Universal Power Supply - Silicon Chip

[boylesg]

The first part of this circuit up to the 2200uF caps works well. From a 24VAC wall plug I can get -36VDC and +36VDC unregulated. I can then pass these through my LM317 and LM337 voltage regulators to get -18VDC and +18VDC.



But what's with the 36V? I can't conceptualise how the 24VAC is ending up as +/-36VDC.

I know the capacitors 'fill in' the gaps between the voltage humps after the AC passes through the half bridge rectifier. But how is the AC voltage being almost doubled as DC?

Also, if I was to build an adjustable voltage source with this system, how would you go about adding a digital display to show the voltage? Has anyone come across an easy to understand webpage on this?

 

[godfreyl]

But what's with the 36V?

The wall plug gives 24V RMS, so the peak voltage should be 24 * sqrt(2) = 34V. There's a bit of voltage lost across the diodes, so you should get about +-33.4V DC unregulated.

So it's only 6% higher than expected. Two possible reasons:

• With light loads, a transformer's output voltage is normally a bit higher than the nominal value. When higher current is drawn, the voltage drops.
• The mains voltage may be a little high e.g. 240V instead of 230V.

 

[boylesg]

The wall plug gives 24V RMS, so the peak voltage should be 24 * sqrt(2) = 34V. There's a bit of voltage lost across the diodes, so you should get about +-33.4V DC unregulated.

So it's only 6% higher than expected. Two possible reasons:

• With light loads, a transformer's output voltage is normally a bit higher than the nominal value. When higher current is drawn, the voltage drops.
• The mains voltage may be a little high e.g. 240V instead of 230V.

The RMS voltage of a changing waveform is that voltage which would produce the same heating effect in a purely resistive load as would a pure DC voltage of the same level.

OK I sort of get the idea. I have always thought of the 24 in 24VAC as the height of the sine wave.

 

[godfreyl]

No, it's the RMS value. It's the same with mains voltage. 220V mains is over 300V peak.