StepDown Transformer forcing MCB to TRIP

Discussion in 'General Electronics Chat' started by BluePhantom, Nov 16, 2015.

  1. BluePhantom

    Thread Starter New Member

    Nov 16, 2015
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    Hello
    I'm New to this forum..
    I decided to make a high power 25V power supply for running Mazzilli ZVS Driver.
    I got an old Microwave from scrap and took out the Transformer and removed the 2KV Secondary.
    I added new Secondary for 25V and Max. 40A, but when I connected the primary to AC Mains without any load on secondary
    the MCB tripped.
    What is causing the problem?
    Mains is rated 240V~ @50Hz
    Transformer is rated for MAX 1200W
    Thanks in Advance!
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You have a short on the new wound secondary.
    Max.
     
  3. BluePhantom

    Thread Starter New Member

    Nov 16, 2015
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    I didn't short it but I tried to test the output with the multimeter..
    Also I have seen people who wind only 2 turns on secondary to use it as spot welder... There is a short too.,so why doesn't the circuit breaks in that case?
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    What size is the MCB?
    Did you drive the magnetic shunts out?
    Maybe that was the reason the MW was scrapped in the first place?
    Max.
     
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
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    did you remove the origional secondary windings?
     
  6. BluePhantom

    Thread Starter New Member

    Nov 16, 2015
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    Mcb is rated for 20Amps
    Do you mean anything else by the size?
    shunts are still there..

    I also used a same mcb which was lying around as a switch with the transformer. But that didn't trip
     
  7. BluePhantom

    Thread Starter New Member

    Nov 16, 2015
    20
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    Please read and understand the question before replying
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    After the initial inrush the current should be very low when unloaded.
    You could try a high wattage low resistance in series with the primary on initial turn on, short the resister after the short inrush period, if it blows the breaker, you have a short on the transformer.
    Max.
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    As a variation on Max's suggestion, you can put an incandescent bulb in series with the primary.
    If the bulb stays other than dimly lit with no load on the transformer it would indicate there's a short somewhere in the windings.
     
    MaxHeadRoom likes this.
  10. BluePhantom

    Thread Starter New Member

    Nov 16, 2015
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    Is there a specific value for that resistor?
    Highest wattage in case of resistor I have 10w 500ohm resistors... Will it do?
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The lamp as in @crutschow post is better option, but you can try it, although if the transformer has a short circuit the resistor will heat up rapidly.
    Max.
     
  12. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Before you modified the transformer, was it working ok, whats the primary winding resistance?
     
  13. BluePhantom

    Thread Starter New Member

    Nov 16, 2015
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    Yup it was working and giving 2kv output..
    Resistance is around 17.1ohms
     
  14. BluePhantom

    Thread Starter New Member

    Nov 16, 2015
    20
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    Yup I will try this..
     
  15. BluePhantom

    Thread Starter New Member

    Nov 16, 2015
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    I was also thinking that resistor would burn away
    Is there any specific wattage for the bulb or i can use any lamp?
     
  16. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Try a 40 to 80w bulb, so what's the primary and secondary resistance now after modifications?
     
  17. BluePhantom

    Thread Starter New Member

    Nov 16, 2015
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    Primary resistance is 17ohm
    Secondary resistance is 8.9ohm
     
  18. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Ok, maybe the primary winding is shorted to earth and that's causing it to trip, can you monitor the primary current with an ammeter?
     
  19. BluePhantom

    Thread Starter New Member

    Nov 16, 2015
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    How can I do that if the circuit is broken just after turning it on
     
  20. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    With the bulb in series with the primary winding, and no secondary load.

    Measure the resistance of both primary connections to earth one at a time.
     
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