Testing low voltage transformer. - UK Vintage Radio Repair and Restoration Discussion Forum

Andrew B · 28th Mar 2017, 4:08 pm

Hi All.
Is there a relatively easy way of testing the safe continuous output current capability of a *large" low volts (30v) toroidal transformer?

I have been donated this rather large item (which is potted) so I can't even get any clues from the wire gauge.

Thanks
Andrew

Nickthedentist · 28th Mar 2017, 4:13 pm

Its weight might be a useful indicator - compare it with data for similar ones in suppliers' catalogues etc.

Nick.

28th Mar 2017, 5:05 pm

A five percent drop on output voltage on load for large transformers is about right, more for smaller ones. It is fairly linear, load, measure and calculate, e.g. if it drops by 2% at 1 amp then 5% / 2% (2.5) gives 2.5A. When done load at the calculated maximum, a 60 centigrade rise is OK for modern units somewhat less (40) for older stuff.

Skywave · 28th Mar 2017, 9:40 pm

It's the V-A rating of the transformer which you really need to know. As a guide, measure the core cross-sectional area, A, then use one of the two formulae:

1. If A is in mm.², V-A = A²/16042.
2. If A is in in.², V-A = 26*A².

Either of those are approx. valid for conventional 'E-I' transformers at 50 Hz., in the range 10 - 100 V-A.

Al.

Andrew B · 28th Mar 2017, 9:59 pm

Hi Al.

It's a "big beast" encapsulated toroid 10" dia, 4" dia hole and 8" high.

I got a "standard" E I transformer given at the same time. 18 volts 100A continuous.

Radio Wrangler · 28th Mar 2017, 11:28 pm

Gauging transformers by weight has a degree of legitimacy. Jim Williams of Linear Technology wrote a lengthy section on it as part of LT's application note on switch mode PSUs...... Something like Power supplies for Poets.

David

28th Mar 2017, 4:08 pm	#1
Andrew B Heptode Join Date: May 2015 Location: Bradford, West Yorkshire, UK. Posts: 512	Testing low voltage transformer. Hi All. Is there a relatively easy way of testing the safe continuous output current capability of a *large" low volts (30v) toroidal transformer? I have been donated this rather large item (which is potted) so I can't even get any clues from the wire gauge. Thanks Andrew __________________ worried about the electrons entering the circuit and the smoke leaving Andrew

28th Mar 2017, 4:13 pm	#2
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,846	Re: Testing low voltage transformer. Its weight might be a useful indicator - compare it with data for similar ones in suppliers' catalogues etc. Nick.

28th Mar 2017, 5:05 pm	#3
Guest Guest Posts: n/a	Re: Testing low voltage transformer. A five percent drop on output voltage on load for large transformers is about right, more for smaller ones. It is fairly linear, load, measure and calculate, e.g. if it drops by 2% at 1 amp then 5% / 2% (2.5) gives 2.5A. When done load at the calculated maximum, a 60 centigrade rise is OK for modern units somewhat less (40) for older stuff.

28th Mar 2017, 9:40 pm	#4
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Testing low voltage transformer. It's the V-A rating of the transformer which you really need to know. As a guide, measure the core cross-sectional area, A, then use one of the two formulae: 1. If A is in mm.², V-A = A²/16042. 2. If A is in in.², V-A = 26*A². Either of those are approx. valid for conventional 'E-I' transformers at 50 Hz., in the range 10 - 100 V-A. Al.

28th Mar 2017, 9:59 pm	#5
Andrew B Heptode Join Date: May 2015 Location: Bradford, West Yorkshire, UK. Posts: 512	Re: Testing low voltage transformer. Hi Al. It's a "big beast" encapsulated toroid 10" dia, 4" dia hole and 8" high. I got a "standard" E I transformer given at the same time. 18 volts 100A continuous. __________________ worried about the electrons entering the circuit and the smoke leaving Andrew

28th Mar 2017, 11:28 pm	#6
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,876	Re: Testing low voltage transformer. Gauging transformers by weight has a degree of legitimacy. Jim Williams of Linear Technology wrote a lengthy section on it as part of LT's application note on switch mode PSUs...... Something like Power supplies for Poets. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done