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Old 29th September 2003, 12:02 AM   #1
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Default Transformer question

Most amplifiers use a power supply
with a center tapped transformer on
the secondary windings. For example,
let say the transformer is rated for
1KVA, about 100v output with 10 amperes
roughly.

If I use the center tap, I can get
(50v @ 10A = 500VA) * 2 = 1KVA

Hypothetical;

What if I don't use both halves of the
secondary, what if I use only the
50V @ 10A section and don't use
the other half. Can this section
output 50V @ 20A because the other
side is not used ?

If you design a dual rail power supply
with a 1KVA transformer, can I get
1KVA out of each rail when only
one side is used at a time ?
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Old 29th September 2003, 12:15 AM   #2
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Default Re: Transformer question

Quote:
Originally posted by thylantyr
What if I don't use both halves of the
secondary, what if I use only the
50V @ 10A section and don't use
the other half. Can this section
output 50V @ 20A because the other
side is not used ?

No. The amerage output is limited by the wire gauge. In essence, you have a 500va transformer.

However, if the CT isn't soldered internally (so there are four wires on the 2ndary), you can parallel the two windings (make absolutely sure that they are phase matched).
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Old 29th September 2003, 12:18 AM   #3
AJT is offline AJT  Philippines
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Default depends on the wire used

i doubt that you can get 20amps, but i am sure you can get more than 10amps if you just use one-half, the magnet wire used may not be able to carry that much current, if you can have the 2 windings in parralell, then no doubt you can have it....
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Old 29th September 2003, 02:33 AM   #4
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I would bet you could get close to 900 VA but with a little looser regulation than 2x50 setup. Temp rise is the real limit. Surface area on the transfo will be the same, resistive heating will be doubled in the one winding if current is doubled, but there will be no heating in the unused coil, so you'll have the same total temp rise by only loading one coil. But that coil may not be able to dissapate as well and could risk cooking varnish, so don't expect UL approval.

This is not the best way to set up a good psu, but you will have much more current reserves than a 50V 500VA transformer, but don't expect 1200 VA continuous.

Brian/ putting on flameproof suit and awaiting attacks from all sides.
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Old 29th September 2003, 02:37 AM   #5
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This is just a theoretical question,
wire gauge is not factored in.

Hypothetical;
Assume 100% efficient amplifier.

If your ideal amplifier is 1000 watts
and you need to design a power
supply, do you need just a 1KVA transformer (500VA per rail)
or do you need a 2KVA
transformer (1KVA per rail)
to supply power to your 1000w amp?
Only one rail operates at any given
moment in time.

I realize this is not the ideal way
to design a power supply as most
good engineers factor in safety margins,
but I just want to know if it works
this way.
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Old 29th September 2003, 04:25 AM   #6
djk is offline djk
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A normal amplifier is ~70% efficent, to put out 70W it will need 100W in.

A normal amplifier power supply will have the rectifiers feeding large filter caps, a brute force pseudo regulated supply if you will.

The power factor on this will be ~.7, so you will need a 100VA trafo to output 70W.

So we need a 140VA trafo for a 70W amplifier.

Highly compressed rock/rap music has a 50% duty cycle, but a real speaker may look like a 45* load. Call it a wash.

Rule-of-thumb = 2VA for 1W out.

I have built as low as 1VA for 1W out. It sounded fine until driven into clipping (no one ever does this do they?). Going to 2VA for 1W made the amplifier sound much more 'solid'. Going above 2VA for 1W gave diminishing return.

Running a 50mA regulated supply for the front end of the amplifier did far more than going above 2VA for 1W, and cost far less.

Amplifiers I have owned that were built like this:

McIntosh (series regulator for front end)
Electrocompaniet (series regulator for front end)
Rowland (shunt regulator for front end)
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Old 29th September 2003, 05:08 AM   #7
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Thanks for the replies, here is a
picture to help me figure out the
mystery.

1KVA secondary, 100v@10A.

Above the center tap, the output
is 50v@10A, below the center tap,
the output is also 50v@10A.

Positive and negative rail voltage,
+50v and -50v.

If I don't use the 'negative' rail
at all, is it possible to get
50v@20A from the positive rail
assuming ideal conditions.

50v@20A = 1KVA since the bottom
1/2 of the transformer is not used,
is this theoretically possible ?

Lets vote

For those who wonder, the answer
I seek is related to this post.
H Bridge 'subwoofer' amplifiers
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Old 29th September 2003, 08:05 AM   #8
djk is offline djk
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"Lets vote "

You counting wrong answers?

The best I would figure on long term would be an increase in current of 1.414, IOW about 70% of what you could get if you ran the trafo the way it was designed.

Study this:

http://www.amveco.com/Technical_Notes_3.htm
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Old 29th September 2003, 05:11 PM   #9
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Quote:
Originally posted by djk
"Lets vote "

You counting wrong answers?

The best I would figure on long term would be an increase in current of 1.414, IOW about 70% of what you could get if you ran the trafo the way it was designed.

Study this:

http://www.amveco.com/Technical_Notes_3.htm

Thanks, that works for me. An unsual question that has
some folks scratching the brain
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Old 29th September 2003, 06:41 PM   #10
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Quote:
Originally posted by thylantyr
[snip]If your ideal amplifier is 1000 watts
and you need to design a power
supply, do you need just a 1KVA transformer (500VA per rail)
or do you need a 2KVA
transformer (1KVA per rail)
to supply power to your 1000w amp?
Only one rail operates at any given
moment in time.
[snip]
You have here a hughe safety factor. Unless the amp is used to drive a lamp or something at 1kW constantly, the real output of the amp over time may be no more than 1/10th of that, even with rock music. During the 1kW peaks, it's up to the supply caps to supply the current to the output stage. I mean, the bridge diodes only conduct 10% or so of each mains cycle anyway, so the xformer is effectively disconnected from the amp 90% of the time. Go figure.

Jan Didden
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