Help sourcing a good Solid State relay

Discussion in 'The Projects Forum' started by mpooley, Dec 18, 2013.

  1. mpooley

    Thread Starter Active Member

    Mar 3, 2010
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    Hi all
    I have been controlling a 3kw immersion heater using a solid state relay - DC input 3-12v with 24-340VAC output 25A load.
    This is mounted in a metal case with a large heat sink but I have got through two of them in 18 months as they seem to overheat.
    Now I have to admit they are Chinese and very cheap so I'm thinking for safety's sake I would be better off getting better quality.

    could any one advise me on a good quality replacement please?

    thanks For any help

    Mike
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Opto22:
    But measure the actual current draw to make sure you have the rating right.
    Max.
     
  3. inwo

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    Nov 7, 2013
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  4. mpooley

    Thread Starter Active Member

    Mar 3, 2010
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  5. crutschow

    Expert

    Mar 14, 2008
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    What is the mains voltage?

    Did you use thermal grease to mount the SSR on the heatsink?
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Is it mounted inside an enclosure or in free air?
    What voltage on the output side?
    Is it failing "shorted"?
     
  7. mpooley

    Thread Starter Active Member

    Mar 3, 2010
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    Thanks for your interest :)
    NO I didn't use thermal paste before but I will do next time .
    The Mains Voltage is 230vac output ,12v dc control side
    It is mounted in it's own aluminium case Horizontally ,back to front alignment with ventilation holes top and bottom to allow free air movement. It is in the same cupboard as the hot water tank though, no way round that!
    and Yes it fails shorted!

    Mike
     
  8. crutschow

    Expert

    Mar 14, 2008
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    The lack of thermal paste could be your problem. Any small air gaps between the SSR and the heat sink greatly increase the thermal resistance.
     
  9. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    I'd highly suspect the aluminum case/vent holes are not sufficient oh and its in another cupboard and its in there with a hot water tank.. good luck with that.. Might need to slap a fan in there to force air in/out.. or get it out of the box
    I'd bet its a nice little oven inside that case.. that is 100% your problem
     
  10. tcmtech

    Well-Known Member

    Nov 4, 2013
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    One thing to consider when using the cheap chinese SSR's is they tend to have marginal suppression systems built in so it helps to add some extra protection over the outputs.
     
  11. mpooley

    Thread Starter Active Member

    Mar 3, 2010
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    So are you saying buying a good name brand SSR won't solve the problem?
    I can add more ventilation and heat transfer paste.
    I have a spare cheap relay so could have another try and also monitor temperatures in the case.
    I would have thought that the relay should stand up to a reasonable temperatures though.
    especially a better quality one?

    the only other place I could put the relay would be in the loft!
    would be nice and cold but a bugger to work on. :rolleyes:
    thanks
     
  12. mpooley

    Thread Starter Active Member

    Mar 3, 2010
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    thanks for your reply, could you tell me what you mean though as I'm not sure how to do that :confused:
     
  13. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Its possible that "better" SSR has more of a tolerance for "higher ambients"
    But I'd HIGHLY suggest increasing ventillation
     
  14. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Any idea of how hot these are actually running?

    Can you touch the heatsink after they have been running for a while or is it above that?

    If you can keep your hand on it for more than a second or two its running well below the SSR maximum ratings.

    Given rough estimates on the power dissipation you should be seeing around 2 - 3 VAC drop across the device at the 13 amp load which would put your total thermal load at around 30 - 45 watts which the heatsinks you show should be able to easily handle that.
     
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You can get adhesive labels to attach to semi conductors etc, and they record by colour what the surface temperature has reached at some point in time.
    Max.
     
    inwo likes this.
  16. crutschow

    Expert

    Mar 14, 2008
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    Better to touch the SSR, since that includes the thermal resistance of the SSR-to-heatsink. If that's not low then the SSR could be a lot warmer than the heat sink.
     
  17. mpooley

    Thread Starter Active Member

    Mar 3, 2010
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    Ok I'm going to put my spare SSR in tommorow, with the paste and I'm going to drill as many new holes in the case as I can without it falling to bits.
    I will then monitor it while it's on for a while by touching it for a rough guide but I have a laser temp meter as well so If all goes well I'll be posting the results some time tomorrow !

    thanks for your help it's much appreciated.

    Mike
     
  18. MaxHeadRoom

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    You can get a Large PC processor H.S. and attached fan for cheap.
    Max.
     
  19. inwo

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    Failures in heating circuits is common.
    A similar situation is a laminator I work on.

    Less than 20a load on 208.

    The 25a ssrs would fail.

    On newer models, manufacturer switched to 30 then 50 now up to 70a, with larger heat sinks than you'd expect.

    Controlled by a pid process controller, but that shouldn't matter.
     
  20. mpooley

    Thread Starter Active Member

    Mar 3, 2010
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    :cool:
    I think I have one knocking around somewhere. I'll try getting the temperatures today first and see what's what.
     
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