Power Supplies

I have been reading a lot of questions pertaining to power supplies.
There are a few simple formulas that anyone can use to size the components.
Consider an unregulated DC supply best for most drives, certainly for the
steppers. I will show formulas for 10% regulation which should be OK.
I also assume you know how to hook the parts up (since I can't draw a schematic
here)

Transformer; Primary V should match your power available, so in the states
that would normally be 110-120 or 240V if you have it and so does the
transformer. Secondary voltage should be about .7 X the desired DC level.
(This is because the AC voltage is in RMS and the rectifiers will produce
the peak levels, or 1.4 times higher than the RMS). The rectifiers will
cause you to lose another 2 volts approx. So a 24V transformer will create
a 32V DC supply, 36V > 50DC, 48V > 65DC. You get more volts free :)
The secondary amps should be rated for AT LEAST the total amps being drawn.
In a conservative design, the amps rating really should be 1.4 times (or more)
the DC current. It is the RMS thing again, this time you lose :(
So add up the current required by your motors, the 5A servos draw 5A each.
If you got 3 axes all the same, that's 15 A DC or at least 20 A AC rating.

Rectifier; Buy a bridge module, they are readily available in sizes up to 30A.
Three things to consider, Volts rating at least 2X the DC value will
be safe. Rectifiers are readily available with higher ratings for almost
no more money, cheap insurance. 200 V will be fine for our use.
Ampere rating should be at least the DC
draw, more is better, 1.5 X is reasonable. So our 3 axis supply will be
happy with 20, 25, 30 or more. You do pay for amps so don't go overboard unless
you get a deal. The third is a surge ampere rating, get the highest you can find.
Typically there will be numbers like 150-400 A. The 150 may be marginal, depending
upon the transformer. I like 300 and higher, usually found on the 30 A rectifiers.
These size rectifiers will need a heatsink so bolt the module to an aluminum plate.
If it gets hot enough to boil spit, use a bigger plate.

Capacitor(s); These things smooth the pulsating DC coming out of the rectifier.
Two ratings to figure out; Voltage needs to be greater than the DC value. At a minimun
consider 1.2 X but conservative supplies will use up to 2 X. Capacitors have a life
(usually specified at 1000 hours under test) and will last longer if they have the higher
voltage ratings. Finally is capacitance- here is where I suggest a 10% regulation, or
more accurately, 10 % ripple.Here's the basic formula

capacitance[uF] = DC current[amps]* time between peaks [sec] * 1,000,000 / ripple [volts]

where time between peaks is 8.33 mS in the states (60 Hz) or 10 mS if you have 50 Hz.
ripple is 10 % of the DC voltage say 3-6 V. You can always connect capacitors in parallel
to add their capacitance values. If you have more capacitance than calcualted its ok too.
That means your ripple voltage will be lower.

I hope this helps out.

Tom in NY.

>I would like to hear opinions on what "ideal" components would be
>for a typical 3 axis 5 amp at 36 or 40 volt system such as Dan's and some
>stuff aimed at the Nema 42 guys amongst us. A power supply for the people
>that have 70 or 80 volt, 7 to 10 amp chopper drivers. I'm sure there would be
>a difference between chopper driven systems and series resistor driven
>systems. What is required? Three times rated motor current? I'm assuming for
>one axis, not three. Dan mentioned a 15 amp supply would be good but was that
>for a three axis setup or for one? I'd like to get the best parts so that I
>can avoid heart ache later....
>Peter