StepDown Transformer forcing MCB to TRIP

Thread Starter

BluePhantom

Joined Nov 16, 2015
20
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!
 

Thread Starter

BluePhantom

Joined Nov 16, 2015
20
You have a short on the new wound secondary.
Max.
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?
 

MaxHeadRoom

Joined Jul 18, 2013
20,866
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.
 

Thread Starter

BluePhantom

Joined Nov 16, 2015
20
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.
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
 

MaxHeadRoom

Joined Jul 18, 2013
20,866
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.
 

crutschow

Joined Mar 14, 2008
25,103
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.
 

Thread Starter

BluePhantom

Joined Nov 16, 2015
20
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.
Is there a specific value for that resistor?
Highest wattage in case of resistor I have 10w 500ohm resistors... Will it do?
 

Dodgydave

Joined Jun 22, 2012
9,197
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?
 

Dodgydave

Joined Jun 22, 2012
9,197
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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