HB-415M Stepper Driver Heatsinking: Lack Thereof

Just to see what’s inside, I took those HB-415M drivers apart. They’re not all identical inside:

HB-415M Driver - interior top
HB-415M Driver – interior top

The other side shouldn’t come as much of a surprise:

HB-415M Driver heatsinking
HB-415M Driver heatsinking

Now, admittedly, I’ve applied a heatsink to the top of an epoxy package, but that DIP package has thermal tabs that should connect to the heatsink through a low-thermal-resistance path. A dab (!) of heatsink grease and what might be a thermally conductive plastic sheet atop the package seem, well, insufficient.

The driver chip sports an Allegro A3992 marking that might be genuine. The datasheet goes into some detail as to how you should lay out the PCB; none of its recommendations made it into the finished product. In particular, the hulking current sense resistors surely have more inductance than you’d like.

The resistor color code seems odd: black red red silver brown.

HB-415M current sense resisors
HB-415M current sense resisors

Using black as the first band is unexpected, but it’s probably the only way to indicate a low-value resistance without printing the numbers: 0.22 Ω ±1%.

Ah, well, the peak current isn’t as high as they claim, so it all probably works out in the end.