"Spehro Pefhany" wrote in message
...
On Wed, 24 Nov 2004 16:16:16 -0800, the renowned "PrecisionMachinisT"
wrote:


"Spehro Pefhany" wrote in message
.. .
On Wed, 24 Nov 2004 14:40:26 -0800, the renowned "PrecisionMachinisT"
wrote:


"Nick Hull" wrote in message
...
In article ,
Gunner wrote:

Ok...Im way way out of practice on my basic electronics...blush

I found a 12vt dc diaphragm pump of the same type thats in the tig
coolant unit I was given. (Lincoln Magnum)

The old pump and all the gizmos were fed 24vt ac. I did find a
tiny
full wave rectifier inside the pump, which apparently only had bad
brushes. Unfortunately..I didnt get all the parts to the motor,
else
Id simply stick in a new set of brushes and voila.......sigh

The new pump is 12vdc at 7 amps. The rectifier is no biggie.

What is the best way to drop the 24vac down to 12v?

You could just cheat; use the 24 vac and half-rectify it. Instead
of a
steady 12 volts you would be supplying a pulsed 24 volts, but I
doubt
the motor would notice. Make sure the rectifier can handle the
higher
pulses, bet even that is probably no big deal since it is the
average
current that limits diodes.


And add an electrolytic capacitor to smooth out the pulsing.

No, that will make the motor burn out and maybe the capacitor explode.


Oh, then I guess that explains why virtually every large non-switching dc
power supply I own happens to have a large bank of them connected in
parallel with the load, excepting perhaps a battery charger or two.

That's a different situation. You notice you don't ever see output
caps on SCR motor controls.

A small (like a few thousand uF) capacitor will likely have the
*ripple current* rating exceeded, causing it to overheat and possibly
vent. It won't smooth the output voltage much with 7A load. To get 10%
peak (2.4Vp-p) ripple would take C ~= (1/60) * 7A/2.4 ~= 48,000uF

A large capacitor will charge to close to the peak voltage, which is
1.4 * 24 ~= 33V. In the above case (48,000uF) the average output
voltage would be 32VDC. That is an awful lot for that poor 12V motor.

Half-wave rectifying it gives an average voltage of 12V, which is okay
(maybe a bit hard on the brushes, and the motor may be relatively
noisy but it's okay) for the 12V motor.


Yeah okay, thinking it through the vdc does rise to near peak, its been a
very long time since I built any power supplys, and when I did it was
generally for B+ supply in tube amps.

Also, its probly a brush motor and wouldnt give a hoot whether its running
off ac or dc anyways, so a 2/1 stepdown would probly work fine too.

But im not in the habit of jerry rigging things to work via use of scavenged
parts these days and also got no idea what he might have in his junk
box.......

Gunner, give us a complete inventory of all your odd bits and Im sure
someone will come up with a workable solution G

--

SVL