# Bypass capacitor for reistors in step motors

Discussion in 'The Projects Forum' started by jamaral, Dec 4, 2013.

1. ### jamaral Thread Starter New Member

Nov 11, 2013
2
0
Hi,

I found an step motor driver where each motor winding is driven by an output of l298 and it also has a resistor in series ( a picture is attached).

I have seen a lot of posts here where people use resistors to limit the current. The problem is the only power resistors I can find to buy are wire-wound. What are the other types of power resistors that I can use that are not wire-wound?

In the jones tutorial about step motors, he says if the resistor are wire-wound I have to use a capacitor to bypass the resistor. How can I calculate this capacitor?

Thanks,

Jorge

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Jul 18, 2013
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As you can see the series resistors are usually relatively high wattage, so W.W. are commonly used, the capacitor is usually used to cancel the inductive effect of the wound resistor, I would take a guess that anything around .5μf would be ample.
That is an old method to control the current by the way, now it is done principally by PWM switching.
Max.

3. ### DickCappels Moderator

Aug 21, 2008
3,812
1,032
In addition to that, you can use any resistor as long as you don't exceed that resistor's power rating. The optimum capacitance will be a function of the motor, power supply, and the motor's load, so start with a small value (such as 0.5 uf or thereabouts) and increase the capacitance until the motor reliably steps.

4. ### THE_RB AAC Fanatic!

Feb 11, 2008
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1,309
Modern stepper motors have very large inductances, in the order of tens of mH.

Even a large wire wound resistor might have only 30 turns on a 1/2" ceramic rod, and it's inductance is many thousands times less than the stepper motor coil, so it would not make any significant difference.

Jones was a good reference many years ago but it is very outdated now and some of the issues mentioned are specific to very old stepper motors and very old drive techniques.

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