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Old 11th July 2006, 09:57 PM   #1
Bonsai is offline Bonsai  Taiwan
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Angry 2kVA Torroidal blows fuse

I ordered a 2KVA torroid for my amp - low noise and low magnetising current (30mA no load). When I apply the mains it pops the fuse and sometimes trips the mains circuit breaker. Anyone got experience with big transformers like this - what's the inrush current? I see some people recomend putting a 10 Ohm resistor in series with the primary for a second or two. I had a 220 Ohm 20watt resistor and tried this. Fuse still blew when I shorted the resistor out (with a zero crossing triac switch). Is my series resistor too high in value.

By the way, I sent the transfoermer back to the factory - they checked it and apparently its ok.

Feedback/suggestions appreciated.
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Old 11th July 2006, 10:04 PM   #2
poobah is offline poobah  United States
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What is the load on the secondary?
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Old 11th July 2006, 10:09 PM   #3
jleaman is offline jleaman  Belgium
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I wonder if you tried to connect the tranny with out any thing on the secondary side at all. Just to see that it is working. We know now it is. But you should also practice this too.
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Old 11th July 2006, 10:10 PM   #4
Bonsai is offline Bonsai  Taiwan
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Poobah - there is no load on the secondary - they are open circuit
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Old 11th July 2006, 10:14 PM   #5
poobah is offline poobah  United States
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Geeze! What size fuse are you blowing?

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Old 11th July 2006, 10:15 PM   #6
Netlist is offline Netlist  Belgium
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Dual primary connected the wrong way?

/Hugo
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Old 11th July 2006, 10:19 PM   #7
poobah is offline poobah  United States
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Bet you nailed it Hugo...
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Old 11th July 2006, 10:25 PM   #8
Bonsai is offline Bonsai  Taiwan
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no - I think the primaries are wired correctly. I checked this by wiring a 40W lamp in series. If I measure the seondaries I get 60 - 0 - 60. The lamp is OFF because only the magnetising current is being drawn thru the primary. I short one of the secondaries out and the lamp goes on - I assumed therefore that I had phased it correctly. I checked again with the guy at the factory - I got the primaries right.

I am using a 10A anti surge fuse from Farnell

I spent a fortune on this transformer so I am pretty jerked off that I've got this problem now.

Anyone know what the in-rush (initial magnetising) current is for these things?
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Old 11th July 2006, 10:33 PM   #9
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how about a soft-start circuit?
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Old 11th July 2006, 10:42 PM   #10
jaycee is offline jaycee  United Kingdom
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For a toroid that huge, you will absolutely need a soft-start circuit.

That is one hyoooge mofo

edit: I see you have one. 220 ohms is too high and probably isn't allowing the transformer's magnetic field to stabilise. Go with a lower value like 68 ohms. Also I didn't see what the primary voltage is in your country.
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