Opto-isolator to energize AC relay

Discussion in 'General Electronics Chat' started by mikeysela, Jul 29, 2010.

  1. mikeysela

    Thread Starter Member

    Jul 24, 2010
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    Is it possible to use an Opto isolater to isolate from a DC line and an AC line. In other words, I would like it, that when DC current is passed via the diode, it will shine a light on a transistor connected on a 6VAC line, that will short and cause a relay of a coil to energize with that same 6VAC. Hence, i only want to relay to energize when current passes through the diode of an optoisolator. And this is why my transistor output is located on ac line of relay. What must i consider? is this doable? thanks fellas.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Transistors are DC devices, so you can't use one for controlling the AC voltage to the relay.

    How much DC voltage have you got to work with? If it is close to that 6 volt figure, you can use it to pull the relay - they are really DC devices, as the coil has a diode bridge feeding it. You can even use that transistor to control it.
     
  3. mikeysela

    Thread Starter Member

    Jul 24, 2010
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    I am currently working with 9VDC out of a supply i have plugged into the wall, but the actual circuit should only have 6VDC feeding the Optoisolator. SO i am using 6VDC for the diode of an optoisolator and 6VAC for the relay coil. So are you saying i can energize a 6VAC relay with 6VDC?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    You can do a quick experiment with the 9 volt supply to energize the relay as a proof-of-principle. Just don't keep 9 volts on the coil for more than a few seconds. If it clicks, or your meter shows continuity between the common and NO contact, it works on DC.
     
  5. mikeysela

    Thread Starter Member

    Jul 24, 2010
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    should i maybe do a small circuit and do a test and add some resistors to reduce the supply from 9 to about 6 for the relay?
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Do the test, by all means. One way to reduce the voltage that is a bit better than resistors is to use diodes. Grab 4 1N4001's and run them in series. They will drop around 2.8 volts, reducing the 9 volt source to so close to 6 as to not matter. If the relay coil uses less than 25 ma, smaller diodes like 1N914's will work. This counts as your first kludge.
     
  7. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    What are you trying to do with the relay? (ie. what are you switching on the contact side)
    Any reason you can't simply use a DC coil relay?
     
  8. mikeysela

    Thread Starter Member

    Jul 24, 2010
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    The contact side is complicated and controls many things, but for now lets just say i must stick to the 6AC relay...is it not a possibility to energize the AC relays?
     
  9. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Just use your 6VDC to turn on the opto and your 9VDC on the transistor side to energize a suitable 9V relay. Transistors are are not for AC. Don't forget the diode across the relay coil and to ensure your opto is suitable for the current requirements of the relay..
     
  10. mikeysela

    Thread Starter Member

    Jul 24, 2010
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    Yea but again, I will need to stick to AC relays so that does not solve my problem.
     
  11. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Why? Almost all relays have an AC (and DC) rating for the contacts and can be either DC or AC rated coils. Simply switching to a relay with a DC rated coil will solve your problem (and save money in the process). Not sure how much your AC relay costs but for $1 or less you can have a suitable DC rated relay.
    I frequently use a relay that I energize the coil with DC and switch an AC load on the contact side.
     
  12. mikeysela

    Thread Starter Member

    Jul 24, 2010
    87
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    thanks for the input mcgybr but i think i have a possible alternative: Phototriacs. Input = dc, AC output using a Triac.
     
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