Boosting the power for a stepper motor without breaking things

I have a NEMA-17 stepper motor (a 12V Mercury Motors SM-42BYG011-25 - and am trying to pull a slider across a track for a motion time-lapse. I am using a Raspberry Pi Zero with a motor hat: I had an 8-pack of AA batteries that I was initially using to power the slider, but was not happy with the batteries running low after two runs down the track. I purchased a 7A, 12V rechargeable lead-acid battery, but when I hooked it up, instead of stepping in one direction the motor was fluttering back and forth with significant strength. After about ~10 seconds, a flash and smoke came out of the driver's IC.

I have a spare channel in this motor hat, so it is still usable (I only need one stepper motor attached to it). What went wrong? The motor's max current (according to the Adafruit page I bought it from: is 350 mA and the driver should be able to drive 1.2A at 4.5 to 13.5 volts. Is there any way to hook up a beefier battery to the motor?

Thanks in advance!

When you doubled the voltage you doubled the current (Ohm's law etc.) and that caused it to fry. The Adafruit motor hat doesn't regulate current; the driver chips are just simple H-bridges.

The better option for driving small steppers are "Stepsticks". Aside from being just generally a better way to run steppers, they allow you to set the current and in that respect guarantee that you won't go overcurrent using any input voltage up to ~40V. And of course they're cheap and easily replaced when you do blow something up.

There is a "PI CNC Hat" that would work for you just for mounting purposes; it is not specifically required if you're OK with creating your own carrier board.

The motor is rated at 350 mA. That is the maximum steady state current allowed.

The motor will burn up if you try to force more current through it, so if you need more motor power, pick a beefier motor.

If the motor power is not a problem, the driver may be, because many of them cannot be set to a current limit as small as 350 mA (the EasyDriver is one of those). You can use higher voltage if you put a small, high wattage resistor in series with each motor winding, to limit the current.

I have some of those motors and I control them with Pololu A4988 stepper drivers which similar to the Sparkfun BigEasydriver. There is a potentiometer on the driver that allows the current limit to be set to protect the motor.

Stepper motors work better with high voltage power supplies. I power mine with a 19v laptop power supply. But a higher voltage would be even better.

...R Stepper Motor Basics Simple Stepper Code

Wondering if you used a powerful lead-acid battery without a fuse? Always have a suitable fuse when using high current
sources like lead-acid or lithium batteries, they can dump huge amounts of power without current limiting.

Some fault was present to cause the failure, I wouldn’t expect it to struggle at 12V as its a 34 ohm
motor. Its designed to thermal cut-out too.

I suspect much higher current was flowing, or significant voltage spikes were happening (that motor
driver is only rated to 13.5V, its not designed for lead-acid batteries which go upto 14V or more fully

I would always have a healthy voltage margin in a motor driver, 100% margin would be ideal, but at least 50%,
since spikes and over-voltage are common. Good decoupling (lots of electrolytic) is good practice.

With most motor drivers when you decelerate the motor it dumps power back at the supply, maybe raising the
voltage - usually a good battery as supply soaks up that reverse power nicely, but its something to be aware of.

Thank you so much for the replies! I am used to dealing with electronics that don't really care how much current you put through them - after looking into it more and taking a look at the diagram for the stepper motor, I can now see that, each pair of wires is just a coil - the more current you put through it, the hotter it will get. Without any regulation on the stepper driver side, I could have easily burnt out the motor - the driver ended up failing before the motor.

I do have a spare EasyDriver chip which I was going to use for another project! Given the max amperage of the motor (350 mA) and the ability for this chip to regulate amperage from 150mA/phase to 700mA/phase, I think this will work (I won't need the "Big Easy Driver" for this motor). It even looks like the pins on the EasyDriver line up nicely with the proto hat for a pi, making everything a nice tight package.

In terms of a fuse with the 7A battery - If I can regulate amperage on the board, would I need a fuse?

Thanks again for the help! I've learned more about stepper motors in the last 24 hours than I ever thought I would!

I am used to dealing with electronics that don't really care how much current you put through them

Circuits DRAW current according to the input voltage and the circuit characteristics.

It is a good idea to use a fuse on a battery or power supply that can supply a large amount of current. The fuse can prevent fires in the case of shorted wires or parts.

mdorfma2: In terms of a fuse with the 7A battery - If I can regulate amperage on the board, would I need a fuse?

A lead-acid or LiPo battery can produce currents of 100 amps or more for a short time if there is a short circuit. The purpose of having a fuse close to the battery terminal is to protect the rest of the wiring from melting and/or causing a fire.

People have suffered serious injuries when metal jewelry (such as a ring or bracelet) has caused a battery short circuit - another risk to watch out for.


mdorfma2: In terms of a fuse with the 7A battery - If I can regulate amperage on the board, would I need a fuse?

Have you seen a high current short-circuit? metal melts/vaporizes, bad things happen, wiring catches fire, batteries get permanently damaged, etc etc.