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

On Thursday 19 February 2004 04:10 pm, plastiguy wrote:

> 80,000x12amps/36 VDC = 33,333 microfarads for the filter cap.
>
> Queries:
>
> Is it OK to use a filter capacitor between 33,333 microfarads and
> 100,000 microfarads? (I think rule of thumb is 1 to 3 times required).

Yeah, probably. Be aware that tolerances on those parts is extremely loose,
and also biased -- numbers like -20% / + 80% (!) are fairly typical.

> Is a 50 volt capacitor adequate for the 36 Volts out of the
> transformer? (I think 50 volts is a standard capacitor rating, the
> next lower one being 35 volts)

The rule there is that the output rating of the transformer is RMS volts,
while the cap needs to be able to handle peak volts -- and you probably want
to allow for the highest line voltage that you're likely to see. I'm
guessing that the 36 volt output spec on the transformer is for something
nominal like 117vac in, and I've seen line voltages as high as 125v. Figure
out the ratio...

Then take the same ratio, and apply it to the output voltage, that gives the
highest RMS voltage you're likely to see.

Then take that figure, and multiply it by 1.414, and that gives you your
peak voltage. Then add another 20% or so for a safety factor...

> Though I understand what a capacitor is and does, what's the function
> of these filter capacitors (e.g. what would happen if I didn't use
> them?)

What you'd get for an output would be "pulsating DC", the waveform of which
depends on whether you're using a full-wave or half-wave rectifier. I'm
assuming that you're using a full-wave bridge, so take the normal AC
sinewave and fold it in half -- that's what your output waveform would look
like, pulsating at 120 Hz, or twice the line frequency. Probably not real
good for what you're trying to run here. Also some circuits dump power back
into that capacitor from the motors, and if it's not there you'll end up
with problems.

Contact me off-list when you figure out what you need, I've got a couple of
boxes of big old electrolytics. I may not have what you need, or I may,
I'd have to look.

> What rating or capacity besides the 100 or so volts do I need in
> order to ensure the bridge rectifier is sized asequately (I really
> don't want this to be the weak link in the circuit, as bridges are so
> cheap).

I'd worry about the amperage rating more than the voltage. And I hate for it
to be the weak link too, as so much stuff can get damaged if it shorts
out...

> Is a 100 volt bridge rectifier OK for the 72 volts total between plus
> and minus 36 coming out of the transformer?

No reason not to go somewhat higher, or even significantly higher, as it
doesn't add all _that_ much to the price unless you're looking at
manufacturing quantities. :-) Plus a higher voltage rating also tends to
survive longer in encountering power surges.

> What's the functional difference between a toroidal transformer and
> a....well, square? one? Is one better suited than the other for this
> application?

Toroidal transformers keep the magnetic field confined inside things better.
The only advantage besides that I know of for them is they're more compact,
so they're used in situations where space is a major issue -- low-profile
cases and such,

> Any great clues on finding good prices for quality components like
> bridges, capacitors and transformers would also be used and
> appreciated.

I've had good dealings with Digi-Key. Watch out for the minimum order,
though. I've also seen people speak well of Mouser, though I've never dealt
with them myself. There's a "repair.faq" mentioned pretty often in the
newsgroup sci.electronics.repair done by Sam Goldwasser, and he's done one
heck of a job on it. There is a list of suppliers in there along with URLs
for most of them. Browsing the sites oughta tell you whether they'll have
what you want or not.

Another good source for stuff is hamfests and computer shows, if they're run
by hams (Timonium is one of those), but we're not quite into the season for
those just yet. You can find all sorts of stuff at those.