Re: [CAD_CAM_EDM_DRO] Re: Power Supply for Gecko 320 and servo motors for 3 axis router

On Friday 20 February 2004 04:48 pm, caudlet wrote:

> I'm not sure what the purpose of a OVP SCR across the output is for
> since under most failure conditions the primary switching components
> will shutdown rather than run wild. In most supplies if there is an
> OVP it is for the voltage that supplies the logic (+5 or +12) So it
> would never see the higher reverse voltage. Maybe if you lost the
> feedback loop to the output, but we used to design in shutdown if the
> loop feedback was out of normal ranges.

I guess there's lots of different designs out there for switchers, but the
stuff I've seen crowbars in put it in there for regulator failure, more or
less. Yeah, things won't get out of hand normally, but sometimes stuff
doesn't work the way it should.

I can still remember getting called out to an industrial site, looking at
massively huge stacks of input product (rolls of paper) all over the place,
and me and several others trying to help this guy get this machine running
again. It was four boards full of CMOS, and the supply had hit something
like 24V. He got real wide-eyed when I explained the idea of a crowbar...
;-)

> I think the biggest reason not to use switchers is that you just
> can't find them in voltages above 48VDC (and those are less common
> than the 5, 12 and 24 V units.)

Excepting the ones that are built into every tv and monitor out there...

> Linear regulated power supplies are actually less desirable for running
> inductive loads than switchers.

I wouldn't have thought of this.

>All power supplies have to be designed for transient loads (non-
> linear current surges). The output filter section of a switcher
> takes that into consideration based on its loop charactersitics and
> the electronics in a switcher can make some pretty quick adjustments
> when the switching times are sub millisecond.

And linear supplies can't?

> There are hybrid designs that have switching front ends and use
> active regulators on the some of the secondary voltages. You often
> see this on multi-output supplies where there have to be several
> tightly regulated voltages. You have to treat them as a linear type
> regulated supply with all of the reverse (back EMF) warnings.
>
> One last observation: If the outboard caps are too large the inrush
> current will be more that the current limit will allow and the supply
> will never stay on long enough to get them charged. I doubt under
> most circumstances if more than a few hundred MFD are needed if any
> at all. Just enough to keep the feedback loop dampened below
> oscillation.
>
> Okay, I'll duck back down and wait for the slings and arrows :-O

Nah... :-)