Mains power puzzle

Phil Allison wrote in news: snipped-for-privacy@googlegroups.com:

It has a shorted turn on a secondary or tertiary winding.

The toroid has a big crack in it.

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" Nothing wrong found with the amp when a new fuse was fitted. "

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Runs right round the core about 120 times.

Strewth...

.... Phil

mpm wrote in news: snipped-for-privacy@googlegroups.com:

He never said it had a ferrite core. But even if it did, being cracked would not hurt a 60 or 50 (or even 400) cycle power transformer one bit.

Phil-

I assume there is some marking that specifies a 5 Amp fuse is to be used. That sounds marginal for a 600 VA transformer at 120 VAC, but about right if you have over 200 VAC mains.

When a power supply is first turned on, there is an in-rush of current as power supply capacitors charge. A normally blowing fuse may not always hold out long enough. Be sure you have the T5A a "slow blow" fuse.

Your measurements do not make sense.

Fred

Phil Allison wrote in news:4af515d1-9eb0-4ba8- snipped-for-privacy@googlegroups.com:

You said 120 A inrush.

That hardly qualifies as "nothing wrong".

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I think pissed off users uprate the fuse to T8 or T10 amps to make it work.

Remember, max inrush surges do not occur every time.

..... Phil

I'm just making stupid guesses, but I'll guess anyway just for giggles.

High inrush current might be OK, but might indicate a defective (or missing) inrush thermistor, or abused thyristor or other EMI/surge protection.

DMM and analog meter use low current/volts, the DC supply uses higher volts, looks like effective resistance goes down as volts goes up... semiconductor? Is it reading the bridge rectifier as a resistor?

It would be interesting to see the amp ratings at 2V and 3V *AC* instead of *DC*.

Sounds like a leaky/damaged varistor, which sometimes causes surges instead of waiting for them. Nonlinear behavior, i.e. 'resistance' depends on test voltage.

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There is nothing damaged in the amp.

..... Phil

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FYI:

There are TWIN 400watt clas sB output stages using BJTs inside.

.... Phil

Transformer was probaby last unplugged with the transformer flux such that the next time it is plugged in, at just the right time of the AC cycle, the transformer is further brought into sautration (the core goes away) which becomes a short circuit to the mains until the flux works its way back to center and operates normally.

Either that or filter capacitors charging up but this sounds more like xformer saturation.

boB

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.... Phil

Especially toroids are nasty, since depending on the mains phase during switch-off determines what kind of magnetism remains in the core.

Since the switch-on mains phase is also random, so in the worst case some current is needed to neutralize the remaining core magnetism before starting to follow the mains waveform.

To avoid such high peak currents, VDRs or NTC resistors are often used in series with the toroid primary to limit this current. Switching on again immediately after switching off and the resistor might still be hot and resistance is low allowing large peak currents.

Of course the inrush limiter may be faulty.

sustain the 120 A peak current.

How many measurements did you make in each case for each polarity ? Did you connect the amplifier into mains between each DC measurements to get different switch-off phase ?

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** It ain't got one.

Yet aging, it has no faults.

Magnetism has no known affect on the DC resistance of copper.

... Phil

Noise suppression capacitor or MOV on the mains. With the provided informat ion those are the only two logical assumptions.

That is provided it has a regular iron power supply, with an SMPS you would not get those low readings.

An analog meter has a little different formula. Most DVMs are 2 or 3 volts but will read open on anything with a drop more than 2 volts, though I foun d it will actually light an LED and still read open...

An analog(ue) meter can be any voltage, maybe you should measure the open c ircuit voltage. you might find you are reading an non-linear device or that it is just a matter of the math.

The info you gave is a bit limited but I think somewhere there is a non-lin ear device in there you are reading. And the nuisance fuse blows (MOV) coul d be related to heat, the mains voltage at the time and all kinds of s*it.

That is all I can say with the information provided and really, I can't tel l you what to check first (you almost probably ever need that) so just chec k into it.

Remember rule, I think it is 27, when there is a overload or a short start disconnectiing things, Rip (well disconnect) the primary wires clean out an d just apply to them, meters as well as volts.

And then measure the rest of it.

That's what I would do first.

And I haven't read this whole thread so don't tear me up if I missed someth ing. No, in fact tear me up all you want. If you knew my family you would k now I don't really mind. Like, how many times did your Father pull a gun on you ? (well I mean when you didn't do anything, other times OK)

But I told you what I would do, take it or leave it. Disconnect, disconnect , disconnect.

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FYI:

the primary resistance of the 600VA, 6kg tranny is 2.4ohms.

.... Phil

Sounds like roughly 1-2 diode drops and 2 ohm primary in series.

Is it a circuit to remove small DC components from the mains input that might cause transformer saturation ?