MOSFET question

Discussion in 'The Projects Forum' started by Jassper, Feb 16, 2009.

  1. Jassper

    Thread Starter Active Member

    Sep 24, 2008
    84
    1
    I have this simple voltage divider cicuit,
    [​IMG]

    It works good, and if the voltage goes over 30 volts, LED1 begins to glow indicating over voltage(corrected LED in drawing). However, what I would really like it to do is to shut down, or significantly reduce the voltage threw the voltage divider when the input is over 30 volts and the zener begins to conduct.

    I am thinking a MOSFET could be used to shut off or reduce the voltage going into the resistors R1, R2, & R3, but not sure how to insert it into the circuit.

    Anyhelp?
     
    Last edited: Feb 16, 2009
  2. Jassper

    Thread Starter Active Member

    Sep 24, 2008
    84
    1
    I guess never mind, I made this simple modification to my circuit,
    [​IMG]
    And it keeps the Zener at about 30volts so even with an applied input voltage of 48 volts, the Vdrop across each divider resistor is approx 10.9v which is acceptable.

    So I guess I had the R4 in the wrong place originally, I didn't think the Zener was acting right before.

    thanks anyways.
     
  3. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    You can a transistor controlled by an op amp with negative feedback as to produce an error voltage depending on the amount of Vcc and drive the transistor according to this error voltage. Like the circuit of a simple voltage regulator.

    Here is an even simpler circuit which can provide a moderate accuracy:

    http://www.satcure-focus.com/tutor/page5.htm
     
  4. Jassper

    Thread Starter Active Member

    Sep 24, 2008
    84
    1
    Thanks for the reply, but I am not sure I follow. I'm not wanting a constant voltage, just want to prevent the voltage across R1,R2,and R3 from going much over 30 volts if the input rises above 30 volts.
     
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    You can use a comparator to sense when the voltage rises above 30V which will enable a voltage regulator to keep it at 30V.
     
  6. Jassper

    Thread Starter Active Member

    Sep 24, 2008
    84
    1
    I see, I'll look into that. I'm hopeing to keep this as cheep and low component as possible however.

    Thanks
     
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