Re: current limiting power supply for stepper?

> Message: 9
> Date: Wed, 26 Feb 2003 04:07:30 -0000
> From: "grantfair2001 <grant.fair@...>" <grant.fair@...>
> Subject: current limiting power supply for stepper?
>
> Is there any reason not to use a current limiting power supply with a
> stepper, so that higher than rated voltages can be used? (Of course
> the stepping sequence must use the same number of coils for each step).

There is absolutely no problem using constant current supply to drive a
stepper in unipolar or full-step bipolar mode. The performance
( assuming the same power supply voltage and adequate current ) will be at
least as good as the "standard" chopper drivers we use now. In fact, it
could easily be argued that a CC drive would outperform a chopper drive,
under those circumstances.

There are three real problems with CC drive, one easily manageable, one not
so, one just costing money.

The first is heat. For example, using easy numbers because it's early in the
morning, we go with a 25 volt supply and 1 volt 1 amp unipolar steppers,
there is 24 watts of heat to get rid of. This is not a real problem for a
TO-220 LM317, but it is a bit of a concern and does require a bit of costly
aluminum.

Problem two is that for the above example, the power supply needs to be a
25 watt power supply, as it will be constantly supplying 25 watts. In a
chopper drive, the supply still needs to be 25 VOLTS, but really only needs
to be 1 watt. ( This is a bit of a simplification, but only a bit )

So what you gain - dollar wise - with the simpler circuitry, you lose by
virtue of the much larger power supply.

The biggest problem is this .. you USUALLY don't want to run your steppers
unipolar ( because you'll only get half the power out of them ) nor do you want
to run them bipolar full step because resonance issues will severely limit
your maximum speed.

And the conversion ( especially with an LM317 solution ) to current shaped
half step or finer is REALLY troublesome.

HOWEVER, if your steppers are of adequate power in unipolar mode and if the
max speed so wired is adequate to the task and if you happen to have suitable
electronics and heatsinks laying around, there is NOTHING wrong with your plan.

You get a genuine stepper drive for pretty low cost and complexity.

Alan

--

Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta