High voltage opto-isolator driver

Discussion in 'General Electronics Chat' started by Crispin, Aug 4, 2011.

  1. Crispin

    Thread Starter Member

    Jul 4, 2011
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    2
    Hi folks,

    How can I drive an opto-isolator when my source is 350VDC (max).
    I thought of a simple voltage divider but that would not drive the LED.
    A simple series resistor would end up dissipating about 3.5W (source is a PV array) as well as unnecessary heat.

    Is there a clever way something like this can be done? Different isolator?

    I’ve not got any isolator in mind but was looking at 4N25. With an If of 10ma, V of 1.25, I am still looking at 3.5W I need to throw away.
    It’s worth noting that I would like it to be linear from about 100V to max of 300 (350 is absolute and other things will start breaking) and I would prefer a passive (or a very low power) option.

    I was wondering if there is a way I could turn on a transistor (using a voltage divider) _just_ enough to allow x amount of voltage to build up on a cap. The transistor would be controlled through some form of feedback. This however is beyond my knowledge limits :(

    Another idea just popped into my head:
    How about if I drove the LED with a battery, biasing it with the result from a voltage divider? The divider would not have to do all the hard work but would it be enough to alter the LED? Don’t think so… :(



    thanks
    CP
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    What exactly do you need to do (circuit operation)?
     
  3. Crispin

    Thread Starter Member

    Jul 4, 2011
    88
    2
    I need to measure the voltage of the panels (is there an easier way?)
    When I hooked up the divider it worked as expected. Before hooking it to the netduino, I checked for stray voltage between it and the battery and got around 165V and decreased slowly (like a cap charging?)
    Seeing as there is nothing common between the PV Gnd and the battery GND, I thought it a bad idea to make them common and opted for an isolator.

    See attached sketch for a better explination :)
     
  4. mik3

    Senior Member

    Feb 4, 2008
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  5. Crispin

    Thread Starter Member

    Jul 4, 2011
    88
    2
    ohhhh, that's what I need.

    So that with the normal shunt, a isolated DC-DC convertor to power it and I am done.(?)

    Thanks!
     
  6. mik3

    Senior Member

    Feb 4, 2008
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    Probably! but it can be quite expensive.
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    1,605
    As long as you are going for an isolated supply you could just as well use a simple cheap comparator or such to detect the high voltage thru a high impedance divider, one that could draw. Just by dropping the divider current to 1mA you only burn off .35W, and 1mA is a large current.

    Then the cheap comparator can drive the opto isolator.

    Most any wall wart can provide the power, doesn't necessarily have to come from the netduino.
     
  8. Crispin

    Thread Starter Member

    Jul 4, 2011
    88
    2
    Farnell have them, not that bad if you only after 2 (one to burn out, second one the real one)

    The simple point though of using the DC-DC isolator and then a normal opamp > Isolator will be tried today as I have all the bits here.


    Thanks Guys (and Gals?)
     
  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Hey, who you callin a gal! :mad:
     
  10. Crispin

    Thread Starter Member

    Jul 4, 2011
    88
    2
    no one... :whistle:
     
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