Re: My power supply calculations and a zener question

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "ghidera2000"
<ghidera2000@y...> wrote:

> I'm trying to figure out a power supply to use with a three axis
> A3977 based driver. I read the Gecko white paper and its pretty
> straight forward.
>
> Anyway, The A3977 can handle 35 volts at 2.5A for each driver -

7.5A

> total. While I'm not planning on adding a fourth axis, just in case
> I'll add on another 2.5A - 10 AMP minimum. So I want the power
> supply to be able to handle at least 12A to give me a little room.
>
> Unfortunately the 35V includes back EMF so the limit is really

about

> 30V
>
> If I use a 22VAC transformer, that'll give me about 30.8 VDC out.
>
> The bridge is a no-brainer but the cap takes a little math
>
> Using the Gecko capacitor formula, I should use at least a
>
> C = 80,000 * 12 / 30.8 = ~31,000 uF
> V=Vin+Vin*0.2 = ~37v
>
> Closest I can find is 33,000uF @ 50v Electrolytic
>
> The only thing I'm not sure about is what they say about protecting
> the cap from back-emf voltages when voltage is near the voltage
> rating of the drive. Since we run all these motors at 5-20 times
> their voltage ratings, isn't this always a requirement?
>
> Since I can't exceed 35 volts on the A3977, a 33 volt zener should
> do the trick. Above my desired voltage but below the A3977's limits
> But what power rating do I need? The whole 12 Amps? If so this

could

> be tough, 5W is the highest Digikey has.

Shouldn't be a problem as this isn't a continuous load, providing
that the mains voltage doesn't flutuate wildly for long periods of
time. The other alternative you might consider is using Mariss's
circuit "Power Supply.pdf" in the files section.

Bernard