5-60V Opto pull low

Discussion in 'The Projects Forum' started by mcgyvr, May 4, 2012.

  1. mcgyvr

    Thread Starter AAC Fanatic!

    Oct 15, 2009
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    Does this look OK before I commit it to a PCB?
    Task is to pull a microprocessor pin low when the Vin voltage is between 5V and 60V.

    R4 is just to keep LTspice happy and will not be included.

    R1 will be 1/2W all others 1/8W

    Q1 is showing 562mW dissipation @ 60Vin (it will actually be a ON semi BCP53 series PNP 1.5A--12mW/degC)... Is that ok???

    U1 opto (LiteON corp LTV-816S) is getting approx. 9.3mA @5V and 13.6mA @60V.. Is that ok???
     
  2. #12

    Expert

    Nov 30, 2010
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    I believe R1 should go to ground, not the opto input. To avoid current through the opto during "off" time.

    I calculate 72 C on Q1. Looks acceptable but if you add more current after moving R1 to ground, you might get too hot on Q1.

    The opto has a 35 volt limit. You're going to have to use an external transistor to survive that voltage.
     
  3. mcgyvr

    Thread Starter AAC Fanatic!

    Oct 15, 2009
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    Circuit doesn't work with R1 to ground..
    and really SGTWookie did this (almost) for me for a very similar project.. He just had an LED instead of a true zener diode and it was pulling high versus pulling low before


    Opto--You pulled the LTV817 datasheet..I'm using LTV816 with an 80V limit. and actually the transistor side of the opto is 5V (Vcc not Vin) so 80V is plenty and the 817 would work just fine too..
     
    Last edited: May 4, 2012
  4. #12

    Expert

    Nov 30, 2010
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    That seems to clear up my doubts.
     
    mcgyvr likes this.
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