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  1. #1
    Join Date
    Apr 2008
    Location
    il
    Posts
    19

    transformer issues

    i ran a call that the furnace would not come on, on a call from the stat. i walk in and find a blown transformer, i test all high voltage wires in the cabinet and find no short. so i put a new transformer on, give it a call for heat and all is fine, give it a call for fan on all is ok, when i call for cooling the transformer blows it was raining ou so i could not go check the out door condensing unit. im thinking possible short in contactor but then again shouldnt it pop th 5 amp fuse a/c is on its on circut.

  2. #2
    Join Date
    Dec 2007
    Location
    East central Indiana
    Posts
    477
    I would guess that a 24 v 40 vac transformer could handle about 9.87 minutes under amperages of 4.99 before popping. So, in this case, no fuse would be blown, but the transformer would.
    Last edited by ECIndHVAC; 09-16-2008 at 08:29 AM. Reason: too much diy info

  3. #3
    Join Date
    Dec 2007
    Location
    East central Indiana
    Posts
    477
    BTW, I'm guessing the reason you checked the high voltage side for a short was because your transformer windings were open on the high side.

    Remember, whatever current exists in the low side is magnified on the high side. So the high side windings are the ones more likely to take the hit. So chances are overwhelmingly good that the short is on the low side.

  4. #4
    Join Date
    Feb 2006
    Location
    Tn.
    Posts
    1,344
    You could always keep your hand on the transformer ,if it gets hot its going to burn......J/K . Apply for pro

  5. #5
    Join Date
    Aug 2008
    Location
    San Clemente, CA
    Posts
    1,212
    Low voltage wires to outdoor unit are shorting to ground. Look at all the exposed wiring outside and at wall to see if dog or rats chewed on wiring. Could also try a 3 amp fuse till resolved.

  6. #6
    Join Date
    Dec 2006
    Location
    Middle Tennessee
    Posts
    11,347

    *

    Quote Originally Posted by ECIndHVAC View Post
    I would guess that a 24 v 40 vac transformer could handle about 9.87 minutes under amperages of 4.99 before popping.
    when a homeowner goes and buys an ampclamp and shocks the hell out of himself he can thank-you

    quit giving DIY advice in the residential section



    .
    Last edited by Airmechanical; 09-16-2008 at 09:20 AM. Reason: took out diy advice

  7. #7
    Join Date
    Dec 2007
    Location
    East central Indiana
    Posts
    477
    Quote Originally Posted by Airmechanical View Post
    when a homeowner goes and buys an ampclamp and shocks the hell out of himself he can thank-you

    quit giving DIY advice in the residential section



    .
    I wouldn't guess this guy is a DIYer. He said he was "on a service call." And if he's willing to brave the dangers of changing out a transformer, the simple use of an amp clamp shouldn't be too over the top

    But anyway, I'll edit out the post, I guess

  8. #8
    Join Date
    Dec 2006
    Location
    Middle Tennessee
    Posts
    11,347
    Quote Originally Posted by ECIndHVAC View Post
    I wouldn't guess this guy is a DIYer. He said he was "on a service call." And if he's willing to brave the dangers of changing out a transformer, the simple use of an amp clamp shouldn't be too over the top

    But anyway, I'll edit out the post, I guess


    good man;

    if i told you i had some water out in the pacific ocean for sale, would you think about buying it

    a lot of DIY'ers have started their post/question with "when i was at a service call today"

    i am not saying the o.p. on this particular post is a DIY'er

    but, don't beleive everything you read!



    .

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,125
    He likes York mods.
    He can't be BSing us.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  10. #10
    Join Date
    Jul 2006
    Location
    NW burbs of Detroit
    Posts
    6,058
    Contactor on ac probable.

  11. #11
    Join Date
    Dec 2007
    Location
    East central Indiana
    Posts
    477
    What you say is very likely true.

    But look where the OP said that the 5 amp fuse was not blown. A short on the contactors sufficient to cause an evidently rapid transformer pop out would have surely put more than 5 amps in the low circuit.

    That leads me to suspect that he has a "minor" short, causing under 5 amps of current, and that it was just a coincidence that the tranny popped shortly (no pun intended) after he turned on the AC.

  12. #12
    Join Date
    May 2000
    Location
    Monmouth Junction-NJ-USA
    Posts
    6,006
    If it is a 240v transformer in an airhandler, it does not take much to pop them at all.
    If you really know how it works, you have an execellent chance of fixin' er up!

    Tomorrow is promised to no one...

  13. #13
    Join Date
    Dec 2007
    Location
    East central Indiana
    Posts
    477
    A few moments ago, I decided it would be fun to short out one of boss's old transformers (came off an old unit).

    It was only 40 vac, but it managed to handle a whopping 100 seconds of a lethal current of 6.5 before it burst into a dazzling green flame.

    We have a few 240 volt transformers, but I don't really want to risk an @ss whooping.

    BTW, the coil opened on the 120 volt side.

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