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Step-down transformer suddenly stopped working - but why?

Discussion in 'General Electronics Discussion' started by mylop, Oct 5, 2013.

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  1. mylop

    mylop

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    Oct 5, 2013
    I have a Condor 240v AC to 24v AC transformer which just stopped working.

    Taking the cover off, my multimeter confirms 240v is going into the primary coil, but nothing is coming out of the secondary.

    Unless I'm wrong, there is no connection between the two coils, so I don't see what can break. If the primary coil has a current, I don't see why the secondary coil would be dead.

    Any help or advice appreciated.
     
  2. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

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    Aug 27, 2013
    really? Really? you should take the multimeter back and buy another transformer, the MM has already put you in danger once and is likely to do so again.

    Fish
     
  3. mylop

    mylop

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    Oct 5, 2013
    Would you care to explain further?
     
  4. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

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    Aug 27, 2013
    You have a bad transformer.

    Get a new one.
     
  5. duke37

    duke37

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    Jan 9, 2011
    Transformer windings can go open circuit for various reasons. The transformer may have a thermal fuse in the primary to stop current if the transformer gets too hot. This not the case if you have a connection through the primary.

    Corrosion can affect either winding. Old transformers suffer from 'blue spot or green spot' where the wire has rotted through due to contamination when wound. The bad connection may be just a bad termination.
     
  6. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

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    Aug 27, 2013
    Transformers provide galvanic isolation from the mains supply. ( http://en.wikipedia.org/wiki/Galvanic_isolation ). This is an essential safety feature.

    It is highly probable you are working on an HVAC system as this is the primary use of 24Vac. This implies that you have opted not to call a trained service technician and are attempting to repair a system that you clearly do not understand using a tool (multimeter) whose readings lead you to the incorrect conclusion that voltage across a transformer primary mandates an output on the transformer secondary under all circumstances. The fact that you are probing an exposed, live, mains connected system psuedo-randomly suggests that you are in danger regardless of your perception.

    If you suspect the transformer is the culprit you should install a new one, they are cheap. You should ABSOLUTEY pull the service disconnect and/or turn the breaker off before you attempt replacement. If your MM has an "ohm" setting, you can test both the transformer primary and secondary with the power disconnected.

    Fish
     
  7. mylop

    mylop

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    Oct 5, 2013
    The equipment the transformer powers is very expensive and produced only in the US. It clearly states to only use a transformer as supplied by them. A replacement is $110 but that doesn't include shipping. Before requesting a new transformer simply for that to blow, too, I was hoping to assertain why the transformer had blown. There was no smell, no noise - it just stopped.

    My assumption was a cable had come loose/fuse blown.

    Upon opening the transformer, I could see nothing replaceable but used the mulitmeter to try and detect how far the current was getting. I'm sensible enough to understand not to touch anything I don't understand. But I do understand a blue and brown cable on one side and the neg, pos and centre tap on the other.

    I'm still confused why I need a new MM - if you're suggesting my actions were foolish, a new MM wouldn't have changed that.

    Opting to call a technician isn't really an option as they're in Los Angeles.
     
  8. mylop

    mylop

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    Oct 5, 2013
    Thanks for this. The transformer isn't even 2yrs old, so I'd be surprised if it's corrosion.

    As I've mentioned above, a replacement is $110 and I'm concerned that purchasing a new one without figuring out the reason it blew will just mean a very expensive third replacement!
     
  9. pebe

    pebe

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    Sep 3, 2013
    I don't think he has established there is no current going through the primary. He only mentions a multimeter set to 240V range, so he is only checking that it has a supply.

    @mylop. Remove the unit from the supply, ie. pull the plug, and using your MM set to an ohms range measure for continuity across the 240V winding of the transformer.
     
  10. mylop

    mylop

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    Oct 5, 2013
    You're right - all I did was check the supply was making it that far. Thanks - I'll give this a go. The transformer no longer gets warm or hums, so I've always assumed it's an issue on the primary side.

    Considering I'll never use the transformer again, is it worth pulling it to pieces to see what's gone wrong?

    EDIT: Yes, it's dead. The primary circuit is broken. Strange that nothing changed - it's worked happily for 2yrs and it's suddenly stopped.
     
    Last edited: Oct 5, 2013
  11. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

    464
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    Aug 27, 2013
    mylop,

    I woke up grumpy this morning; that's not your fault.

    Transformers are generally reliable devices. As previously mentioned occasionally environmental factors cause failures. In addition to these failures power spikes can cause failures. While most transformers can withstand the secondary being shorted for some period of time, if a transformer is under a hi load for an extended period of time it might get hot enough to damage the insulation in either the primary or the secondary. I have honestly seen very few consumer items with failed transformers; however, I have seen a fair number of HVAC transformers go bad; and most of the time there was an excessive load on the transformer (ie the transformer might have lasted considerably longer if there weren't another problem.)

    I do not understand why the company that only has service techs in LA is so concerned over a 24V transformer, unless there is something unusual about it. It would appear from what you have said that they aren't particularly worried about your device except to insist that you use their $110 transformer. Are there any markings on it? If so, I would bet dollars against donuts that you can source it somewhere else for a fraction of the price, especially if it can be replaced by a standard HVAC transformer, but even if it is a fairly high-powered transformer, you can almost certainly source it for less than $110.

    Again, sorry for being grump this morning.

    Fish
     
  12. davenn

    davenn Moderator

    13,620
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    Sep 5, 2009
    sounds like the thermal fuse has blown as suggested by some one earlier
    this could happen because of excessive secondary load .... ie.... there's a fault in the circuit that the transformer is powering

    but more likely ....

    that there was as 240VAC mains power surge that popped the fuse

    cheers
    Dave
     
  13. duke37

    duke37

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    Jan 9, 2011
    If you know the correct voltage (24V ?) then get a cheapo transformer and tack it in to test the rest of the circuitry. If that works then look for a transformer to fit or get the original rewound.

    I have never been able to repair a primary with a thermal fuse. Wall warts are subject to overheating.
     
  14. mylop

    mylop

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    Oct 5, 2013
    I don't doubt you're right - it's purely the value of the equipment it powers that makes me overly cautious.
     
  15. mylop

    mylop

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    Oct 5, 2013
    The transformer is powered from the mains in a film stage where the mains is clean and surge protected, so I would be surprised if this had happened (although I'm not a spark so of course I don't know for sure).
     
  16. mylop

    mylop

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    Oct 5, 2013
    No worries about being a grump - thanks for the honesty.

    I always thought transformers were reliable, hence my questions. Seems odd that it would just 'pop' when nothing else has changed and it's being powered from very clean mains.

    I reckon they insist on 'their' transformer because they make a nice profit, but I can't be 100% sure there's not another reason. From what I can gather, it's this one:

    http://www.condorelectronics.net/catalog/detailac.php?ID=286

    but I can't find it available in the UK anyway
     
  17. pebe

    pebe

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    Sep 3, 2013
    The link shows a 24V centre tapped winding, so the output probably goes into a bi-phase rectifier inside the equipment. If so, you may be able to use something like this, which would run cooler than the original transformer.

    http://www.ebay.co.uk/itm/Daewoo-DL...ctronics_PowerAdaptors_SM&hash=item2ec946016c

    I can just make out the words "Made in China" from the photo of the transformer. The quality of exported electrical items from China can be quite variable, so it is not unusual to get a failure.
     
  18. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

    464
    105
    Aug 27, 2013
    Now that we know a little more about the transformer, use the following search terms for an ebay search:

    24v transformer 40VA

    Not sure about the UK, but in the US there are a lot of choices under $20.

    Fish
     
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