Old power supply for GC

[Vikash]

I'm going through the learning process to make my first GC, and starting to hunt down parts. If possible I'd like to use parts that I can pull out of old equipment for my first one. So far I've opened up an old Pionerr SA-540 amplifier and pulled out a transformer.

I think this is an E-l type (which doesn't mean anything to me), but is it suitable?

[officeboy]

It could be, depends on voltage, and some of that stuff. Chances are the VA rating is ok, (just guessing by size).

About the only way to know for sure if this will work ok is to plug it in and measure the secondaries voltage. (Be sure to fuse it or something to keep yourself relatively safe)

Each leg should be the same voltage when measured against the middle, and the 2 against each other should be double.

Anything from about 14V to 30V (each leg) should work

[Vikash]

The wires on the primary winding are brown = live, white = neutral, and yellow = ?. I first assumed it as ground and blew the fuse. I don't want to connect it again without some confirmation on how to connect it...

[Vikash]

For now I'll assume the yellow leads to the second of two primary windings. Connecting with the standard brown and white works ok.

I measured both legs: 28.08V and 28.10V, and then between legs: 55.05V. It seems to be at the ceiling of what the chip will handle (LM3875).

Can I, and how do I wire the transformer up to power both channels (two chips)?

[azira]

55 * 1.414 -1.4 = about 75 Volts. With +/- 10% mains variation that is RIGHT on the edge of what a GC will handle.

You can make a standard dual rail powersupply and then just run both chips off of it in parallel and it will work fine. If you really want to isolate the channels you can put rectifier circuits in parallel and run each channel from it's own rectified source.

I would suggest that you find some way to burn off a few volts to be a little more cautious. Rod Elliots cap multiplier circuit might be a good method, it takes about 4Volts to run and would provide a cleaner powersupply.
--
Danny

[jackinnj]

here's a good way of checking unknown power transformers:

find the two common leads with the highest resistance. Connect these to a signal generator with a 1kHz, 1 volt sine signal. the impedance should be such that this doesn't appreciably load the sig generator.

measure the output on the other pairs of leads (center taps, etc.)

the voltage ratios will give you the turns ratios, etc.

you can get a reasonable approximation of the VA rating of the transformer from the core cross-sectional area, I must have posted this a year ago or so...

[tpenguin]

Originally posted by jackinnj
here's a good way of checking unknown power transformers:

find the two common leads with the highest resistance. Connect these to a signal generator with a 1kHz, 1 volt sine signal. the impedance should be such that this doesn't appreciably load the sig generator.

measure the output on the other pairs of leads (center taps, etc.)

the voltage ratios will give you the turns ratios, etc.

you can get a reasonable approximation of the VA rating of the transformer from the core cross-sectional area, I must have posted this a year ago or so...

I have an even quicker method that provides (in my experience) about a ±5% accuracy. I Simply measure the DC resistance of the primary, and divide by the DC resistance of the secondary. This is (assuming the same wire gage on all the windings) almost always going to be equal to the primary:secondary voltage ratio. I have found this very useful for measuring unknown transformers, and it requires nothing more than a multimeter. Then, you can of course connect it to the AC mains with a 1-3A fuse in series with the hot wire, and (very carefully) measure the voltages.

This is definitely also the best method to use if you come across a transformer, and it is unclear when is the primary and which is the secondary. I once quite stupidly plugged in such a transformer to test it (also quite stupidly with no fuse), and I instantaneously tripped the circuit breaker, because I had connected the mains to the secondary winding. Later I had the idea to measure the DCR's, and found the ratio to be ~10:1, and when I properly tested it later, I read about 13V on the secondary, which means my approximation was pretty accurate.

[Vikash]

I'm not sure what to do with that yellow wire. help!

primary side
yellow---white---brown

secondary side
red---black---red

The resistance between yellow and white is 20 ohms, and the resistance between brown and white is 22 ohms (2 ohms between yellow and brown).

The resistance between each red and black pair is 0.6 ohms (and 1.2 between both reds).

I've connected the brown and white to the switch, but I don't know what to do with the yellow. Can someone tell me if this is a single primary and single secondary transformer, or something different?

[maylar]

Seems like a tap on the primary for different voltages. Leave it disconnected.

[officeboy]

Might be a way to drop your secondary voltage a bit.
What is the secondary voltage with the yellow and white hooked up to 115V?