favorite is a R//C sense resistor diode-clamped to GND, just sink current from the resistor until it matches ground. The capacitance of the diodes and error amplifier is swamped by the bypass capacitor on the high value R.

Reply to
whit3rd

But this changes ground. The bjt essentially becoming a low side resistor with the load being "lifted" off ground by the the bjt. This causes a problem with transfering signals between circuits since they will be shifted. If could "restore ground" somehow then I guess it would work.

What do you mean by "power-sequence issues"?

I'm not sure I follow. Simply adding capacitance across the bjt and emitter to ground is not any better(although maybe slower)?

Reply to
Jon Slaughter

It's just intended as a level-translator for whatever series pass element you have at the high voltage side; (as long as you don't want to step down the 500V to below 5V). The collector of the translator transistor can drive the gate of your mosfet...

Mainly, that the 500V might be unregulated in the first seconds of startup, until your DAC gets a programmed value. Miller effect could cause a startup heat pulse, too.

er

Sensing 500V with a (for instance) 1/10W resistor means you need that resistor to be over 2.5 Mohm; that can be irritating if there's any slewing of the source, partly because the attenuator loses gain, and partly because stray capacitances aren't negligible. Since your goal is regulation, the AC sensitivity from a parallel swamping capacitor isn't likely to be a problem.

Sense resistor to pseudo-ground of an inverting amplifier seems the obvious configuration, and is easy to diode-clamp to ground (needed, if the

500V is present when the low-voltage parts are powered down...).
Reply to
whit3rd

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