# Bridge rectifier output voltage at no load?

(First posting)

I am building a simple PSU with a torroidal transformer and bridge rectifier. I'd like to know the output DC voltage from the rectifier while under load. e.g. charging a battery. I see that the formula is in here but these things do my head in (sometimes) so if I quote the numbers some kind souuratingl may help.

The transformer is labelled as being 22 VAC at 16.9 amps but at a less than constant duty cycle I fear. It is centre tapped and grounded. At least there's a green etc. ground wire along with the red and yellow secondaries. However, my DVM is registering 28 VAC and the DVM if not Fluke, isn't rubbish no worse than 2-5% I'd say.

Connected to a bridge rectifier rated at 1000 V and current rating > 20 A, what DC voltage should I expect to see with/without a resistive load?

Note: I am assuming no filter capacitor is connected.

Well, it sort-of makes sense but you won't get a good reading without a resistive load.

The average voltage of a (full wave rectified) sine wave is 0.9 times the RMS voltage, or about 19.8V in your case (minus a volt or so because of two diodes with little loading), so maybe 19V.

However, with no loading the output voltage of a transformer will be higher than the rated (full current) load, typically by 10-20% for a smallish transformer.

There's another 'however'- the input to your voltmeter will actually have a small capacitor across it and it will act to filter the voltage (since the back leakage of the bridge will be small). So, you'll get a higher than expected voltage, but not reliably.

I suggest you try putting a resistor such as 10K across the output and measure again. I would expect a reading in the 20-22V range with a basically negligible load.

• Thanks a lot. Will work on that info and report progress. Regards TT Commented Sep 26, 2016 at 11:49
• It is unclear what @Experimenter is doing with that centre-tap. A bridge rectifier suggests the centre-tap could be left un-used. Or two diodes of the bridge could be left un-used. You get far different DC voltage out for those two cases. Commented Sep 26, 2016 at 23:08

If you add 40% for the conversion from RMS to sine peak or x root(2)