help with mosfet b+ reducer

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by drycounty71, Jan 23, 2012.

  1. drycounty71

    drycounty71 Member

    Messages:
    21
    Joined:
    Dec 20, 2011
    Location:
    Dallas, TX
    I am trying to test this on breadboard and using a IRF830 power mosfet with 2k on the source, 220 on the gate and a 43v zener on the drain with all wired like the below diagram. I achieve the voltage drop from ~480 to 440 but shortly after taking first readings, and nothing has changed, the reading is back at ~480. Can anyone tell me what I am doing wrong? Is it the wrong mosfet? Credit for diagram to R.G. Keen.

    BTW the source is going to + on filter capacitor after rectifier bridge and the drain is going to B+ (the diodes on the RB)

    [​IMG]
     
  2. GaryE

    GaryE Member

    Messages:
    528
    Joined:
    Feb 2, 2009
    Location:
    Van. BC
    Sounds to me like you may have blown the Mosfet.Did you heat sink it enough?
     
  3. ked

    ked Supporting Member

    Messages:
    896
    Joined:
    May 18, 2006
    From what I can tell your doing it right. Check the zener and make sure it's still good. Other than that I'd guess the MOSFET is bad.

    Ken
     
  4. donnyjaguar

    donnyjaguar Member

    Messages:
    4,243
    Joined:
    Nov 4, 2005
    Location:
    Canada
    Given the G-S/D maximum voltage is 30Vdc, a 45V zener would appear to be a bad choice. I think you have it in the wrong spot too. I'm not in front of my notes but I think you should have it with the cathode to Gate and anode to Drain, and as noted above appreciable less than 30V. A 45V would make more sense strapping S-D too, but many MOSFETs have this built into them.
     
  5. drycounty71

    drycounty71 Member

    Messages:
    21
    Joined:
    Dec 20, 2011
    Location:
    Dallas, TX
    Thanks guys for your replies, Ked I believe your right-about the mosfet. I had of course bought more than one and the second withstood repeated power on/off cycles. All other factors being unchanged, I believe is was a bad or out-of-spec fet. Both are the same part number, guess it was just luck of the pick.
     
  6. drycounty71

    drycounty71 Member

    Messages:
    21
    Joined:
    Dec 20, 2011
    Location:
    Dallas, TX
    Donny,

    that isn't my diagram but it is the one I am basing my b+ reducer on. I actually have a 43v zener in there now and I will be purchasing a 30v to dial it in exactly were I need it. But I did review the spec sheet closer and I think your right the D-S/v is 500 but the G-S/v is +/-20v ....Doh!
     
  7. reaiken

    reaiken Member

    Messages:
    1,798
    Joined:
    Jan 4, 2002
    Location:
    Greenwood, SC

    That's not the problem, you can put any value of zener in there as long as the mosfet in the safe operating area. It is what controls how much voltage is dropped.

    What Donny is alluding to is that there should be a second zener from gate to source (with the cathode to the gate) to limit the max Vgs to no more than 20V (not 30V) for the IRF830. A 12V zener would be a good choice, as the voltage drop in normal operation will be around 4V or so. You should also increase the 220 ohm resistor to something in the range of 2-10K to keep turn-on surge currents from killing your grid resistor and 12V zener. Also, make sure you resistors that can handle a 425V surge voltage (most 1/4W are only rated for 250V). The turn on surge voltage across the grid resistor will be 425V, dropping to zero as the output capacitor charges. The 2K resistor will also see a surge to 425V on startup.

    What is happening is startup transients are killing your mosfet.

    Rule number 1: It's never a "bad part".
    Rule number 2: Except in those rare occasions where it is a bad part.

    Randall Aiken
     
    Last edited: Jan 23, 2012
    TweeDLX likes this.
  8. WesKuhnley

    WesKuhnley Member

    Messages:
    2,241
    Joined:
    Mar 9, 2007
    Location:
    Minneapolis/St Paul
    Classic.
     
  9. drycounty71

    drycounty71 Member

    Messages:
    21
    Joined:
    Dec 20, 2011
    Location:
    Dallas, TX
    Thanks Reaiken for the clarification and helpful suggestions. Your response is appreciated!

    DC
     

Share This Page