Are A/V receivers good source for transformers for PSU?

[Hammy]

I've been looking for a suitable transformer for building a 300W PSU
0-30V @ 0-10A. My 2.5A one just isn't cutting it anymore. I already
have everything I need with the exception of the transformer.

I was wondering if older A/V receivers would be a good place to
scrounge a transformer. There's a couple of pawnshops in town that
sell old ones for anywhere from $20 bucks and up. How could I tell
without opening up and measuring the secondary of the transformer what
the secondary is. I know the question is a bit weird but I'm guessing
an audiophile would know what a typical secondary is. What are good
models if any that I'm likely to have success?

Ideally I would like a 25.2V (stancor makes these) secondary but this
is probably reaching but is a 28VAC secondary common?

Another alternative is to use two of the seventy watt Flyback
transformers I have to construct an interleaved flyback at 140w. Then
put a linear reg on the output to adjust the voltage down.

Any suggestions?

Thanks

 

[Bob Engelhardt]

Hammy wrote:

...
Any suggestions?

Get 2 microwave oven transformers of approximately the same physical
size. Separate the E & I sections (they are not interleaved) by cutting
the welds. Remove the secondaries & one of the primaries. Remove some
of the turns on that primary & install it on the core with the other
primary. Reweld the sections, clamping tightly to avoid an air gap.

The number of turns you'll need to keep can be calculated, but should be
checked under power before the reweld.

I just did this to get a 40v @ 10a xformer. Finding one to buy wasn't
easy & the prices offended my chea..., er, frugal nature. MOT's are free.

Bob

 

[Bob Masta]

On Tue, 13 Jan 2009 11:29:44 -0500, Hammy
<spamme@hotmail.com> wrote:

I've been looking for a suitable transformer for building a 300W PSU
0-30V @ 0-10A. My 2.5A one just isn't cutting it anymore. I already
have everything I need with the exception of the transformer.

I was wondering if older A/V receivers would be a good place to
scrounge a transformer. There's a couple of pawnshops in town that
sell old ones for anywhere from $20 bucks and up. How could I tell
without opening up and measuring the secondary of the transformer what
the secondary is. I know the question is a bit weird but I'm guessing
an audiophile would know what a typical secondary is. What are good
models if any that I'm likely to have success?

Ideally I would like a 25.2V (stancor makes these) secondary but this
is probably reaching but is a 28VAC secondary common?

Another alternative is to use two of the seventy watt Flyback
transformers I have to construct an interleaved flyback at 140w. Then
put a linear reg on the output to adjust the voltage down.

Any suggestions?

Thanks

In order to estimate the transformer specs, you'd
need to know the receiver's rated power output.
Then you can use the relationship that power
P = V^2/R and also P = I^2 * R.

Let's say each channel of the amp is rated at 100
Watts into 8 ohms. To find the RMS voltage the
amp is putting out at that rating, you use

V^2 = P*R = 100*8 = 800

V = sqrt(800) = 28.28

But remember that's RMS, so peak is 1.414 times
that or 40V. And also remember that you have plus
and minus supllies, so 80V total. That's the
minimum the transformer is putting out after
rectification and filtering. (NOTE that this
assumes the specs were on the level... probably
true for a decent name-brand receiver, unlikely
for a no-name.)

As for the current:

I^2 = P/R = 100/8 = 12.5

I = sqrt(12.5) = 3.53 amps

Again, remember that this is RMS current, so the
peak will be 1.414 times higher. However, the RMS
is probably the value you really want anyway.
Note that if the spec is for N channels driven,
you multiply the per-channel current by N.

In the Good Old Days of simple stereo, that would
be all you'd need. But modern A/V amps may spec
different powers to different channels, so you may
have to do some extra calculations and a bit of
guesswork.

Best regards,


Bob Masta

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