Re: Question on Power Supply

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Luke1027 <luke1027@p...> wrote:

> Thanks Dave...as always.
>
> These motors are going to be used a Table Top,
> Dremmal Tool CNC for Wood. First CNC to get
> some knowledge before making the CNC Plasma machine
> I want.
>
> I contacted the Electrical Engineer (who designed it) on the Drvier and
> this was his responde to
> my question on the power supply needed for this Driver and Motors.
>
> "You will need two separate power supplies.
>
> One for logic, regulated 5V DC (+/- 5%) 150mA min.
> The second power supply will be used to provide the power to the motors.
> This one doesn't have to be regulated but at the same time CANNOT be
> switching type.

Read the White paper. Cannot stress that enough.

> They don't work well with any step motors.
> I would recommend the cheapest AC-to-DC LINEAR 24V DC and at least

3A per

> axis. For example
> if you want to run 3-axis then 9A min , for 4-axis 12A min.
> The 24V is an industry standard so it will be easier to find one. The
> problem is to find LINEAR power supply
> with so much current. Since it doesn't have to be regulated I would use
> just
> transformer, bridge rectifier and
> a large electrolytic capacitor. I'm assuming you are going to mount
> everything in some cabinet of the machine."
>
> A few things are still unclear, but I am hoping Mr. Robert Campbell will
> make them clear.
>
> Luke

Here's how it works. (forgive me for being overly simplistic, but we
are talking concetps, not chasing minute details that don't have much
bearing on this use)

your transformer changes the voltage of AC, not the current, not the
watts, just the voltage. 110AC in, 24AC out 100 watts, in, 100
watts out.

Now, you want DC, so you add a set of 4 diodes that are used to pass
the DC part of the voltage to either the + side, or the netural side.

These 4 make up what is called a bridge. you can buy a premade unit
called a bridge rectifier if that suits you . most of us do.

Diodes are not free and eat about 1 volt. so, your 110 VAC dropped to
24VAC thru the tranformer, and the 24VAC dropped to 23 vols DC thru
the diodes/bridge.

But, the DC will be what they call noisy. it will be lots of peaks
and valleys. So, you need to add a filter.

the two choices are inductors or capacitors. Inductors do not change
the voltage, but capacitors do. Capacitors charge like batteries, but
instead of being the average of the peaks and valleys, you get all the
peaks. so, the voltage is increased to 1.414 times the DC voltage.

now, your 23 volts would would increase 1.414x, now, you are at 33.39
volts. too high for use.

OK, so what about inductors ? well, no one has defined them or
detailed how us mechanical types can use them, besides they are hard
to find and are expensive. there is just no demand like for capacitors.

We can rule out a 24 volt AC with capacitors.

Looking at it in reverse, 30 vDC divided by 1.414 is 21 volts. so you
need to find a transformer to handle something less than 22 volts. 16
volts is common with doorbell transformers. Three 6 volt units will
give you 18 volts. a 6 and a 12 will also give you 18 volts.


18 less 1 volt = 17, times 1.414 = 24 volts. not a bad choice.

Then the white paper details how to size capacitors.
(80,000 x total current for all motors in amps)/ voltage = capacitor in uF

In this case, ( 80k * 4.5A ) / 24 = 15,000 uF cap with a rating of at
least 35 volts, maybe 50 volts. Two 10k caps would work fine.

Now, buying a trasnformer, bridge, capacitors, some terminal strip to
screw all the parts to.... or buy a power supply and learn how to
adapt it for CNC use (from the white paper)

But, as we mentione earlier, motor voltage times motor amps = watts.
double for the two phases, then multiply times # of motors.
IIRC, you needed 21 watts ?

www.MPJA.com
P/N 14822 Price: $10.95
+5V/3A, (more than enough for logic
+24V/ 2.2A, (52 watts )
-12V/ .17A (who cares, it is unused)


But, looking around, you will find a power supply to be less work,
sized closer to what you want and MUCH easier !


Dave