Re: Getting 4.2V from a 5V PSU for a Stepper Motor Driver Board

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Kev Pearce \(kevp.com\)"
<lists@k...> wrote:

> Hi all,
>
> I'm trying to put together a stepper motor driver board using L297/8's.
>
> The stepper motors I'm using are rated bipolar 4.2V/phase at 1.2A/phase.
>
> I wanted to use an old PC power supply, as it can nicely do 5V at

upto 22A ready done.

>
> But what's the best way to get 4.2V from 5V at about 3A per driver

board and be able to run it with a L298???

> I've tried looking up voltage regs at 4.2V with no luck, or zeners...
> Would a simple series resistor or voltage divider do the job... but

at 15W could they handle it???

>
> Anyone any ideas...
>
>
> Cheers all
>
> Kev/.
>

I *could* tell you how to get 4.2V form a 5V power supply but it
really won't help you with what you are trying to do. Steppers are
not voltage devices. They are a series of coils that run on current.
What the voltage specs tells you is IF you apply 4.2V with no
limiting the coil will draw 3A (12.6 Watts). That does not mean you
run them that way. At 4.2 volts, they will turn VERY slowly. The
secret to running steppers is to limit the current electronically and
punch them with as much voltage as your driver can handle (or at least
10 times the voltage spec on the label). It charges and discharges
the coil faster which spins the motor faster. You just need to make
sure you control the width of the current pulse (the 297 298 set has
provisions for that) and use at least 24VDC (40 would be better but
will overload the chips)

Honestly, if you are doing this as a learning experience it will teach
you a lot (and you will spend a lot of time doing it) and you will
also see why that chip set is no longer used in most designs. If cost
is your driver you need to look at the Xylotex new unipolar
microstepping chopper kit. For less than 90 bucks you can drive 4
(~20 bucks per axis) of the 3A motors at 40V directly from your
parallel port.

Okay, I will answer your origianl question anyway.

Now: HOW TO GET 4.2V FROM A 5V POWER SUPPLY:

1. Find the adjustment pot and crank it down to 4.2
OR
2. Put a power diode in series with the + output. Cathode pointing
towards the load. It will drop between .6 and .8 volts based on the load.
OR
3. Feed the motors with 5 volts using the 297/298 chips since they
are choppers and couldn't get 100% of the DC to the coils anyway.