# Can I drive a low voltage stepper motor with an A4988 driver?

I have this stepper motor with the following specs:

Model: 17HS4417
Rated voltage: 2.55V
Rated current: 1.7A
Phase resistance: 1.5 ohms
Phase inductance: 2.8Mh
Holding torque (min): 40N-cm. Detent torque (max): 2.2N-cm.

I want to know if an A4988 stepper motor driver would be appropriate for this motor. I suspect not because the driver specifies an 8-35V rating, while the load is rated for 2.55V.

If not, should I look for a driver with a lower voltage rating?

• It is customary in English to capitalize "I" when referring to yourself. I have edited your question accordingly. – JYelton Jan 9 '14 at 6:55

That driver will work fine, provided you use a proper power supply.

The allegro stepper-drivers are current-limited chopper stepper drivers. As such, you only have to ensure the power-supply voltage for the driver is > then the rated voltage on the stepper, and you have set the current limit properly.

Basically, chopper-stepper-drivers actually modulate ("chop") the drive voltage to the stepper in real-time to maintain a fixed coil current.
The ratings for your motor are steady state. Basically, it says that if you apply 2.55V DC, 1.7A of current will flow though the motor coil.
However, the Allegro drivers don't apply DC, they apply a duty-cycle modulated square wave, which limits the overall power delivered to the motor.

Functionally, the driver will vary the applied voltage to the stepper to maintain a fixed current (it's not quite that simple, motor inductance is involved, but it's a reasonable simplification). As such, as long as you're not applying more then 1.7A of current to the motor, it will work fine.

Basically, the simple version is the motor ratings are basically constrained by the thermal behaviour of the motor. If you apply too much power, it'll get hot enough to damage the motor.

With the A4988 driver board you link, you can vary the motor current by adjusting the tiny pot, which allows you to adjust the motor power to whatever you'd like.

If you run the driver off input DC within it's operating range, you will be fine.

The driver will work fine, you just need to set the current limit to an appropriate level for your motor. The voltage rating of the motor is just specified based on the resistance and the given current.

In this case 2.55v/1.5 Ohm = 1.7A

Stepper motors like this are intended for constant current operation rather than constant voltage so the voltage range of the power supply for A4988 is not a limiting factor and it doesn't prevent the use of the particular motor. On the contrary driving the motor with higher voltage increases the torque of the motor as the speed increases.

Running a motor at higher voltages leads to a faster rise in the current through the windings when they are turned on, and this, in turn, leads to a higher cutoff speed for the motor and higher torques at speeds above the cutoff.

Yes, you can drive a stepper with a low voltage rating using a higher voltage and any current limiting driver, provided you set the current limiter according to the motor´s rating.

But there are limits to this.

The load presented by the stepper coils is inductive. If you drive these with steep high voltage steps, the magnetic flow can become unpredictable. The higher the voltage, the higher the change in flow, the higher the induced eddy currents in the materials.

Using an A4988 and 12V on a Nema8 Motor (3.9V), I had the little motor jump up to 1/8 rotation at times. To avoid this, I had to reduce the current limit so low there was next to no holding torque left: less than 10% of the rated torque.

• Interesting. I can run the the stepper using any voltage within the range of the driver, provided that the current limiter is set to the rating of the stepper. However, higher voltages can lead to unstable magnetic flow. I surmise then that i should drive the stepper with lower voltages. In this case, with a voltage closer to the driver's lower limit of 8V. Is this correct? – Paul Jan 9 '14 at 14:53
• As far as I understand, yes. An alternative might be inductivities in series with the stepper - or a second stepper. – posipiet Jan 10 '14 at 6:53

Edit: The load supply voltage need not be the same as the stepper's rated voltage, it can be 8 Volts or more. This invalidates the "won't work" part of this answer.

The Allegro A4988 will not work at 2.55 Volts load voltage.

The datasheet states:

Load Supply Voltage Range VBB Operating 8 – 35 V

Also,

VDD Undervoltage Lockout VDDUVLO VDD rising 2.7 2.8 2.9 V

This means the device will simply go into under-voltage lockout if the Vdd pin is supplied by 2.55 Volts.

So yes, look for a stepper motor driver designed for lower load voltages. Alternatively, if microstepping and other enhancements of the driver are not important, roll your own driver using discrete MOSFETs or BJTs for the output stage.

• -1 I strongly disagree. Using the particular driver is not a problem if current limiting is set to an appropriate level. These motors are intended for current limited drive rather than constant voltage. – alexan_e Jan 9 '14 at 7:58
• He doesn't need to apply a low voltage to Vmot, he can use 9v or 12v etc. That is the point of constant current driving, to be able to apply initially more voltage to the coils in order to counteract the inductance effect that tries to react to the current change. – alexan_e Jan 9 '14 at 8:29
• @alexan_e Actually, you know what, you're right. I've just tried it for myself with a 7.2 Volt supply and a 3 Volt stepper. OP needs to un-accept my answer. – Anindo Ghosh Jan 9 '14 at 9:01
• Just to be clear, my objection wasn't to the A4988 Vmot range (8-35v) but to the statement that it can't be used with the specific motor. If the question was "can I use the driver with 2.55v supply?" the answer would be no but the question was "can I use it with a motor of the given specs" and the answer is yes. Here is an example, the Rep-Rap schematic using A4988 with 12v supply for Vmot and the motor recommendation for 3-5v models. – alexan_e Jan 9 '14 at 9:13
• @alexan_e Yes, I did get that, eventually. Until that point I was thinking of the supply being constrained to say 5 Volts or whatever the OP had available - simply because that happened to me once. – Anindo Ghosh Jan 9 '14 at 9:16