Transformers Shorting Out

Discussion in 'General Electronics Chat' started by Bart Fey HVAC LLC, Mar 12, 2015.

  1. Bart Fey HVAC LLC

    Thread Starter New Member

    Mar 12, 2015
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    I have an old Janitrol furnace that used the old gas valve which opened at approximately 22 Volts. I am up-dating the furnace with a new 24 volt gas valve. My problem is that when I hook up a standard 120 volt to 24 volt step down transformer the transformer is popping, smoking, and shorting out after 120 volts are applied to the primary. This process takes several minutes. I have tried the standard 40 VA transformers. Any suggestions?
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    How do the current ratings (or coil resistances) of the two valves compare? Is the new valve AC or DC?
     
  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    What is your mains rating ?
     
  4. koibhi

    New Member

    Mar 12, 2015
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    dear check out the transformer name plate..... rated values will be written there. for rated current.. n coil resistance.... and rated frequency...
    And..also check out the output connections also....
     
  5. Reloadron

    Active Member

    Jan 15, 2015
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    Starting with a new transformer I would connect it to the 120 VAC supply, leave the secondary open and check the secondary output voltage, just letting the transformer sit there. Then I would connect the secondary to the system and monitor the under load voltage, I assume you have a meter. The secondary voltage should not drop other than maybe a volt or two. If the secondary voltage drops way down close to zero volts either the gas solenoid is shorted or something to the gas solenoid is shorted. Finally I assume 120 VAC 60 Hz mains power and a transformer rated for the same.

    Oh wait a min, I just saw where you were updating the gas valve. My slowness this early. Make sure you are not operating a AC coil gas valve solenoid with DC. Then too transformer secondary is AC and solenoid should be AC.

    Ron
     
    Last edited: Mar 12, 2015
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The DC version would be much higher in resistance so I would not expect any where the same load for a DC coil?.
    Max.
     
  7. alfacliff

    Well-Known Member

    Dec 13, 2013
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    how about the diode across the coil of a dc solenoid? half wave short on ac.
     
  8. Bart Fey HVAC LLC

    Thread Starter New Member

    Mar 12, 2015
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    I have not even tried hooking the new gas valve into the 24 volt transformer circuit. I can NOT get the transformer operational because it blows out..shorts out after a few minutes
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Wired backwards!?
    Max.
     
  10. Bart Fey HVAC LLC

    Thread Starter New Member

    Mar 12, 2015
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    If I am blowing out the primary of a 120 volt circuit could it be surging more than 120 volts to the primary?..And why does it take several minutes for the transformer to blow?..I have not tried hooking the new AC gas valve up..because I can not keep the transformers from blowing.
     
  11. Bart Fey HVAC LLC

    Thread Starter New Member

    Mar 12, 2015
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    I just need a 40 VA ..120 Volt stepped down to 24 volts secondary..I have shorted TWO NEW ONES OUT..w/o even hooking up the secondary..to the new AC gas valve..
     
  12. Bart Fey HVAC LLC

    Thread Starter New Member

    Mar 12, 2015
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    What do you mean by 'Mains Rating'?
     
  13. Reloadron

    Active Member

    Jan 15, 2015
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    I did manage to get that backwards. Thank you Max.

    Ron
     
  14. Reloadron

    Active Member

    Jan 15, 2015
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    No way you are applying 120 VAC to the transformer secondary is there? Meaning primary and secondary backwards.

    Ron
     
  15. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    measure the AC you are connecting to. if secondary is open, I can only see over voltage causing this. be sure it is 120 and not 240.
     
  16. alfacliff

    Well-Known Member

    Dec 13, 2013
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    most of the transformers sold for hvac use have two black wires and two screw terminals. the screw terminals are the 24 volts and the black wires are 120.
     
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