Mosfet as variable linear HV resistor?

Discussion in 'General Electronics Chat' started by coinmaster, Jun 26, 2017.

  1. coinmaster

    Thread Starter Member

    Dec 24, 2015
    I want to use a mosfet as a variable resistance load as a replacement for a mechanical potentiometer using real resistors because of the power dissipation I expect to have, it would just be impractical using a resistor pot.
    2017-06-26 (2).png 2017-06-26 (1).png
    So basically I want M3 to be in the ohmic region. Unfortunately I get all manner of weird results when I use any kind of RC filter or bootstrap capacitors pretty much anywhere in this circuit. So I forced it into compliance with that 1F cap to shut it up.
    After that it works fine except for the fact its missing half its voltage headroom.
    I tried bootstrapping M3 so it sees the same DC voltage across all 3 of its terminals by relaying the signal to the input of U1 and the gate of M3 but that just gives weird results. I tried many things but it just doesn't work.
    If I remove the U1 circuit and just attach the source of M3 to B+ it appears to work fine but something isn't right with that because that would put the IRF9640 at 100v drain-source voltage and there's no way it can be in the ohmic region with that much voltage across it.
    Any ideas?
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    Those opamps won't like 250V up 'em.
  3. coinmaster

    Thread Starter Member

    Dec 24, 2015
    Yeah I know, one problem at a time.