Reversing mains toroids for HT

andyjevans · 8th December 2005, 11:49 PM

I've been ruminating on how to use two transformers back to back, let's say two mains toroids with 230v primaries (I'm in UK) and twin 12v secondaries.
If these are 60VA toroids, then with the first transformer I can use one 12v winding of 30VA for heaters, leaving the other winding to feed the second transformer.
The second transformer is reversed, so 12v in gives 230v out.
My question is how to calculate the VA rating of that 230v out. Do I just say "it's 30VA" so at 230v this gives me 130mA?

How do I calculate other factors - winding losses? Is there any point in using anything larger than a 30VA transformer for the second (reversed) one?

If I can get all this to work, it's a cheap way of constructing a mains transformer for a preamp. I even thought about taking the minus -15vDC bias for the CCS off the 12v heater winding, assuming that's AC. Is all this feasible? Andy

Tom Bavis · 9th December 2005, 12:46 AM

Sure, it'll work. Expect a bit lower voltage than the 230 nominal for the second transformer. In your example, you're limited to 30 VA from the second transformer since that's all the first transformer has left. A larger transformer for the first one would allow the full 60VA from the second one.

Double the resistive losses, double the weight of a single transformer of the right ratings. But who cares - if they're cheap enough!

nafunga · 9th December 2005, 08:09 AM

I agree.Although frowned on by some - Iv'e used this back to back method on several occasions for both heater and ht supplies with pre-amp and DAC o/p stages. One has been in use for around 6 years, with no ill effects.

Regards,

David.

dhaen · 9th December 2005, 09:39 AM

All perfectly feasible. As for VA reduction, I've used -20% and not come unstuck yet. But it's give and take all round. A twin secondary transformer can supply slightly more than 50% current if just one winding is used (with some voltage droop) etc..
What is useful, is using different transformers EG a 12v into a 15v, for reduced final voltage out. Of course you can't do it the other way round.

phase_accurate · 9th December 2005, 10:24 AM

I used this trick on a bass-preamp which is mainly solid-state except a triode input stage.
It works reliably.

I wouldn't use the principle to feed tube power-apms though !

Regards

Charles

andyjevans · 9th December 2005, 11:21 AM

A further question - which of these is the best method? Assume a 12-0 12-0 toroid.

a) Use one half of the secondary to feed the second transformer and the other half for the 12v heaters?

b) Parallel up the secondaries and take both the 12v heaters and the second transformer off them?

c) Take the second transformer off the start of the first winding and the end of the second, and connect the second transformer in series. Does this then mean I can use both the 12v windings on the first transformer seperately?

Tom Bavis · 9th December 2005, 01:53 PM

(b) and (c) are best as you use all the copper - (b) will be easier to load evenly, (c) can give you as much as -35V if you rectify it. Use a resistor divider to make a center tap if you're using AC filaments, or if you're rectifying, ground one side of the DC.

AudioFreak · 9th December 2005, 02:03 PM

Don't do C!

8th December 2005, 11:49 PM	#1
andyjevans diyAudio Member Join Date: Sep 2003 Location: London	Reversing mains toroids for HT I've been ruminating on how to use two transformers back to back, let's say two mains toroids with 230v primaries (I'm in UK) and twin 12v secondaries. If these are 60VA toroids, then with the first transformer I can use one 12v winding of 30VA for heaters, leaving the other winding to feed the second transformer. The second transformer is reversed, so 12v in gives 230v out. My question is how to calculate the VA rating of that 230v out. Do I just say "it's 30VA" so at 230v this gives me 130mA? How do I calculate other factors - winding losses? Is there any point in using anything larger than a 30VA transformer for the second (reversed) one? If I can get all this to work, it's a cheap way of constructing a mains transformer for a preamp. I even thought about taking the minus -15vDC bias for the CCS off the 12v heater winding, assuming that's AC. Is all this feasible? Andy

9th December 2005, 02:03 PM	#8
AudioFreak Warp Engineer On Holiday Join Date: Oct 2001 Location: Queensland, Australia	Don't do C! __________________ - Dan

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9th December 2005, 12:46 AM	#2
Tom Bavis diyAudio Member Join Date: Jun 2002 Location: Macedon NY	Sure, it'll work. Expect a bit lower voltage than the 230 nominal for the second transformer. In your example, you're limited to 30 VA from the second transformer since that's all the first transformer has left. A larger transformer for the first one would allow the full 60VA from the second one. Double the resistive losses, double the weight of a single transformer of the right ratings. But who cares - if they're cheap enough!

9th December 2005, 08:09 AM	#3
nafunga diyAudio Member Join Date: Jul 2005 Location: Whitstable , Kent. UK	I agree.Although frowned on by some - Iv'e used this back to back method on several occasions for both heater and ht supplies with pre-amp and DAC o/p stages. One has been in use for around 6 years, with no ill effects. Regards, David.

9th December 2005, 09:39 AM	#4
dhaen diyAudio Moderator Emeritus Join Date: Jun 2002 Location: U.K.	All perfectly feasible. As for VA reduction, I've used -20% and not come unstuck yet. But it's give and take all round. A twin secondary transformer can supply slightly more than 50% current if just one winding is used (with some voltage droop) etc.. What is useful, is using different transformers EG a 12v into a 15v, for reduced final voltage out. Of course you can't do it the other way round.

9th December 2005, 10:24 AM	#5
phase_accurate diyAudio Member Join Date: May 2002 Location: Switzerland	I used this trick on a bass-preamp which is mainly solid-state except a triode input stage. It works reliably. I wouldn't use the principle to feed tube power-apms though ! Regards Charles

9th December 2005, 11:21 AM	#6
andyjevans diyAudio Member Join Date: Sep 2003 Location: London	A further question - which of these is the best method? Assume a 12-0 12-0 toroid. a) Use one half of the secondary to feed the second transformer and the other half for the 12v heaters? b) Parallel up the secondaries and take both the 12v heaters and the second transformer off them? c) Take the second transformer off the start of the first winding and the end of the second, and connect the second transformer in series. Does this then mean I can use both the 12v windings on the first transformer seperately?

9th December 2005, 01:53 PM	#7
Tom Bavis diyAudio Member Join Date: Jun 2002 Location: Macedon NY	(b) and (c) are best as you use all the copper - (b) will be easier to load evenly, (c) can give you as much as -35V if you rectify it. Use a resistor divider to make a center tap if you're using AC filaments, or if you're rectifying, ground one side of the DC.

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