Re: SP-3/HT (dropping resistor calcs)

Woodknack,

Reducing the voltage will DECREASE motor speed. Look elsewhere for
the speed "limit", IMO.

Ballendo

P.S. This drive uses the Allegro 5804 driver chip. It is a unipolar
drive. You should have large resistors in series with each motor
winding to limit current. Without these, the motor can overdrive, and
have poor performance. You can see the circuit location of these on
pg. 5 of the data sheet at:

http://www.allegromicro.com/datafile/5804.pdf (look just below the
motor which is at the top of the drawing, for the two "zig-zag" lines
which are the ballast resistors).

On the Stepperworld site, they do not show any ballast resistors. Did
you get any with your board/kit? (they may be mounted under the
board, but are not visible in the pics online). They will
be "squarish" and likely "gritty" looking, and big enough that you
would notice them. (could also be "finned" aluminum, with a wire
sticking out each end). They will also get quite hot when the drive
is on and the motors are idling.

The motors with this kit should work fine, and I don't think you'll
get much, if any speed increase with diofferent motors. You "could"
increase the supply voltage up to 35v MAX (i'd use 30-33), but I
don't think this is worth the effort.

Here are the calcs for figuring these ballast resistors (also
called "dropping" resistors). I'm assuming here that you have the
motors listed on the stepperworld site.

Basically, the needed value is: Vs - Vm - Vd/Am, where:

Vs= Supply Voltage (24 in your case)
Vm= Motor "rated" Voltage (5.1 in your case, use 5)
Vd= Voltage Drop across the driver output (use 1; will vary with
drive from about .7 to 2volts)
Am=Motor "rated" Amps (1 in your case)

Simplifying we have 18/1= 18 ohms

Now we need to be sure the resistor has enough power capability.
power is expressed in watts, and the formula is:

Volts x Amps = watts

So:

24 x 1 = 24 watts (you need AT LEAST this value, but can use higher
value.

So you're looking for 18 ohm, 25watt power resistors. You need 2 per
motor, so you will need 6 total.

Be aware that this type of l/r drive will not be the fastest, no
matter what you do. But lots of people use it/them successfully.

> --- In CAD_CAM_EDM_DRO@y..., "woodknack" <wjdupont@h...> wrote:
> > So how fast would it be if I just hook up to 12 volt power supply
> > instead of the 24 volt that came with it? Could I hook up faster
> > motors to this setup? Thanks