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Topic: Will adding resistance to stepper make the h-bridge run cooler? (Read 940 times) previous topic - next topic

charlieb

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?

Grumpy_Mike

Yes t will run cooler but it will not produce as much torque.

LarryD

#2
Jul 05, 2013, 12:46 am Last Edit: Jul 05, 2013, 12:48 am by LarryD Reason: 1
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.
The way you have it in your schematic isn't the same as how you have it wired up!

zoomkat

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.
Consider the daffodil. And while you're doing that, I'll be over here, looking through your stuff.   8)

charlieb

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.

LarryD

The way you have it in your schematic isn't the same as how you have it wired up!

charlieb


Grumpy_Mike

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.

dc42

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.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

MarkT


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?
[ I won't respond to messages, use the forum please ]

sbright33

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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