# Will adding 1/2 watt resistors reduce the current load of a stepper motor?

I have a stepper motor rated at 1.8 ohms or resistance per phase, a driver rated at 750mA and a powersupply that gives 12V rated at 1A. This would mean that the motor will draw 6.6A per phase.

If I wanted to limit the stepper so that it cannot draw more power at 12V than the driver can tolerate then can I add 14.2 ohms of 1/2 watt resistors (if there is such a thing) to acheive this?

Is my math correct? 1.8 Ohms + 14.2 Ohms = 16 Ohms 12 Volts / 16 Ohms = 0.75 Amps?

I know that the stepper will not be at full power but it doesn't have to be.

EDIT: Sorry wrong resistance per phase. It supposed to be 3.6 Ohms per phase; would I require 12.4 Ohms of additional resistance per phase?

• Please note that you will be having problems finding such resistors as they are not standard. Since your driver is the problem, go for resistors with a bit higher resistance. Commented Mar 17, 2011 at 23:02

well not half watt resistors .... 0.75 amps though a 12.4 ohm resistor P=IIR = 0.75*0.75*12.4 = 7 watts - half watt resistors are going to smoke - maybe some 10 watt wirewounds?

• So then it would work?
– user3045
Commented Mar 18, 2011 at 0:17
• probably - but you're only talking about DC issues here - you may see some LR issues slowing down edges Commented Mar 18, 2011 at 9:38

There actually is an advantage of the resistor with a higher than required voltage, it make the drive more like a current drive. This can improve stepper performance particularly at higher speeds. Efficiency is not good. PWM is a more efficient current drive. If you do not mind the waste, go ahead. I did not check your math.

• So Chopper Driving is better than current driving?
– user3045
Commented Mar 18, 2011 at 16:01
• Yes, the stepper motor can't tell the difference between chopper drives and constant-current drives, and chopper drives pull less power from the power supply. Commented Mar 18, 2011 at 19:13
• There fore I don't need the resistors, just a chopper driver. Will the power suppply be able to tell that a chopper driver is in use, or do I need to get one that can supply more amperage than the motor can consume?
– user3045
Commented Mar 18, 2011 at 22:18
• The power supply can tell. Output caps on the PS can supply the current, if it does not have enough already. A scope on the power supply should not show too much variation when the chopper is on. Commented Mar 30, 2011 at 16:16

Do you have a choice between (a) a chopper drive that automatically turns everything off when it overheats, and lets you set the maximum current allowed through to the motor to prevent the motor from overheating, or (b) a L/R stepper drive that turns on its internal power transistor between the stepper and the power supply, and if you happen to stop sending step pulses it leaves that transistor on indefinitely, even if it makes the motor or the power transistor melt?

If you have a choice, I recommend the chopper drive. Also, I would look at the data sheet for your motor and find the maximum recommended current for that motor, and set the chopper drive to limit its output to that current.

If your datasheet doesn't say what the rated current is, divide the rated voltage (hopefully at least that is printed on your motor or listed in the datasheet) by the coil resistance (which you can measure with an ohmmeter) to get the rated current.

If you are stuck with the older L/R stepper driver, then resistors in series with the stepper winding are an excellent idea. The maximum current before the motor is damaged is different than the maximum current before the L/R stepper driver electronics is damaged -- use the smaller of the two currents in your calculations. Is the smaller of the two is "750mA" in your case? Russell Laidman's tutorial "Stepper Motors and Control: Part III - Current Control of Stepper Motors" describes how to calculate the appropriate resistor -- it appears that you are doing that calculation correctly.

You may find the RepRap Motor FAQ useful and the RepRep Stepper Motor page informative. The RepRap alternative electronics page has a list of many chopping -- or even better, microstepping chopping motor drivers. The Open Circuits motor driver page has an even longer list.

EDIT:

If you have a chopper driver, connect it directly to the motor. Any resistance between a chopper driver and the motor reduces performance and wastes power.

When a chopper motor driver "is rated at 750 mA", it means "The chopper motor driver will control the power sent to the motor to at most 750 mA; it is designed to supply that amount of current indefinitely". That rating does not mean that you need to do something to limit that current.

It's kind of like a light bulb "is rated at 100 W", it means that the light bulb will control the current pulled from the power grid so that it consumes 100 W; it is designed to run at 100 W for its entire lifetime. That rating does not mean that you need to do something to limit that current.

• My stepper is rated at 1.4A while my chopper driver is rated at 750mA. The driver does have thermal shutdown however.
– user3045
Commented Mar 20, 2011 at 1:57
• So he doesn't even need resistors because he has a chopper? Commented Aug 3, 2013 at 12:52