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Transistor switching on another voltage supply

Discussion in 'General Electronics Discussion' started by Juzzac, Jun 20, 2015.

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  1. Juzzac

    Juzzac

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    Mar 1, 2015
    Hi, Can someone please help point me in the right direction, I am wanting to switch on a separate 12v circuit with a transistor or similar, the circuit i wish to switch it on from is about 0.2 volts when off and about 1 Volt when on, I would use a solid state relay but can't find one that will switch on with just 1 volt. The circuits don't and can't share the same earth, what would be the best solution ?

    Thanks Juzza
     
  2. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    Add a transistor to drive an optocoupler and the other side of the optocoupler switches the 12v circuit.
     
    chopnhack and hevans1944 like this.
  3. cjdelphi

    cjdelphi

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    Oct 26, 2011
    Or better still, use an NPN power transistor voltage of 1v is too low for a mosfet , 12v + side to your device to power, eg 12v bulb and the other side to the collector and the emitter tied to ground

    that leaves say a 300ohm resistor from the 1v to base of the npn.
     
  4. cjdelphi

    cjdelphi

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    Oct 26, 2011
    Opto's are over kill for this imho....
     
  5. Martaine2005

    Martaine2005

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    May 12, 2015
    Very clever.
    Using the bulb as an opto isolator.
    And bloody pricey..
    Martin
     
  6. cjdelphi

    cjdelphi

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    Oct 26, 2011
    Alright fine 2am never read it properly regarding it can't share grounds...

    But why?
     
  7. Martaine2005

    Martaine2005

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    May 12, 2015
    I think you read me wrong..
    I meant VERY clever. They used to use light bulbs to isolate circuits.
    Martin
     
  8. cjdelphi

    cjdelphi

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    Oct 26, 2011
    How?

    Then it occurred to me, there's only 1v which is not enough for the optoisolator to light the LED, you'd need the transistor to share grounds with at least a 3vish source to fire an opto at which point why bother and tie grounds with the 12v source ...
     
  9. Martaine2005

    Martaine2005

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    May 12, 2015
    The whole point of isolating circuits is to completely isolate them.. No 'common' either.
    It would be an independant circuit controlled by the first low voltage circuit.
    For safety reasons no voltage/current can be fed back to the controlling circuit.
    Martin

    Ps, I will try and find something I read years ago about the light bulb isolator.
     
  10. Martaine2005

    Martaine2005

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    May 12, 2015
  11. Martaine2005

    Martaine2005

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    May 12, 2015
    Sorry can't find any of my old city and guilds books yet.
    But possibly davenn, Bobk or Colin will be able to shed more light on this..Oh and of course my favourite encyclopedia Howard.

    Martin
     
  12. Minder

    Minder

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    Apr 24, 2015
    Set a LM311 to switch over at 1v operate into a 4n25 (25¢), if you need to condition the output for a little more power a 2n7000 will do it up to 200ma.
    Total ~ $1.00
    Max.
     
    davenn likes this.
  13. BobK

    BobK

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    Jan 5, 2010
    Colin gave the answer in post #2. It is the same one I would have given.

    Bob
     
    Martaine2005 likes this.
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