Using the secondary of a normal transformer as the primary to get a HT supply

baggystevo82 · 2nd April 2004, 03:18 PM

Hello,

Im being a skinflint and don't really want to fork out 120 quid for a mains transformer. I have a 500VA toroidal in mind, with twin primaries and twin secondaries. primaries are 2x 0-115v and secondaries are 2x 0-30v
If the mains is fed in across the secondaries, then this should result in a 440-0-440 output, on what used to be the primaries. Is the insulation likely to be up to this? Or any other factors I haven't considered, like the power handling of the primary. It would be running the HT supply of the WAD Kat6550 circuit, which is a push-pull 6550 power amp.
World Audio Design
The secondaries are rated at 8.5amps, so I guess the primary should match this power handling (8.5x30 = 255W??). If the primary wasn't up the the current loading, which I can't imagine too easily, I guess it would be possible to parallel two toroids. The think is these toroidal transformers are only £30, so it's very tempting!!

Thanks,
Steve

markp · 2nd April 2004, 03:26 PM

I've done it before with no problems but make sure you've got good insulation on your new primaries(secondaries). If its a 120/240 transformer then you've got a center tap on your new secondaries! Remember to fuse the input.

speaker · 2nd April 2004, 03:28 PM

I've heard of taking 2 identical filament transformers and driving the secondaries of one into the other, in essence making it 1:1. Then rectfying/doubling that to get suitable B+, but never what you propose.

speaker

leadbelly · 2nd April 2004, 03:32 PM

NO! This is a dangerous practice. Never put ANYTHING that is not specifically mains rated on your mains: transformers, capacitors, coils, etc.

baggystevo82 · 2nd April 2004, 03:34 PM

Quote:

I've done it before with no problems but make sure you've got good insulation on your new primaries(secondaries). If its a 120/240 transformer then you've got a center tap on your new secondaries! Remember to fuse the input.

yes it is a 120/240, thats what drew me to it

toroidals
it's the 500VA 2x 0-30v one here I was considering.
Gotta be worth a go, i suppose nothing can go that stupidly horrendously wrong if its fused....
Cheers
Steve

baggystevo82 · 2nd April 2004, 03:36 PM

hmmm, yes i thought it was a bit dodgy, which is why i asked here for some opinions. if it worked ok in a lab (im still at uni so i can test it easily enough, and overtest just to make sure) then how much of a hazard would it really be? if any?
Thanks again,
Steve

leadbelly · 2nd April 2004, 03:40 PM

IMHO, this discussion has gone far enough and the mods should send it to Texas.

baggystevo82 · 2nd April 2004, 03:40 PM

hehe ok fair enough

HDTVman · 2nd April 2004, 04:02 PM

Quote:

Originally posted by leadbelly
IMHO, this discussion has gone far enough and the mods should send it to Texas.

I agree very dangerous.

BZ

dhaen · 2nd April 2004, 11:02 PM

If a winding, whether primary or secondary has higher than its rated voltage applied, there'll be trouble. So your original idea is no-go.

However, there is nothing wrong with using a transformer to step-up, just as speaker described.
There is no way though, that you'll get the voltages you want, without a proper transformer.

You don't have to pay the money you are thinking. They sometimes crop up on eBay second hand.

I'm closing this thread before any really dangerous suggestions are made

2nd April 2004, 03:18 PM	#1
baggystevo82 diyAudio Member Join Date: Dec 2003 Location: Herefordshire	Using the secondary of a normal transformer as the primary to get a HT supply Hello, Im being a skinflint and don't really want to fork out 120 quid for a mains transformer. I have a 500VA toroidal in mind, with twin primaries and twin secondaries. primaries are 2x 0-115v and secondaries are 2x 0-30v If the mains is fed in across the secondaries, then this should result in a 440-0-440 output, on what used to be the primaries. Is the insulation likely to be up to this? Or any other factors I haven't considered, like the power handling of the primary. It would be running the HT supply of the WAD Kat6550 circuit, which is a push-pull 6550 power amp. World Audio Design The secondaries are rated at 8.5amps, so I guess the primary should match this power handling (8.5x30 = 255W??). If the primary wasn't up the the current loading, which I can't imagine too easily, I guess it would be possible to parallel two toroids. The think is these toroidal transformers are only £30, so it's very tempting!! Thanks, Steve

2nd April 2004, 03:26 PM	#2
markp diyAudio Member Join Date: Oct 2003 Location: L.A., CA	I've done it before with no problems but make sure you've got good insulation on your new primaries(secondaries). If its a 120/240 transformer then you've got a center tap on your new secondaries! Remember to fuse the input. __________________ If it sounds good... it is good!

2nd April 2004, 03:32 PM	#4
leadbelly diyAudio Member Join Date: Dec 2002 Location: Calgary, Alberta	NO! This is a dangerous practice. Never put ANYTHING that is not specifically mains rated on your mains: transformers, capacitors, coils, etc. __________________ Aerodynamics are for people who can't build engines. Enzo Ferrari

2nd April 2004, 03:40 PM	#7
leadbelly diyAudio Member Join Date: Dec 2002 Location: Calgary, Alberta	IMHO, this discussion has gone far enough and the mods should send it to Texas. __________________ Aerodynamics are for people who can't build engines. Enzo Ferrari

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2nd April 2004, 03:28 PM	#3
speaker diyAudio Member Join Date: Mar 2004 Location: USA	I've heard of taking 2 identical filament transformers and driving the secondaries of one into the other, in essence making it 1:1. Then rectfying/doubling that to get suitable B+, but never what you propose. speaker

2nd April 2004, 03:36 PM	#6
baggystevo82 diyAudio Member Join Date: Dec 2003 Location: Herefordshire	hmmm, yes i thought it was a bit dodgy, which is why i asked here for some opinions. if it worked ok in a lab (im still at uni so i can test it easily enough, and overtest just to make sure) then how much of a hazard would it really be? if any? Thanks again, Steve

2nd April 2004, 11:02 PM	#10
dhaen diyAudio Moderator Emeritus Join Date: Jun 2002 Location: U.K.	If a winding, whether primary or secondary has higher than its rated voltage applied, there'll be trouble. So your original idea is no-go. However, there is nothing wrong with using a transformer to step-up, just as speaker described. There is no way though, that you'll get the voltages you want, without a proper transformer. You don't have to pay the money you are thinking. They sometimes crop up on eBay second hand. I'm closing this thread before any really dangerous suggestions are made

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