Electronics > Beginners
Determine Transformer details & safe testing method?
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Robbie010:
My project amplifier has dual power supplies, one for the left channel and one for the right channel.

The amp was german made and the markings on the transformers were hand typed rather than a printed label, they read as follows:

300VA - 50-60Hz

Primary 1 - 120v
Primary 2 - 220v

Secondary - 2 x 23.5 volt 6A

Can anyone tell me how to work out the winding ratio of the transformers and also, 23.5 volts seems an odd number, is there a safe way to test the transformer when disconnected from the amplifier circuit?

Thanks

TERRA Operative:
Winding ratio is basically input voltage:output voltage + a fudge factor for copper and iron losses (which will be dependant on the transformer in question).
Like a 120V to 12V transformer will be about 10:1 turns ratio, plus a bit on the secondary side, so more like 10:1.1 or 10: 1.2 or so.
To be 100% sure of turns, you'll need to unwind the transformer or find a datasheet.

To test the output voltage, just disconnect the output of the transformer from your circuit and turn it on with a voltmeter actoss the secondary.
You'll find the open circuit voltage is a bit higher than under load, so you can 'back probe' the secondary connector while it is still connected to the circuit board (or another set load) to get the voltage under load.

Knowing the input voltage, and loaded (and load current) and unloaded output voltages with a little math can help you find more of the transformers parameters.
Robbie010:

--- Quote from: TERRA Operative on February 07, 2019, 12:40:06 pm ---Winding ratio is basically input voltage:output voltage + a fudge factor for copper and iron losses (which will be dependant on the transformer in question).
Like a 120V to 12V transformer will be about 10:1 turns ratio, plus a bit on the secondary side, so more like 10:1.1 or 10: 1.2 or so.
To be 100% sure of turns, you'll need to unwind the transformer or find a datasheet.

To test the output voltage, just disconnect the output of the transformer from your circuit and turn it on with a voltmeter actoss the secondary.
You'll find the open circuit voltage is a bit higher than under load, so you can 'back probe' the secondary connector while it is still connected to the circuit board (or another set load) to get the voltage under load.

Knowing the input voltage, and loaded (and load current) and unloaded output voltages with a little math can help you find more of the transformers parameters.

--- End quote ---

Sorry, another stupid question........

The the secondary wires are soldered to a bridge restifier which then leads nowhere as I have disconnected it from the smoothing capacitors. Would I need to de-solder the secondry wires from the rectifier before testing or could I take the readings from the soldered joint?

Would this potentially damage the rectifier?
mariush:
The output of the rectifier is still connected to whatever's AFTER the smoothing capacitors, removing the capacitors shouldn't disconnect the rectifier from the rest of the circuit.

If you want to test the no load voltage, you'd have to desolder or disconnect the transformer wires from the board. Load voltage should be tested with capacitors in, you don't want the amplifier chips and everything else to break down due to voltage fluctuations.


Robbie010:
Thanks all.

I used the method mentioned and hooked the transformer up to the mains, I also tested the resistance of the windings and got the following results:

Voltage:

Input Voltage 249 Volts
Secondary to Secondary 55 Volts
Centre Tap to Secondary 28 Volts
Centre Tap to Secondary 28 Volts

Resistance:

Primary to Primary 4500 ohms
Secondary to Secondary 400 ohms
Centre Tap to Secondary 200 ohms
Centre Tap to Secondary 200 ohms

Am I correct in saying that the windings ration is 250/28 so roughly 9:1

Or

As this is a centre tapped transformer is it 250/55 so roughly 4.5:1??
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