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.

Interesting, There was a post some months ago that you figure total
motor watts and size the power supply for 2/3 to 80% of that. since
your total is much less when you size on watts, you may be surprised
just how small the power supply need be.

regarding caps, oversizing is not a problem and getting voltage
rating 20% (or more) higher is allways a plus. use 50V for the 35V
power supply.

I seem to remember using a diode across the motor to limit voltage as
a no-no.

Dave