Re: regulated power supplies???????????

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Phil Mattison"
<mattison20@...> wrote:

>
> I think it would be more accurate to say that if you are using a chopper
> type motor driver there is no NEED to regulate the power supply,

because the

> driver does it. The PWM motor driver is essentially a current regulator,
> whereas most conventional switching power supplies are voltage

regulated.

> Using a voltage regulated power supply with a current regulated

driver is

> just overkill, but not inherently bad, so long as you have enough of a
> capacitor to buffer the switching supply from the current spikes

induced by

> the driver's chopper. Clear as mud.
> --Phil M.
>
> ----- Original Message -----
> From: <wanliker@...>
> To: <CAD_CAM_EDM_DRO@yahoogroups.com>
> Sent: Wednesday, April 26, 2006 3:30 PM
> Subject: [CAD_CAM_EDM_DRO] regulated power supplies???????????
>
>
> >
> > In a message dated 4/26/2006 9:56:36 AM Mountain Daylight Time,
> > hwhacker@... writes:
> >
> > I believe the motor power supply SHOULDN'T be regulated.
> >
> >
> >
>

Phil is right. There is no reason not to use a regulated supply, and
several arguments for it. The best reason may be one of expense:
there are lots of surplus regulated supplies out there.

Another factor is the regulated supply output will not change with
varying line voltage and may prevent an overvoltage condition in the
event of a line surge (more common than you think). Almost all have
over current protection that could be of help too. Instead of things
having to get to 200 % overload to blow a fuse the current limit will
just shut down the supply. A nice little 28VDC regulated supply
tweeked to 29 or 30VDC would be perfect for units like the Xylotex
where voltages much over 30 can let the blue smoke out.

There are some things to be aware of if you are using a regulated
supply (switcher or linear). On the Linear supply the regulation is
done after the transformer and filter, and usually has active
components (power transistors) that are lothe to handle reverse
current/voltage (i.e. driving a heavy inductive load) or have voltage
presented into the output that is higher than the regulated voltage
(like from back EMF). Transistors conduct poorly in reverse. The
problem is the base-emitter junction. Back biasing that tends to
destroy the device. The cure is to place an isolation diode (of
sufficient size and voltage for the output) and a small filter cap on
the outside of the diode. If you are running close to the max voltage
on the drive this can possibly result in too much DC at the drive.
Some supplies have overvoltage protection that will shut things down
if the voltage goes over the preset point.

One thing you can do if you can find a linear supply cheap is to
"steal" the RAW DC right off the filter cap and just don't use the
regulated section at all. The RAW DC will be higher than the
regulated out so that at low line voltage (95VAC) you still have at
least 3VDC more than the highest target regulated output. It
typically works out to about 120% so a 24VDC supply might have 30 to
32 VDC Raw at the cap.


On switchers the regulation is done on the primary side and there are
no tender components on the output. Typically just rectifiers, some
coils and filter caps. The reason you hear not to use switchers is
because they operate at 30 to 100KHZ and the filter components are
much smaller. Filter caps can be a from a few hundred to something
like 3300 mfd for effective filtering. Small values of filters and
coils have small amounts of stored energy to handle spikes. The
feedback response of the supply (since it is able to adjust in one or
two cycles at 50,000 hz) is faster and regulation can be tight BUT
that works against you driving a load that is itself swtiching in
nature. There can be circumstances where the feedback gets unstable
trying to control a load that consists of high frequency current
spikes. Ususally just the addition of a moderate sized outboard cap
is all that is need. Some don't even need that.

Never rule out using a supply just because it is regulated or a
switcher. Almost all of the self contained motor drives that run off
120VAC have a swtiching power supply inside for motor DC.

Tom Caudle
North Texas

Http://www.CandCNC.com
"We just made CNC easier!"

Hardware for Home Shop CNC:
Low cost, powered/isolated, breakout card
AC and DC "Drive Saver" power controls
2nd Generation digital THC
Handheld Manual Pulse Generator (MPG)