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Old 11th Sep 2020, 3:01 pm   #61
mhennessy
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Location: Evesham, Worcestershire, UK.
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Default Re: Toroidal inrush

With this, I can't help wondering if the OP is just "lucky" with his low-impedance mains supply. The acid-test would be to find an exactly identical example to see if it behaves differently. I did suggest a long mains extension lead earlier, and someone else also made the same suggestion subsequently - I think it's worth a try here.

I did note the mention of access to a broken amp with a good mains transformer, and that could prove useful. The most meaningful data would come about if the OP can arrange a current transformer and a digital storage 'scope - I can't recommend that enough. Current transformers are very cheap indeed, and of course have the advantage of not requiring any series resistance. Here's a simple one that I built into a mains distribution unit - I amplified the output to give a more convenient scale, but that's really not required: http://markhennessy.co.uk/mdu/

This does remind me of a similar thing we had at work - though the currents involved were much smaller. We run a course where people get to build and keep a bench power supply - a fairly compact that is only 18V at 1A, but has digital metering and adjustable current limit. Quite nice, really. The design is pretty generic, but has proved to be stable and reliable over the years. The mains transformer is a 60VA toroidal unit - bigger than necessary, but that's another story...

Anyway, the mains fuse is a simple 20mm time-delay type, rated at 500mA. During one course, all worked normally, but would blow the fuse at startup every now and again. We'd already built dozens, so knew the design was OK and that 500mA was a reasonable fuse rating, but for this batch, we had to up the fuse rating to 1A to ensure reliability. Despite that, after the course I did hear reports that a couple of the fuses had failed subsequently. So I upped the fuse rating to 1.25AT, and have had no further complaints.

We checked very carefully at the time, and came to the conclusion that the transformers we were using were different to previous ones. The magnetising current was nice and low and the efficiency was the same, but they had a higher inrush current than the previous models (observed with the setup I've mentioned above). We didn't manage to measure any appreciable difference in the DC resistance of the windings, so the effect must have been magnetic. There were no external clues on the transformers themselves - they came from Farnell, built by the same company as far as we could see, but they (Multicomp) probably sub them out to different suppliers as the mood takes them.

Anyway, stuff like this happens - it doesn't necessarily imply a fault. In many applications, power amps don't always get to enjoy such a low mains source impedance as this one currently does, and there's a chance that this might be all the "problem" is. But without meaningful data about the magnitude and duration of the current waveform, we're into the realms of guesswork here...
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Old 13th Sep 2020, 3:38 pm   #62
Silicon
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Default Re: Toroidal inrush

I've only just seen this post and I have not read it fully -just skimmed through.

Have you checked that the rectifier is still working correctly. Is it still full wave?

Could it be half wave and creating DC current through the primary?
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Old 13th Sep 2020, 4:42 pm   #63
mhennessy
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Default Re: Toroidal inrush

You'd see that at a glance if you viewed the current waveform on a 'scope
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