Re: 10 amps to drive steppers

I was asked recently by another member about how to size up a suitable size
of smoothing capacitor for the linear power supply that most steppers use,
the linear type with a transformer, bridge rectifier etc.and if there is a
formula.

The answer is heavily dependent on the current load demanded by the stepper
driver(s) that the supply must service.
Recently there was a thread here where Dan Mauch gave some guidance in
quantifying the current load on the supply.
When you have the current load look at the specs for the capacitors and you
will gerally see a spec that indicates the max current at a frequency of
120Hz or 100Hz and then another at 20KHz.
The one that most interests us is the one at 120Hz or 100Hz.
When you rectify the AC voltage from the transformer through a full wave
bridge diode, you will get the negative going half cycle flipped positive so
we get two positive peak - so our 60Hz AC frequency has become a 120Hz
varying DC frequency countries which use 50Hz AC will result in a 100Hz
rectified frequency.
So the spec will indicate the max current suppliable at frequency.

For thsoe interested in a formula to derive the capacitance themselves, it
is :
Vripple = I load / ( f x C)
or C= I load / (f x Vripple)

you need to decide what level of ripple voltage you will accept.
As the voltage waveform reaches a peak then falls off, the capacitor will
start to discharge into the load acting as a reservoir during the dip in
rectifier output. Whilst discharging, the capacitor's voltage will drop
until the rectifier volatge rises again and starts to recharge the capacitor
this causes a not so constant final voltage but one with a ripple. The
ripple voltage is the difference between the highest and lowest ebb of the
smoothed voltage.
if you require a capacitor value C to allow a max ripple of say 2 volts, and
need to supply 8A at a rectified frequency f of 120Hz
the calculation would be:
C=8A / (120Hz x 2V)
C= 33,000 uF
The current in the above formula is assumed to be constant which of course a
stepper chopper load is not and as the voltage drops the driver will hold
the phases on just a tad longer until it gets to the trip current for the
chopper but armed with an averaged value for load current with greatest
nummber of phases driven as in Dans message should be a good start - if the
cost difference is not too great then go one size up e.g 47,000 but at least
you have a calculated value range to work with now.

Mo