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mattjuk81 30th March 2012 02:13 PM

LM4780 power supply voltages?
Just wired up two transformers 2x25V to the power supply boards, when messuring the voltages V- PG- and V+ PG+ on both boards they started off at about 44v to 38v dc, and gradually went lower and lower to about 22v dc after one rectifier and about 33v dc on another, on the other board they did kind of the same, one was staying at about 30v dc, is this normal without a load connected?

These are the transformers I am using

If you look at the data sheet on the link, I am not sure if I have wired them correctly, I have connected the Grey and Violoet wires together, and the Blue + Brown are connected to the mains?

Marra 30th March 2012 03:01 PM

Your mains is wired correctly.I seem to remember when I connected mine up without the amp board but with the 10uf caps on the p/s board I had +- 38vdc which dropped to 36v with the chip pcb's wired in.

mattjuk81 30th March 2012 03:03 PM

Ok I'm going to answer my own question, and I am an idiot! I didn't noticed but the fuse had blown.

So replaced the fuse with the correct type and I now messure 40v dc at each point on both boards, but this creeps upto 47v dc on each board, anyone know why this is please?

Surely with 2x25v it should give about 2x35v dc after the rectifier...25 x 1.414 = 35.35 VDC?

woodturner-fran 30th March 2012 03:11 PM

Mine gave 36V + and - after the 1st smoothng cap. Deinfitely no gradual rising of the voltage. Are you sure you're measuring correctly?


mattjuk81 30th March 2012 03:25 PM


Originally Posted by woodturner-fran (
Mine gave 36V + and - after the 1st smoothng cap. Deinfitely no gradual rising of the voltage. Are you sure you're measuring correctly?


I think so, meter set to DC and I placed the meter probes on PG- & V- and then PG+ & V+, they all sit at 40v for a moment, then rise to 47v

woodturner-fran 30th March 2012 03:49 PM


To test things further: change your meter to AC and measure the AC volts at the outputs of the transformer(s). What do you get and does it rise?


mattjuk81 30th March 2012 06:08 PM

Ok tested all the AC points before the rectifier with meter set to AC and yes the voltages do rise, maybe I should disconnect the boards and then see what happens

Are some transformers known to do this?

woodturner-fran 30th March 2012 06:23 PM

Err, no.

I'd suggest going back and checking your wiring. Disconnect the transformer from the rectifier board, and check the ac on the secondaries.

/it'd be a good idea to post a pic as well...

mattjuk81 30th March 2012 06:43 PM

Just disconnected the boards and the AC voltages still rise up, the mains wiring is correct for UK mains voltage, and I am measuring across Black + Red and then Orange + Yellow but it rises from 30v upto 37v AC and they should be 25v, unless my multimeter is broken

woodturner-fran 30th March 2012 06:52 PM

Yes - if you have the grey and violet joined together and the mains wired to the brown blue on the primary side, then you should get a pretty much dead constant AC voltage on the secondary. You mightn't get exactly 25V, but it'll be around that.

It pretty much must be your multimeter. I had one that gave me weird readings once when the battery was low.


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