Can I draw 2A from a .7A secondary (toroid)
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 12th April 2012, 09:01 AM #1 ric-paul   diyAudio Member   Join Date: May 2005 Location: Rotterdam Netherlands Can I draw 2A from a .7A secondary (toroid) Hi, I have a toroid with 2 double secondary windings. I do not know their output, but here's my guess: 2x20V~ 0.75A 2x30V~ 2.0A The current is assumed from the thickness of the windings and the total power ~120 to 150VA. I have a PCB with two TDA7294's. Can I switch between the 30V and the 20V windings for 8 ohm and 4 ohm operation? This would mean I draw much more than the 0.75A from the 20V winding, BUT the 30V will not be in use then. I estimate the wire thickness of the 20V winding at 20AWG or 0.5mm^2. But it has been years since I actually saw it... The 30V is probably 17AWG or 1mm^2. I understand the total power of the toroid (150VA max.) is small compared to the power of the PCB (150Wrms), but it is just for an as_small_as_possible test-amplifier to be build from scrap I have lying around, and I seem to have a severe lack of 18-22V toroids in my collection...
sofaspud
diyAudio Member

Join Date: Nov 2005
Location: San Antonio
Quote:
 This would mean I draw much more than the 0.75A from the 20V winding, BUT the 30V will not be in use then.
Not sure of your line of reasoning here.

You can draw 2 amps from a 750mA winding, but probably not for very long...
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It is error only, and not truth, that shrinks from enquiry. - Thomas Paine

ric-paul
diyAudio Member

Join Date: May 2005
Location: Rotterdam Netherlands
Quote:
 Originally Posted by AndrewT Hi, When you started to open your new thread, how many references to older threads did the Forum suggest you read?
4 or 5 iirc.

Quote:
 Originally Posted by AndrewT Hi, Did you bother to read any of them?
No, the thread titles of the suggestions did not seem to reflect my specific question.
If I was wrong in assuming this, I sincerely apologize.
I did do a Google search before posting, but it also did not turn up a clear-cut answer.

Obviously I know that it is unlikely that my questions have not been answered before somewhere on the net, and I always *try* to find out before posting here or on any other forum. It is very possible that I am simply not that good at searching because of e.g. not using the correct terms.
So despite apparently giving the impression that I am a lazy git that just posted here so I wouldn't have to search for myself I am convinced that I am not that person.

As for your answers: thanks! It has been made clear that I can do what I suggested, as long as I simply watch the temperature of the transformer somehow. If it gets to hot during normal conditions running 4 ohm speakers, I will have to stop using it at 4 ohm.
Whether I will actually go this route has yet to be determined. I have some time before I can complete the PCB's (parts not in yet), so I may find another solution before then.

 12th April 2012, 11:11 AM #5 KatieandDad   diyAudio Member     Join Date: Nov 2011 Location: UK Why not simply wind an extra few turns onto the toroid and switch it in anti-phase when you need the lower voltage. Then you will always be using the higher current windings.
sofaspud
diyAudio Member

Join Date: Nov 2005
Location: San Antonio
Quote:
 made clear that I can do what I suggested
Not sure of that line of reasoning either, but do not forget to insert a fuse between the mains and primary winding. I repeat, do not forget to insert a fuse between the mains and primary winding.
__________________
It is error only, and not truth, that shrinks from enquiry. - Thomas Paine

SoNic_real_one
diyAudio Member

Join Date: Jan 2008
Location: Virginia
Quote:
 Originally Posted by KatieandDad Why not simply wind an extra few turns onto the toroid and switch it in anti-phase when you need the lower voltage. Then you will always be using the higher current windings.
That's a no-no. Those "backwards" turns will act like a short-circuit.
Quote:
 Originally Posted by ric-paul I understand the total power of the toroid (150VA max.) is small compared to the power of the PCB (150Wrms), but it is just for an as_small_as_possible test-amplifier to be build from scrap I have lying around, and I seem to have a severe lack of 18-22V toroids in my collection...
Yes, you can use the transformer as described, but not to the maximum power, only just to some 40%. You could burn up that 20V secondary.

diyAudio Member

Join Date: Nov 2011
Location: UK
Quote:
 Originally Posted by SoNic_real_one That's a no-no. Those "backwards" turns will act like a short-circuit.
WRONG

It is like connecting a battery in reverse it will just subtract from the initial voltage.

 12th April 2012, 01:46 PM #9 ingenieus   diyAudio Member   Join Date: Apr 2011 Location: Pretoria My first thought was that a trafo like this is ideal for a class G project, but the higher voltage windings can't deliver the high current required. Class-G Amplifiers
 12th April 2012, 01:47 PM #10 ric-paul   diyAudio Member   Join Date: May 2005 Location: Rotterdam Netherlands Edit: the below is an answer to KatieandDad and SoNic_real_one. To ingenieus: I know (and hate) the class G concept... (it never worked for me and clipped at 25V with a supply of +/-20 and +/-40V... Never got to the bottom of it). I do have one working and one broken toroid (300VA, the broken one may have a break where the primary and the 10% primary are coupled) left over from that project if you're interested I have considered this before and couldn't work out which theory would be correct... Originally I had transformers wound for a project with 7294's. It's the left-over TDA7294's that led me to this question in the first place. But those transformers were wound for the class-G application with dual supply rails (+/-20 and +/-40 DC) But since getting them custom wired anyway, I added an extra 10% primary winding (again for 4 or 8 ohm operation). If I could wire this in reverse, instead of 'series', it would LOWER my 2x30V~ outputs to 2x27V~, which would make it suitable for 8ohm operation... So who is correct? Last edited by ric-paul; 12th April 2012 at 01:59 PM.

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