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

tom.kay@... wrote:

> Hello
>
> This is more of an electronics question, than a metalworking
> question, but I'm sure no one will object.
>
> I have Dan Mauch's 3 axis, 5 amp boards done and ready to hook up to
> power and stepper motors. The logic part of the boards is 5V, so I'll
> just buy a small wall "wart" from Radio Shack for that.
>
> But it's the high power section that has me a bit concerned, because
> I keep hearing that, for example, if you have a transformer with
> 115VAC primary, and 24VAC secondary, then pass it through a bridge
> rectifier and a 15,000 uf capacitor, you'll end up with 36VDC.
>
> 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.

24 VAC would be the RMS value of the sinusoidal waveform. that means that
24 V DV would produce the same heat in a resistor as the AC voltage.
The AC voltage has a higher peak, in fact 1.41 times the 24 V or 33.84 V.
When you rectify it, the rectifier's exact job is to charge the cap to the
highest voltage that occurs, so the cap will be charged to somewhere
close to 33 V, due to the forward voltage drop of the rectifier.
Now, under load, that voltage will have a lower average, as the cap
loses some charge between peaks of the AC line.

So, any time you rectify an AC voltage, you can expect a capacitor-input
filter to charge to about 1.41 times the AC voltage measured with a
standard voltmeter (which should be the RMS value, if the waveform
is sinusoidal). If the input waveform is NOT sinusoidal, voltmeters
which do not have true RMS reading circuits will give totally erroneous
results.

Jon