Over-working a mains transformer.

[G6Tanuki]

I've decided to bring back into service my Stewart-Warner-built version of the WWII-era Collins TCS12 receiver I've owned for something like 45 years.

It needs a power supply of 250V@60mA and 12.6V@1.2A

To hand I have a rather well-built mains transformer that is 300-0-300V@100mA and has heater windings of 6.3V@3.5A and 6.3V@1A. The 6.3V@1A was designed to feed a valve rectifier; I'll be going semiconductor.

I'll be applying suitable series-resistors in the HT feeds to the rectifiers, and maybe doing regulator-stuff involving a MJE340 series-pass transistor to keep the HT within sensible limits - but I'm nervous about wiring the two LT windings in series.

Given that the HT windings will be significantly under-run, I guess I can get away with pushing the 1A LT winding a bit harder and drawing 1.2A.

[I'm designing for something that can operate 24x7 without getting at all hot under the collar]

Is this sane? I'm really thinking I can get away with it.

[paulsherwin]

Should be OK. Obviously you'll check the voltage isn't sagging and it's not getting overly warm when it's loaded up.

[Aub]

Yes, go for it.

Aub

[Bazz4CQJ]

Do you recall the graph I once posted (attached below) showing the line for VA and mass?

Especially if it's a quality transformer, that does not sound too outrageous.

B

[kalee20]

Should be fine! The primary losses and temperature rise is governed solely by the total load - the primary doesn't 'know' which secondaries are loaded and which aren't.

The 6.3V LT secondary - you're pulling 1.2A out of a winding designed for 1A, so the power losses will be 44% higher than normal (1.2 squared). Against that, the HT secondary is under-run so the heat generation there will be less than normal.

The total secondary heat generation may well be comparable with that that exists when loaded as intended, however it won't be as evenly distributed as in a transformer well-designed for its loads, however it's unlikely to be a showstopper. If in any doubt, try it with dummy loads, and assess its temperature rise.

[turretslug]

I know that you're not a fan of chokes from previous posts, but a choke-input filter here might help with both the HT voltage situation and overall transformer loading, further favouring the heater winding equation.

[Leon Crampin]

Load the transformer as you intend to use it, and measure the voltage of the 1A heater winding. If it's > 5% low, there is a good chance the winding will run hotter than intended. This may not matter much as the underrun remainder of windings will heatsink it. The lower voltage could be a problem. Is there room on the bobbin to wind another 6V winding? If you exactly match the turns, It could go in parallel with the 1A winding and be wound with much thinner wire.

Leon.

[G6Tanuki]

Thanks for the suggestions.

I've done some experiments; I wired boith heater windings in series and used them to feed the series-connected heaters of a pair of EL34 I had lying around; I then hooked up a couple of uF4007 diodes and a 100uF 450V electrolytic as a smoother and switched on.

Got 451V ! Ouch!

Hooking up the anodes of the EL34s and an external -ve bias supply so I could use the valves as a variable load, I switched back on and set the bias so the HT current was 60mA.

The rectified DC voltage was now 322V, heaters 6.4V and I left it like this overnight; the transformer did not get overly warm. So it seems OK.

But I'm worried about that HT peak voltage - shoving an instantaneous 450V into a radio designed for 250V and which is now 70+ years old seems callous and risky. I'm minded to search for a somewhat less-frisky transformer, I'm thinking one of the traditional broadcast-radio 250-0-250V@60mA types with a 6.3V heater winding, and using an additional RS 6.3V@1.5A heater transformer (which I know I have somewhere in the attic) in series to get the 12.6V might be the way to go.

The juicy transformer might be just what I need to power a 6146 Class-AB1 linear amplifier....!

[Electronpusher0]

Originally Posted by Bazz4CQJ

Do you recall the graph I once posted (attached below) showing the line for VA and mass?

B

Thanks for the graph.
I transfered it to Excel and spot checked it with a range of LT transformers of known (marked) VA ratings.
My spot checks yield a linear relationship but have a lower slope, ie a lower mass for a given VA.

Very limited sample I realise but interesting.

Peter

[Richardgr]

I always fit a thermistor like this after the on-off switch.
CL90

They come in different shapes and sizes, but this one seems to be the best all round choice for the primary side of the supply.

[Bazz4CQJ]

Originally Posted by Electronpusher0

Thanks for the graph. I transfered it to Excel and spot checked it with a range of LT transformers of known (marked) VA ratings.
My spot checks yield a linear relationship but have a lower slope, ie a lower mass for a given VA.Very limited sample I realise but interesting.

Peter

That's interesting that you have what looks like a straight line, but with an appreciably lower slope than my data. I guess that neither of us has that much data, and that factors like the type of mounting (soldered on PCB, or bolted down with a hefty shroud) will affect the outcome.

B

[Electronpusher0]

Originally Posted by Bazz4CQJ

That's interesting that you have what looks like a straight line, but with an appreciably lower slope than my data. I guess that neither of us has that much data, and that factors like the type of mounting (soldered on PCB, or bolted down with a hefty shroud) will affect the outcome.

B

Agreed. I will try and add more data points.

Peter

[Herald1360]

Lower mass for a given VA rating might suggest better iron and/or higher insulation temperature rating perhaps?

Or maybe commercial vs professional designs?

[Bazz4CQJ]

Originally Posted by Herald1360

Lower mass for a given VA rating might suggest better iron and/or higher insulation temperature rating perhaps?

Or maybe commercial vs professional designs?

I was surprised that I got a straight line with relatively little scatter. Peter looks to have a straight line too but a significantly different gradient. An impartial observer might suggest that one of us has a faulty balance?

At the end of the day, I guess that operating temperature is more informative than mass.

B

[Electronpusher0]

Originally Posted by Bazz4CQJ

An impartial observer might suggest that one of us has a faulty balance?

B

Fair point, I used the digital kitchen scales (don't tell SWMBO), I have no reason to disbelieve them, I also use them for posting parcels and had no problems. I will double check against another set.
3 of the transformers I tested were RS types with RS stock numbers printed on.

Peter

[AdrianH]

In one of the Wolpert documents about winding transformers it has a guide table based on the area of the stack, I have copied the page here, perhaps this may add some information.

VA rates.pdf

It is quite an old doc and relates to imperial sizes but should give an indication.

Adrian

[Electronpusher0]

Interesting, I added some spot values from the Wolpert data assuming a stacking factor of 0.9 and got an almost linear line that tracks my spot checks but lower. Of course this does not include frames and bolts so if that is added it comes pretty close to my line.

Peter