Can I draw 2A from a .7A secondary (toroid) - diyAudio
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Old 12th April 2012, 10:01 AM   #1
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Default 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...
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Old 12th April 2012, 10:14 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
transformer can in general tolerate a massive short term overload.
The limiting factor is heat. Heat in the wire, heat in the core and the effect this heat has on the insulation.

An audio amplifier rarely runs at full power. Most high power delivery situations are short to medium term.

Transient current delivered to the amplifier load comes from the decoupling and smoothing capacitors, not from the transformer feeding the rectifier.

The transformer re-charges the smoothing capacitors. It does not supply transient current to the amplifier.

Now back to your question.
The generally accepted advice for powering a ClassAB amplifier is to provide a transformer where
VA ~ 1times to 2times the total maximum power that the amplifier/s can deliver to your load.
The amplifier will work when the transformer is outside that VA range. Just ensure a small transformer does not overheat. If it feels hotter than your hand on the surface then try to imagine how hot it is in the middle.

When you started to open your new thread, how many references to older threads did the Forum suggest you read?
Did you bother to read any of them?
All of the information I provided is repeatedly posted, by many Members, in various earlier Threads.
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Last edited by AndrewT; 12th April 2012 at 10:17 AM.
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Old 12th April 2012, 10:16 AM   #3
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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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Old 12th April 2012, 11:28 AM   #4
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Quote:
Originally Posted by AndrewT View Post
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 View Post
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.
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Old 12th April 2012, 12:11 PM   #5
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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.
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Old 12th April 2012, 12:13 PM   #6
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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.
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Old 12th April 2012, 12:19 PM   #7
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Quote:
Originally Posted by KatieandDad View Post
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 View Post
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.
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Old 12th April 2012, 12:22 PM   #8
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Quote:
Originally Posted by SoNic_real_one View Post
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.
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Old 12th April 2012, 02:46 PM   #9
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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
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Old 12th April 2012, 02:47 PM   #10
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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 02:59 PM.
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