Re: [CAD_CAM_EDM_DRO] Power Supply Question, re Stepper Drivers

> Am I missing something? (probably!) How can you end up with a higher
> DC output (36VDC)once you rectify and filter the AC output (24VAC)?
> I thought if anything, the capacitor would bring the average voltage
> down a bit (not up) and just provide a nice smooth 20 volts DC or so,
> from a 24 VAC output.

AC voltage is not constant, but rather varies in a sinusoidal pattern
at 60 [50] cycles per second. The way we usually measure it is to
refer to the 'root mean squared' (RMS) voltage. This is a kind of
time average. Another way to measure an AC voltage is the
peak-to-peak value - ie, the maximum of the sine wave. The
peak-to-peak voltage is the sqaure root of two (about 1.4) times the
RMS voltage.

When you full-wave recitfy you generate a 120hz series of
half-sinusoid pulses whose peak voltage is that of the peak-to-peak
input AC waveform, minus rectifier losses on the order of 1.2 volts.
Ie, you will see 1.4 times the AC RMS input - 1.2v.

Put a capacitor across this with no load and it will float up to the
top of the peaks, giving a DC voltage higher than the AC RMS input
value. Add a load to the supply and the capacitor voltage will sag a
bit, possibly (and I'm not sure on this) as far down as the RMS
voltage when under heavy load.

That's why we call it an unregulated supply...

See if you can find a transformer with a center tap or something.
Depending on how the torroid is wound you might be able to remove some
turns from the secondary to lower the voltage, if it's cheap enough to
make doing so desireable.

Chris