24VDC Relay Driver switching 240VAC 10A Load

Discussion in 'Electronic Projects' started by Ben Halicki, Apr 20, 2017.

  1. Ben Halicki

    Ben Halicki

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    Hi Guys,

    I need someone to help me design a schematic for a basic relay driver (24VDC feed from PLC into optocoupler, which drives a relay). Pretty basic, but my main question is around the relay.

    The relay needs to switch 240VAC loads (up to 2400W / 10A). Is it possible / safe to have the relay mounted on PCB and be able to handle this sort of current, or would I be better off using an externally mounted relay?

    Thanks in advance,

    Ben.
     
    Ben Halicki, Apr 20, 2017
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  2. Ben Halicki

    davenn Moderator

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    Hi Ben

    there are proper SSR ( Solid State Relays) available for this sort of thing. I would strongly suggest you get one of those
    It will simplify your project and be a hell of a lot safer :)

    google 240V switching SSR's

    Dave
     
    davenn, Apr 21, 2017
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  3. Ben Halicki

    Minder

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    Another solution is to use a contactor, these are traditionally a large form of a relay so it would be mounted separate to any PCCT board.
    M.
     
    Minder, Apr 21, 2017
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  4. Ben Halicki

    AnalogKid

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    With proper attention to trace width and copper thickness (weight), a pc board design can handle your voltage and current easily. After all, they are common in many power supplies.

    Since a relay has true galvanic isolation, why do you have an optocoupler between it and the PLC? Very few optos can move the current needed by the coil of a relay with those contact ratings.

    ak
     
    AnalogKid, Apr 21, 2017
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    Minder and hevans1944 like this.
  5. Ben Halicki

    Tha fios agaibh

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    Why not just buy relays with 24vdc coil?

    Btw, If your plc output card has common terminals for internal contacts, you can feed it with other than 24vdc.
    Example; feed common terminal with 120v so it outputs 120v. Of course you need to observe its rating.
     
    Tha fios agaibh, Apr 21, 2017
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  6. Ben Halicki

    davenn Moderator

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    he's dealing with higher voltage and significant current

    SSR's are a very way to handle that
     
    davenn, Apr 21, 2017
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  7. Ben Halicki

    Tha fios agaibh

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    I agree, but SSR's can get expensive and they tend to fail more often than a mechanical relay.
    A relay or contactor (coil) that matches the PLC output voltage is usually the best plan.

    I wasn't suggesting attaching the load directly to the PLC output card. I was merely noting that a PLC can often be rewired to suit the output voltage desired for a relay that one might already have on hand.
     
    Tha fios agaibh, Apr 22, 2017
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  8. Ben Halicki

    Ben Halicki

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    Hi guys,

    Thanks very much for the suggestions. Sounds like using a SSR might be the way to go.

    Out of interest... the current measures around 9.6A. If I have a 10A relay, to me that is probably too close to the upper limit of the relay, but interested to hear your thoughts. Apologies in advance for digging up an old thread, I thought i'd posted this a while ago but apparently not :)
     
    Ben Halicki, Aug 20, 2017
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  9. Ben Halicki

    AnalogKid

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    The standard rule of thumb for electronic components (overrate everything by 100%) applies in part to an SSR. For a 10 A output current, use a 20 A part. You do not have to overrate the voltage spec, because an SSR with a max output rating of 240 Vac is designed to handle that continuously.

    ak
     
    AnalogKid, Aug 20, 2017
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  10. Ben Halicki

    Minder

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    Re-reading the OP, why exactly are you using a Opto coupler from the output of a PLC to drive a relay/contactor?
    Why not drive the SSR or relay direct from PLC output?
    I see it was suggested in #4
    M.
     
    Last edited: Aug 20, 2017
    Minder, Aug 20, 2017
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