Mislabelled transformer?

[coit]

Building a linear power supply I have used a 240V primary;25-0-25V secondary transformer.After the bridge rectifier I measure +/- 25.6V.
GPO measures 251V.Do you think I have an 18V transformer with the wrong label?
I measured it without load.
thanks
Paul
PS: After Connecting to capacitor bank I measured 38.2V. AOK.

[Spiny]

Looks correct, No caps and a normal DMM will show the RMS voltage , add caps and its @ 1.4 times the RMS

[Enzo]

I agree with Spiny, since you mention no filter caps, your DC reading will not be to peak, as it would when filtered. And once you did add filters, the voltage went right up to what we would expect for a 25v AC transformer.


But to answer the original question, if you think your secondary voltages are wrong, why not just measure them and find out?

[coit]

Because I had mounted the bridge inside a U-shaped piece of aluminium as a heatsink and had covered contacts with heatshrink.Why does the meter read RMS before the caps and P to P after?I thought the rectifier increased the voltage by 1.414.
thanks
Paul

[Enzo]

P_P? I think you mean peak, not peak to peak.

On AC, we get the RMS reading. No mystery

ON DC, if it were steady smooth DC, your meter would indicate it. But your "DC" was unfiltered, meaning bouncing from zero up to peak and back 120 times a second. or perhaps 100 times a second where you are. All your meter can do is average out this constantly changing amount. And the average voltage on unfiltered rectified sine wave is pretty close to RMS of the AC.


Rectifying doesn;t increase voltage. If you have 25VAC, that means from zero, each cycles sees the instantaneous voltage rise up to 35v, then drop back down to zero, continue on to -35v, and back to zero. (25v x 1.414 = 35) That is one full cycle. The RMS voltage is an average. it may say 25v on your AC meter, but the voltage is still going up to +35 and down to -35 each half cycle.

When you rectify it,then (assuming positive for discussion) the result is a changing voltage rising from zero up to +35 and back down 100 times a second. Your meter will average that out to about 25v. yes, it is now DC, but it is pulsing DC.

When you add filter caps, that changes things. That pulsing DC charges the cap. The pulse goes up to +35 each half cycle, and that charges the cap to +35. The pulsing voltage drops away as it goes through its cucles, but the cap stays charged, unless something discharges it. SO the cap will be charged to peak, 35v. But this only happens when a filter cap is present.

So rectifying doesn;t increase voltage in the sense that it was always peaking up to 35v, but it is only with filter caps that it can stay there. SO when filtered, the DC voltage reading will be a higher number than the AC voltage reading, but they are measuring different things.