Transformer question.

[Diabolical Artificer]

I have a tfmr which has two separate secondaries of 12 0 12v @ 75VA and 0- 50v @ 200VA. I was using this to power a home made 24v LM317 based PSU with a bridge rectifier and a similar 48v PSU. All was well until I connected the earth strap to one of bolts holding the tfmr in place when it came to tidy the wiring up.

Firstly there was a loud buzzing noise like when a tfmr is overloaded then a smell of burning and the output hit 0v. The bridge rectifier had got very hot and burned out - the cap got very hot too. What I didn't know was that the tfmr had a copper EMI strip of about 3 turns earthed to the body of the tfmr. So obviously that was the cause.

I took off the earth strap, replaced the bridge rectifier but the tfmr was still buzzing and the reservoir cap got hot ( it's not been damaged and isn't leaking - the PSU works ok with a different tfmr).
I'm unable to understand how the tfmr has been damaged. I stripped the tfmr down expecting to see burnt varnish on the windings as I thought the insulation had failed, but can't see any cracked/heat damaged varnish. I thought the core had saturated and had magnetised but when I briefly connected the primary winding on its own on the bobbin, minus the secondaries and the metal I's and E's it still buzzed.

Could someone explain what exactly is happening? I know the copper EMI strip must have somehow been working a bit like a brake but am unable to phrase it better.

I'm reticent to re-wind the tfmr if the I's and E's have, what's the word "magnetic memory"? D*mn, my memory...can't think of the correct term.

Andy.

[pmmunro]

A sketch of the circuit which you used would be useful. Depending on how you wired it, it may have been an interconnection between your 24V and 48V circuits which caused the problem. It very easy to get confused, at least for me, about which points are, or should be, at the same potential. For example, the centre tap of 12-0-12 secondary, if used, is at a different potential to the 0V output of the bridge circuit.

If the cause of the fault had been the EMI screen, why would the bridge rectifier have been overloaded? It would seem more likely that the bridge rectifier would have had a much lower supply than intended as the effective shorted turn, had there been one, would have been reflected into the primary circuit as a very low impedance.

What voltage did you apply to the primary winding of the dismantled transformer? Without the laminations in place, the inductance of the winding would be very low and a much larger current than usual would flow. This is why a contactor whose whose armature does not move fully home due to an obstruction will draw excessive current and may burn out.

PMM

[Diabolical Artificer]

Thanks P for the reply. I'll try and sketch the circuit when I get a mo as I know it's hard to understand a written description.

To clarify, the 24v PSU was connected to the two 12v sec's. the 50v winding wasn't being used at the time. As to your second question I connected the primary windings on the bobbin to the 240v mains, albeit for a split second.

Andy.

[kalee20]

Originally Posted by Dr Wobble

but when I briefly connected the primary winding on its own on the bobbin, minus the secondaries and the metal I's and E's it still buzzed.

It would do, with no core!

Originally Posted by Dr Wobble

I'm reticent to re-wind the tfmr if the I's and E's have, what's the word "magnetic memory"? D*mn, my memory...can't think of the correct term.

Remanance. No, the iron won't be damaged. Transformer iron is tough, anything you do to it magnetically is reversible. Some alloys do have to be treated with respect if you want optimum properties, but this does not apply to mains transformers.

A circuit diagram, showing the connection you made, would help. So.ehow, it sounds like AC is getting onto the reservoir capacitor.

[Radio Wrangler]

The 'connected the earth strap to one of the bolts' bit makes me wonder about shorted turns due to metalwork rather than circuitry. I think photos are needed as well

David

[Diabolical Artificer]

Here's a schematic. Drawing this has made me realise that the collectors of the TIP3055's were earthed as the heat-sink they were on was in contact with the transformer at the time, so in effect earthed. What I can't understand is that with a different tfmr from the one I earthed this PSU worked fine. But after I "cooked" the original transformer, it buzzed and the res cap started to get very hot. What has happened to the original tfmr? It has to be an insulation breakdown.

Sorry for the awful schematic, but hope you get the gist. I used this 0-30v 10A adjustable schematic as basis for my PSU but omitted some of the HV protection diodes and caps. The 24v output (25.70v w/o load) is obviously to the right of the page.The pic is of the PSU when the snafu occurred, but the the black flexible drive is there by accident.

[Amraduk]

If the earth strap was connected to the negative terminal of the 24V supply at the time, that would have placed a short circuit across the bridge rectifier output. It would then pass a very high current which, it would seem, has damaged it so that it is passing AC to the regulator circuit. That would, indeed, cause the reservoir capacitor to overheat and eventually explode!

Regards,

Dave.

[vidjoman]

Take off the yellow cable tie and look at the cable. Transformers often have sharp edges where the frames are punched. Take off the 12 volt cables and try just the transformer for shorts etc.