Re: [CAD_CAM_EDM_DRO] Transformer question

In a message dated 11/19/2002 11:07:06 AM Central Standard Time,
thomasm923@... writes:

> I think it can supply at least 15 amps. More than enough for what I need.
> Unfortunately, the servos that I want to use
> are rated at 24 volts, so before I go looking for another transformer, I
> would like to ask the group if there is a way to
> limit the voltage of this transformer. It seems to me that I remember
> seeing circuits that used a zener diode to reduce voltage. Am I way off
> base here, or can this be done?
>

Tom: First, your servos "want" 24 Volts D.C. not "A.C.", and a transformer's
voltage is the latter! When you rectify and filter N volts A.C., the
resulting D.C. (no load) equals 1.414 x N. So, even if you DID have only 24
VAC from your transformer's secondary, the resulting D.C., assuming minimum
loading to a 15 A. transformer (no more than a couple of amps, say), the D.C.
the poor motors get would be about 33-35 volts!

Now, a "zener" conducts current "backwards" when it's D.C. "zener voltage" is
reached. Thus, zeners would not be applicable to "modify" your transformer!
Besides, all a zener would do to "reduce" voltage, if D.C. only, would be to
"short" some of it to ground, and this means DISSIPATION, so there'd be HEAT,
and that means EXPENSIVE and large zeners, and... Besides, not really
applicable.

Now, if you were to rectify the 30 VAC, filter it with, say, 10,000 to 20,000
µF, you'd have over 40 volts DC to "regulate", and a zener would have to
"sink" 40-24=16 volts @ 15 amps to ground, and this is 240 watts, and... See
the deal? Even if you used heat-sunk "pass-transistors" conducting 15 amps
and dissipating 16 volts or more, you'd need fans, a water-spray tower,
pumps, ... Just joshing, but it WOULD be some serious heat to "get rid of"!

You say "looks like 15 amps". Is the secondary winding about #10 or #9
copper? You'd need that for that kind of current output!

If you had TAPS on this transformer, you could connect for more turns in the
primary, or fewer in the secondary, and get lower AC-out that way, but it'd
take some "measuring and fiddling".

Lotsa luck! See my article about transformer-making I LINKED in
another letter to the group. There's some insight in that about this topic!

Jan Rowland



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