drive input pin low from small voltage?

Discussion in 'The Projects Forum' started by ruprecht, Jul 10, 2006.

  1. ruprecht

    Thread Starter Well-Known Member

    Mar 25, 2004
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    Hi, I'm working on a small project with a PIC and am stuck on a problem. I posted it on the Microchip forum - to save copying here, could anyone have a look and let me know if they can help?
    http://forum.microchip.com/tm.aspx?m=174006
    Thanks!
     
  2. Gadget

    Distinguished Member

    Jan 10, 2006
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    In the circuit you posted as a 2nd post in that forum, reverse the collector and emitter.....
     
  3. ruprecht

    Thread Starter Well-Known Member

    Mar 25, 2004
    73
    0
    Cheers, that helped!
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    You might have better luck if you replace the fullwave bridge with a discrete version that uses four schottky diodes. The lower forward voltage drop associated with the schottky diodes would yield more voltage for use in switching your transistor.

    hgmjr
     
  5. ruprecht

    Thread Starter Well-Known Member

    Mar 25, 2004
    73
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    So just use 4x 1N4004? Thanks I'll try it out
     
  6. hgmjr

    Moderator

    Jan 28, 2005
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    Actually, the 1N4004 diode is a standard silicon diode. What I had in mind was something like the MBR340 available from Digi-key.

    hgmjr
     
  7. ruprecht

    Thread Starter Well-Known Member

    Mar 25, 2004
    73
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    OK thanks, trying to find a local equivalent (Australia)
    Cheers
    Would something like a 1N5822 do the trick?
     
  8. Gadget

    Distinguished Member

    Jan 10, 2006
    613
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    Well, they are schottkeys.... another option would be to use germanium diodes (if you can find them) as they have a voltage drop of 0.2 - 0.3 volts (double that if you go thru a full wave bridge) like the old 1N34, or the OA90 etc etc.
    Another option might be to use half wave rectification.
    When you use a full wave silicon diode rectifier with 1N4001-7 or 1N918/4148 etc, you will need at least 1.8 volts of signal to turn on a silicon transistor (0.6 + 0.6 + 0.6) because the signal will pass thru 2 diodes + the B-E junction of the transistor.
    If it were me, I'd do it like this.... Half wave germanium rectifier, a little smoothing then switching a silicon transistor
    [​IMG]
     
  9. ruprecht

    Thread Starter Well-Known Member

    Mar 25, 2004
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    0
    Cheers Gadget, I'm working on it
     
  10. hgmjr

    Moderator

    Jan 28, 2005
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    214
    I like gadget's half-wave rectification approach. It appears to represent a good compromise for your application.

    I considered suggesting germanium diodes but I decided that they might be tough to find so I went with schottky diodes. A drawback to germanium diodes is the tendency of their forward voltage drop to vary with temperature. This is only a problem if the circuit is going to be subjected to wide temperature variations.

    I think the use of a half-wave rectifier composed of schottky diodes and some good filter caps should provide you with a solid solution.

    hgmjr
     
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