Turn on surge kills 1 2nd winding of transformer?

[Cradle22]

Hi!

I just wanted to finish testing the second channel of my P3A (after setting the bias), carried the unit to my stereo, hooked it up with signal (no speaker yet connected, wanted to check DC offset first), fired it up, and no voltage (LED of P3A not turned on). When I wanted to measure voltage, and therefore moved something a little bit, there was a spark (resulting from a not properly soldered pin on the supply board, the one where one secondary winding connects), and now the LED of the P3A turned on. But DC offset was too high (600 mV), so I turned the amp off. But immediately I decided to measure something else, and turned the amp on (about 1 1/2 secs after turning of).

(Btw:
My toroid is a 500 VA / 30 - 0 - 30 toroid from Reichelt in Germany, look at
http://www.reichelt.de/index_direkt....QsAAGBs-Co+C55
.
)
Upon turning on, the fuse for my living room switched off and I was sitting in the dark. Turned it back on, replaced fuse of amp, and to make long things short:
One of my two secondary windings of my transformer seems to be fried (infinite resistance, no voltage). Why for gods sake?

I have two 225 VA 30 - 0 - 30 toroids for experiments, which I shorted , wired the wrong way, did everything bad imaginable to them, sometimes even about 5 secs long (they produce a nice, crackling sound when that happens...), and they both still work to their full extend ...!!

Why did the 500 VA toroid die immediately on me ? Could the reason for this be my turning off / on without any delay (and remember, there are 2 * 10.000 uF BHC SlitFoil - which were charged - connected to the toroid)?

While those Reichelt toroids still are very cheap (and absolutely hum-free), it still was the most expensive part of my amp...

Normal behaviour, or bad luck?

Bye,

Arndt

[peranders]

If you have fused the primary winding properly, I doudt that you can really "fuse" the secondary windings but this is easy to check. Only two wires (of four) shows low resistance.

[jackinnj]

look like a dead short when the device is first turned on, that's why some power supply manufacturers use brute force inrush current limiting -- as I mentioned elsewhere -- Sorensen for instance uses a resistor which is shorted after a few seconds via time=delayed relay.

[Cradle22]

Originally posted by peranders
If you have fused the primary winding properly, I doudt that you can really "fuse" the secondary windings but this is easy to check. Only two wires (of four) shows low resistance.

Well, the primary was fused (10 A slow), the secondaries were fused, as well, but only directly at ampflifier (2 * 5 A fast), so if the error (the bad solder point?!) happened on the supply, I think those fuses in the amp were useless...

Anyway, I think I will have to stick to one of my experimental transformers (or true dual mono, using both of them), until I have the money to buy a new big one... Sadly one of those 225 VA trafos hums a little bit...

Bye,

Arndt

[AudioFreak]

Using a 10 amp slow blow fuse on the primary would have provided practically no protection. I'm not sure what voltage the mains is in germany but if it is 240V, the fuse should of been at most a 3 amp slow blow and then something like a thermistor or resistor + relay used to limit the inrush.

[peranders]

For a 500 VA 3.15 or 4 AT is on target. With these fuses you can start the transformer without softstart if you have 10 A fast (normal melting fuses) in the wall.

[Cradle22]

Thanks, I will try decreasing the fuse size (will not help me with my killed transformer, though...)

Like I mentioned, since my smaller, cheaper toroids handled torture so well, I always thought that transformers don't die that easily...

I just imagine what my shock would be like if it was, say, a 1500 VA transformer for a BIG subwoofer amp...
Of course I would not use a 1500 VA without a softstart circuit

Bye,

Arndt

[Cradle22]

Hi! Small Update to this thread:

Originally posted by peranders
For a 500 VA 3.15 or 4 AT is on target. With these fuses you can start the transformer without softstart if you have 10 A fast (normal melting fuses) in the wall.

Well, today I got a new 500 VA 30-0-30 toroid (have been using a 225 VA up to now with the P3A, after the first 500 VA melted).

I exchanged the transformers, and found out that a fuse of 4AT or 5AT is not enough. The power surge when turning the amp on (I use a solid state relais with a pushbutton for on/off, no softstart) blows the fuse...

So I will have to increase the value for the fuse again, or use a softstart, which I don't like, since my mains voltage seems to be stable enough for turning on a 500VA toroid...

Or does it make a difference that I disonnected the amps themselves from the supply (so no load connected to the supply / the caps in the supply)?

My supply of fuses is decreasing rapidly... still better than blowing transformers, though


Bye,

Arndt

[janneman]

Unless you have a continued short (how are you rectifier bridges?) it is very unlikely that the turn-on fused the secondary wires. There is simply not enough time for the wires (and insulation) to heat up to 100+ celcius. What might have happened is a voltage punch-through of defective or marginal insulation. That would make it an exemplary problem, and a new transformer should work fine.

Jan Didden

[Cradle22]

Hi!

I think I have a working 500VA toroid surplus...

Maybe you wanna know what went wrong (the first time, and also with my second toroid)?!...

Well, those transformers I used have two secondary winding, no center tap, so there are four connections for secondary windings coming out of the transformer. Each of those connections consists of two solid core wires, isolated with ?laquer?paint? (don't know the word). So, since I want to connect two supplies to one transformer, I simply cut of the isolation, and splitted those four connections into eight solid core wires, isoltaing each one of them.

I don't know the reason for this, but it seems that those two wires of each secondary winding are not connected at all, so if I connect the wrong wires, I have no current whatsoever...

Does anybody know why the secondary windings are split into two wires for each winding?

Now I connected everything properly and it works alright... didn't try with smaller fuse, though... right now it's a 10 AT fuse, little bit oversized...

Well, what can I say, you live, you learn...


Time for bed...

Arndt