Re: [CAD_CAM_EDM_DRO] Re: Power Supply-- more questions

On Wednesday 04 January 2006 04:40 am, Anders Wallin wrote:

> >I can't quite get 25 times my stepper voltage as that would put me over
> > the Gecko 80v rating. So I'm shooting for 78vDC to the drives. Therefore
> > 78/1.4=56vAC 6.0A x 4 steppers =24A.
>
> an unregulated powersupply will run at more than the rated voltage under
> no load. so if the 80V is an absolute maximum limit for the geckos it
> will be a little dangerous to design for 78VDC.

Agreeed! No reason to get that close to the edge...

> I think I use a transformer with 48VAC (2x24 in series) as the secondary
> for my gecko system. remember you will lose the two diode drops at the diode
> bridge.
>
> >I have no idea if the above will work.. ok.. or if I'm totally off base.
> > They want $41 CAD each. I would need two of them wired in Parrellel??
> > Also wondering about what make [material] the proper CAP should be made
> > from.. as there seem's to be several choices? What got me looking at
> > these ones was simply that they "looked" like one's I've seen in others
> > project pictures.... The are Panisonic and are called "Computer Grade
> > Capacitor's"
>
> large caps are always electrolytic (they have polarity and will not like
> being hooked up the wrong way!).

Boom! :-)

> more expensive electrolytics have higher brake down voltage, higher
> temperature resistance, and less leakage current.

And lower ESR, which I noticed was one of the specs. I'd try *one* of those
caps first, rather than two, and see how it works.

> >Next on the list is the Bridge Rectifier.. I don't think there is alot of
> > science for these, I chose one, 35A @ 100V, 400A Max surge, 10.0 uA Max
> > reverse current, 1.1V Max forward volt [peak] They want $4 CAD. [
> > Digikey also]
>
> that's probably ok.

A 100V part is probably at the low end of what they offer there -- and I'd go
higher. Figure that the charged cap is going to be at one end of a
reverse-biased diode, and the transformer going in the "wrong" direction at
the other, and _add_ the two numbers up. Also figure that a spike on the
power line could punch right through a lower-rated part. For OEM quantities
it makes sense to shave costs but for a one-off I see no reason to go with
lower voltages on a bridge if the difference in price is small.

> >Is there anything else I missing? I seem to remember someone putting
> > various other Resistors and what not in theirs.. ??
>
> the filtering caps will retain their charge for a long time when you
> turn off the system unless there is a "bleed/discharge" resistor across
> them. So for example turning off the machine and letting it stand for
> half an hour and then opening the case could still be a shocking
> experience. Choose a fairly big resistor so that the wasted current
> through the resistor is not high. The caps will completely discharge in
> about 5*R*C seconds so on the other hand if you choose a too large
> resistor it won't help very much.
>
> DPST switch for the primary, a fuse !, panel meters for voltage and
> current are nice

A fuse on the output side, too, if there's any chance of a short somewhere.

> If you are afraid of voltage spikes etc.(lightning) in your electricity
> network then put some protection components over the primary.

It's a problem here all right.

> If you are worried that your cnc machine might disturb other sensitive
> equipment around it through the electricity network then probably an RF
> filter on the primary is a good thing.

Probably not a bad idea, and these are easily salvaged in many places, like
dead computer power supplies for one example.

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