Re: [CAD_CAM_EDM_DRO] Sizing a power supply

Jon Anderson wrote:

> Jon Elson wrote:
>
> > Anyway, the input to a chopper drive generally HAS to be filtered,
> or
> > the inductance of the wires from the power supply will cause enough
> > transient spikes to blow transistors very quickly.
>
>
> What constitutes adequate filtering? The PS I'm using now is home made
>
> per specs for my older CyberPak drivers. There's a decent sized
> capacitor in there, would that have the effect of filtering?

Clearly, the power supply itself needs a large energy storing capacitor
to
tide it over between half cycles of the power line. But, because
chopping-type
drives switch the current on and off in the thousands to hundred
thousands
of times a second, they need some local filtering to prevent the
inductance of
the long wires to the power supply from developing voltage spikes. For
the
protection of the transistors, only, a few microfarads would be
sufficient.
To prevent every TV in the neighborhood from going on the fritz, a
larger
capacitance may be needed. A good value might be 1000 uF right at the
stepper driver, or on the board.

> And, should I locate a surplus PS in the 70 volt range, how to I tell
> if
> it's properly filtered for stepper use, outside of finding a dedicated
>
> stepper PS?

70 volts doesn't help. It is the current draw that makes the
difference.
1 Farad of capacitance drops 1 volt per second for every amp drawn
from it. It needs to hold a reasonably steady voltage between line half

cycles, which are 8.33 mS at 60 Hz, and 10 mS at 50 Hz.
Assuming we want no more than 1 V droop at 60 Hz, with full-wave
rectification, and assuming the charging time is zero (a
simplification),
we get C = 8.33 mS * I (amps) / Vdroop = 8.33 mF for a 1 Volt
droop, for every amp of current drawn. Assuming a current draw
of 10 Amps, you need an absolute minimum of 83.33 mF, or 83,000 uF.
It might be a good idea to double that, due to series resistance of
the caps. So, 160,000 uF would be a good capacitance for a 10 A
power supply.

Jon