Making sure your stepper motor wiring is robust and secure is critical. Most (all?) stepper motor driver instructions include the warning “do not connect/disconnect motors while power is applied”. Curious why?

A brief-yet-incomplete intro to stepper motors is appropriate. Stepper motors contain several windings. These windings are very large inductors. By their nature, inductors a) resist changes in current flowing through them and b) store energy in the magnetic field they create.

Steppers are often rated at a very low voltage and fairly high current – like 3 Volts and 3 Amps per winding. Yet we suggest a very large power supply, at 36 Volts (or more depending on the driver). My board (and most others) energize a winding by slamming (a technical term) the voltage to the winding and monitoring the current flow. When the current hits the max setting, the board backs-off the voltage, in my case by using a chopper technique – often you can hear the motors ‘whine’ at the chopper frequency.

 

As the current begins to flow in the winding, a LOT of energy is stored as a magnetic field around the winding. When that winding is turned-off, that magnetic field collapses quickly, in turn generating a lot of power – and this power needs to be dissipated (typically as heat) by the driver board. So far, so good.

So what happens when you mess with the wires while the motor is powered-up? Here’s one example. Take a close look at the solder joint in Figure 1. This is the common for the a-A winding at the terminal strip for the x-axis. The solder is hanging onto the pin, but not making good contact to the PC board trace. It worked for an hour-or-two. At some point, this connection opened-up.

Remember all that power stored in the winding? It’s going somewhere. Since it couldn’t go through the common pin, it had to go through the driver chip – causing enough heat to blast-off a chunk of the driver IC.

For the record, this is NOT covered by warranty.

Pro Tip: Make double-sure your power supply is at zero volts before you mess with any stepper motor wiring!

Bad Solder Joint

Figure 1. Bad solder joint (magnified 40x)

Blown Chip

Figure 2. Blown driver chip

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