piclist 2002\11\08\060509a >
Thread: stepper motor heating up the resistors??
face BY : Russell McMahon email (remove spam text)

> I have a couple of questions about my stepper circuit.
> first, the stepper has a resistance of 1.9 ohm/phase (omega/phi)
> I'm using 10 ohm resistors, little ones, 1/8 or 1/4 watt,
>  and 12vdc for the power.
> (using a 5804 stepper controller)
> These resistors get so hot they melt the plastic proto-board.  I guess
> I need more wattage on the resistors, is this correct?  will higher
> wattage resistors not get as hot??  And how do you calculate what
> watt is required?
> Would it be 1.01 amp * 12 vdc for a 12 watt 10 ohm resistor?  This
> seems or sounds like something up on the pole....

Essentially yes.
Power = I^2 x R or V x I or V^2/R.
I will not quote be as ohms law would suggest (V/R) as the currenyt will be
ramping up for part of the time.
I assume that your stepper is rated for this current?

Here total power is not more than V^2/R = 12^2/(1.9+10) = 12.1 watt.
The resistor will dissipate about 10/(10+1.9) th of this or about 10.2
Even a 10 watt resistor will get very hot at this power level. Note that
this is the power for continuous activation and with a multiphase stepper
any one phase will generally be activated for only part of the time so you
can scale power dissipation per phase downwards accordingly. In some schemes
there is a single resistor used to power all phases and in such cases you
will need to rate it accordingly. Space the resistor off the board somewhat.

A large resistor will dissipate the same energy but will generally run
cooler than a small one as it has a higher radiating area. This sounds like
excessive power to be throwing awy unless you are really really keen on
performance and cannot get it any other way.

> I wanted to make this motor move, it's late(or early), and I only have
> what's in my parts box...so I'm guessing about how to calculate the
> values of parts required.

Correct as above.

> ANY help is much appreciated, thanks

> Second, the motors seems to jump around occasionally, my program
> steps 400 steps in one direction, then reverses and steps 400 steps the
> other way. ( this thing is 1.8deg step angle, set on half step)

Whether this will work depends on what your stepper is specified at for the
given load. At 20/ms/step that's 50 steps/second which is well within the
capabilities of many (most?) stepper motors DEPENDING ON LOAD. What speed
you can step at at a given torque/load and a given power level will be
specced by the manufacturer. Is this essentially unloaded or with some load?
If loaded, what is the load?


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