Determine Transformer details & safe testing method? - Page 1
Forum section: electronics / beginners

author: Robbie010
author_rank: Contributor
timestamp: February 07, 2019, 12:27:05 pm
content: 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 - 50-60HzPrimary 1 - 120vPrimary 2 - 220vSecondary - 2 x 23.5 volt 6ACan 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

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 07, 2019, 12:40:06 pm
content: 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.

author: Robbie010
author_rank: Contributor
timestamp: February 07, 2019, 01:59:58 pm
content: Quote from: TERRA Operative on February 07, 2019, 12:40:06 pmSorry, 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?

author: mariush
author_rank: Super Contributor
timestamp: February 07, 2019, 02:09:14 pm
content: 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.

author: Robbie010
author_rank: Contributor
timestamp: February 07, 2019, 05:34:37 pm
content: 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 VoltsSecondary to Secondary 55 VoltsCentre Tap to Secondary 28 VoltsCentre Tap to Secondary 28 VoltsResistance:Primary to Primary 4500 ohmsSecondary to Secondary 400 ohmsCentre Tap to Secondary 200 ohmsCentre Tap to Secondary 200 ohmsAm I correct in saying that the windings ration is 250/28 so roughly 9:1OrAs this is a centre tapped transformer is it 250/55 so roughly 4.5:1??