AC source with a FET

[tensi]

Hello, I want to use a FET as current source by using its symetrical characteristics; but the problem is that the voltage can variate from 24Vac to 253Vac. I need about 1mA.
All any other suggestions are welcome of course, but nothing with a serial capacitor or resistor divider. Thanks a lot already for the feeback.

 

[Jimttx]

You might be able to use a Supertex LR645 high voltage linear requlator.

See the following web site:

http://www.supertex.com/products/selector_guides/107/100

 

[47gandalf]

Your request is a little confusing. Can I assume since you say you want to use a FET that you mean that you want a DC current source? And will you want to control the current source, or adjust/calibrate it, or just have a fixed-current source with the value determined by component tolerances? And how *about* is your need for 1 mA? I mean, what tolerance is acceptable?

Also, what "symmetrical characteristics" of a FET are you referring to? The basic way to use a FET as a current source is to control Vgs and let that determine the resulting Ids, which would be your current source output. This is not an AC process, the FET will only have correct bias with one polarity of applied voltage from drain to source, and from gate to source.

It sounds like you mean, "I have an AC voltage source that varies between 24 and 253 VacRMS, and I want 1 mA DC current source (fixed) [tolerance not given]". Is that right?

If so, you can get a FET that will withstand voltages in excess of the 357.8 Vpeak of your 253 VacRMS input (plus safety factor), but you will need to rectify the input voltage first so the FET biasing will have constant polarity.

As suggested by Jimttx, Supertex makes high voltage devices like this. As do IR, Vishay-Siliconix and others. A linear regulator might allow you to control the Vgs of your FET and therefore the output current.

Does this current source need to give 1 mA at the voltage of the input AC? So it would vary between 34 V (peak for 24 VacRMS), 1 mA and 358 V (peak for 253 VacRMS), 1 mA? Or do you want a 1 mA DC source at some range of DC voltages that fit with what you are driving? If so, what is that voltage range?

It might help to know what you are driving (or using the 1 mA for).

If you want an AC current source, you'd need at least two FETs, one for each polarity of the input voltage, and a means of protecting each when the input voltage is of the wrong polarity. Like two independent DC sources wired in anti-parallel with each handling one half of the AC wave.

These approaches would have no series capacitor, or resistive voltage divider, as you specified.