Switching polarity

Discussion in 'Lights - Store Bought and Home Made' started by Calloween, Sep 25, 2013.

  1. Calloween

    Calloween New Elf

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    Hello all,

    I bought some 8c hannel ssr's off ebay and didn't realize they were possitve switched. My controller switches negative. How could I swap the polarity? Would a ULN2803 work?
     
  2. David_AVD

    David_AVD Good news, everyone!

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    A link to the device in question would be needed I'd say.
     
  3. damo1271

    damo1271 Full Time Elf Generous Elf

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    I assume you mean by switching positive that the SSR turns on when it receives a high signal. I assume your controller sends a logic low signal.
    If so a simple transistor switch will work. Vin is the link from your controller and VO is the link to the SSR. A PNP version would work if I happen to have the relative 'polarities' the wrong way! The transistor wouldn't be critical and any gen purpose NPN (PNP) would work.
    The ULN2803 IC should also work. Depending on the switched voltages a 2804 could be better. A ULN circuit would be more simple than the transistor. :)

    [​IMG]
    http://www.electrosome.com/transistor-as-a-switch/
     
  4. OP
    OP
    Calloween

    Calloween New Elf

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  5. damo1271

    damo1271 Full Time Elf Generous Elf

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    Looking at the SSR, I take it you want to drive AC elements at 120-240V? If it is standard LED elements @low DC voltages you probably wont need the SSR.

    A typical NPN transistor such as a BC337 should work. They are very cheap (~10cents) and should switch more than enough current for the job and handle C-E voltages up to 40V. I would look to use collector and base resistor values of between 1K and 10K. If you're using 12v power supplies then 10K will give 1.2mA current. That should turn the SSR on.
     
  6. David_AVD

    David_AVD Good news, everyone!

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    You'll need a PNP transistor (not NPN) to convert each channel. Something small like a BC557 should be fine.

    Transistor base goes to DC controller output via a 10K resistor.

    Transistor emitter goes to V+ of SSR board.

    Transistor collector goes to SSR channel input.

    You may also need another 10K resistor from each transistor base to the SSR V+ to keep it off when the DC controller's output is supposed to be off.

    Don't forget to connect the 0V line of the DC controller to the 0V line of the SSR.
     
  7. damo1271

    damo1271 Full Time Elf Generous Elf

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    Thanks David. Are you sure I have it backwards though? I thought the controller sends a logic low level signal for on and the SSR needs a high for on. Or have i got this wrong.
     
  8. David_AVD

    David_AVD Good news, everyone!

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    The DC controller does indeed pull it's output low when on. This is why you need a PNP transistor.

    In the idle (off) state, the controller output is floating. The 10K resistor from the PNP base to emitter (+5V) keeps the base also at +5V and the transistor stays off (collector effectively floating). This means that there will be no voltage on the SSR input, so the SSR will be off.

    When the DC controller channel turns on (pulls its output to 0V), the base of the PNP transistor will be pulled (via the other 10K resistor) 0.7V lower than its (+5V) emitter and turn on. The collector of the PNP transistor (going to the SSR input) will now be near +5V and the SSR channel will turn on.
     
  9. damo1271

    damo1271 Full Time Elf Generous Elf

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    Of course! I re-read it - you're using an an open collector. That will be way more efficient than my method, which will waste power while the controller is off.
     
  10. David_AVD

    David_AVD Good news, everyone!

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    You *could * do it with an NPN transistor but the SSR would be on when the DC controller channel is off and visa versa.
     

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