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Author Topic: Will adding resistance to stepper make the h-bridge run cooler?  (Read 889 times)
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I think it will and here's why:
V = 24
R = 9ohm (just the stepper)
I = 24 / 9 = 2.66
P = IV = 63W

With these settings the h-bridge shuts down pretty quickly (thank goodness it has a thermal shut off or I'd have ruined yet another component!)

If I add a 40ohm resistor between the h-bridge and the stepper:
I = 24 / 49 = 0.48
P = 11.5W

I hope that'll make it run cooler and hopefully the resistor won't catch fire.

Does this sound reasonable? And if so should I add even more resistance?
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Yes t will run cooler but it will not produce as much torque.
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Yes this was often done.
Note: since the motor is inductive, all (most) the voltage is across the coil at turn on and decays down.

Mount your power resistors on a heat sink.
« Last Edit: July 04, 2013, 05:48:16 pm by LarryD » Logged

The way you have it in your schematic isn't the same as how you have it wired up! That goes for me too.

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I think when a stepper motor rotor is not actually being rotated, reduced pulse widths can be provided to the coil to provide a holding current instead of higher current needed to move the rotor.
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I tried it (the total resistance with both the stepper and the resistor was 86ohm => 0.28W) and it still shuts down from the heat. The resistors are rated for 0.5W but they also get too hot to touch but guess that's ok at less than their rating.
It makes me wonder how the printer board actually did it.

I guess I'll probably have to get a heat sink as suggested and develop a more sophisticated software solution. Maybe a fan too.
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This style is probably what you need to look for.


* 7-4-2013 7-15-30 PM.jpg (9.55 KB, 249x153 - viewed 22 times.)
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The way you have it in your schematic isn't the same as how you have it wired up! That goes for me too.

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Damn, that thing doesn't mess about!
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You could get a FET based H-bridge, they do not get as hot. Or as mentioned feed the coils with PWM to reduce the a adage current.
You could reduce the voltage.
Finally you cold get a chopping regulating motor driver, that is how the printer does it.
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Which h-bridge chip are you using? If it doesn't have internal flyback diodes, do you have external flyback diodes connected?

Steppers are best driven from chopper - based drivers. If 1.5A provides enough torque for your application, then an inexpensive A4988 driver available on eBay will probably do the job.
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I think it will and here's why:
V = 24
R = 9ohm (just the stepper)
I = 24 / 9 = 2.66
P = IV = 63W

With these settings the h-bridge shuts down pretty quickly (thank goodness it has a thermal shut off or I'd have ruined yet another component!)

If I add a 40ohm resistor between the h-bridge and the stepper:
I = 24 / 49 = 0.48
P = 11.5W

I hope that'll make it run cooler and hopefully the resistor won't catch fire.

Does this sound reasonable? And if so should I add even more resistance?


The specs of the motor you give do not sound at all plausible.  Which motor are we talking about?
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You can reduce the duty cycle of each step instead of using a resistor.  My code on this forum includes this feature.  It allows a 5v motor to run safely with a 12v supply, at reduced torque and speed.  Or increased compared to 5v supply.  Depending on the circumstances and mechanical setup, you can turn off the coils when it's at rest, or coasting at a constant speed without friction.  In the latter case, the duty cycle can be greatly reduced.  When you need more torque/speed, you can temporarily increase the time the coils are active.  Say for starting a heavy load.
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