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

In a message dated 17-Oct-01 10:48:15 Central Daylight Time, tom.kay@...
writes:

> 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.
>
> I had my sights set on a toroidal transformer from a company in
> Toronto, that outputs about 38 volts AC and 20 amps. Am I going to
> wind up with 50 volts DC once it's rectified and filtered? That would
> fry Dan Mauch's boards, I think. In the end, I want to end up with 36
> to 38 VDC, about 20 amps, so what should my output VAC be from the
> transformer?
>

Tom:

"AC Voltage" is usually read by an AC Voltmeter calibrated in "RMS" volts,
and this spec. voltage (24 volts AC) is again "RMS". The PEAK voltage "at
the crest of the sine-wave" is SQR(2)=1.414 TIMES RMS. The capacitor
"stores" voltage, and if there is little or no "leakage", you would read PEAK
voltage DC, or over 33 volts, assuming you had exactly 24 VAC RMS into the
rectifier (oh, and a no-loss rectifier; most "loose" about 1½ V).

Now, a LOAD across this kind of unregulated rectified-AC source will pull the
"average" DC down, but it actually makes it look like a SAW, if viewed on an
oscilloscope. (No problem for a decent or better stepper-driver; they "do
that"!). A "voltage regulator" add-on to a DC supply "cleans up" this
"raggedy DC". So, yes, if you have a 38 VAC RMS transformer, the
"open-circuit" DC output voltage would be about 54 volts. When you "load"
that, it will drop, and if you load it close to the amp-rating of the
transformer, it may-well measure only 38 VDC, but it will be "ripply, nasty
DC"! Just remember, the "peak voltage of an AC source" is 1.414 times the
"RMS" (Root Mean Squared) AC "measured" voltage.

Jan Rowland, Troll


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