Stepper Motor Driver Bypassing: Mind the Voltage

The supply voltage for that picture came from a bench supply and, having confirmed that the initial slope of the current waveform matched the voltage, I twiddled the knob while watching the slope change.

As expected, lower voltage = lower slope and higher voltage = higher slope. That worked fine, right up until a firecracker popped about a foot in front of my face, launched a missile over my left shoulder, and filled the Basement Laboratory with the pungent smell of electrical death.

Detonated electrolytic cap
Detonated electrolytic cap

While wiring up a hairball test circuit for that Pololu driver, I’d put a pair of electrolytic caps on the +5 and +12 V supply lines, seeing as how solderless breadboards aren’t all that great for power distribution. The brown fur growing just to the upper right of the heatsink is what’s left of a 16 V cap that had 25 V applied for a few seconds: I’d wired in the bench supply in place of the breadboard’s fixed +12 V output and forgot all about the caps.

The cap body departed for the far reaches of the Basement Laboratory, leaving behind shredded cardboard and unrolled plastic strips. I’m sure it’ll turn up some day.

Nothing else took any damage, but for a few minutes I thought I’d killed Eks’ AM503 current probe, which pokes in from the lower right.

The black lump just above the probe is an ordinary AC current transformer that didn’t work well at all: the 1/rev frequency was just too low.

If you don’t always wear glasses at the workbench, start now.

9 thoughts on “Stepper Motor Driver Bypassing: Mind the Voltage

  1. Fully agree on the wearing of safety glasses, I also always do when soldering. I got lucky a few times before I started wearing glasses (blobs of solder hitting my face, not my eye). Don’t laugh, but I also started wearing safety glasses when handling magnets. If you’ve ever (mis-)handled a NdFeB magnet, you’ll know why…. :-|

    As to the electrolytic caps – amazing they withstood the overvoltage for such a short time – recently I was surprized by their (relative) ruggedness: I was modifying a PC PSU to a variable 5-55 V supply, but had made a mistake: the rectifier diodes were miswired, with polarities being reversed (what should have been + was -, and vice versa), causing the electrolytic caps to overheat and blow their top (little fountain of steam and water coming out). Nevertheless, they withstood several minutes of operating with reversed voltage before blowing – which impressed me. The fact that they got quite warm should have given me a clue though.

    1. The fact that they got quite warm

      Miracle of miracles, they didn’t blow right underneath your fingertip!

      One morning, a long time ago in a universe far away, I was working in a humidified and darkened room with laser light dancing all about, when a brace of solid tantalum ‘lytics blew up inside a motor controller, filled the room with fumes, and produced one of those jumping-out-of-your-skin moments.

      Turned out they’d been miswired just like yours the previous afternoon and lasted for an equally surprising time that morning…

      1. When I was working in contract manufacturing, someone loaded a reel of electrolytic caps into the pick-and-place, that went into the boards backwards. (We don’t know if the supplier had loaded them wrong or if it was possible to load a reel backward.) Anyway, we started loading boards at blinding speed, and nobody noticed until they started getting back to functional test. There were about 40 caps per board. I was working in functional test that day so I was helping load them into fixtures. We had about seven boards loaded and powered up when the first caps started to go. It was like a little war: explosions, smoke, spiral smoke trails as debris shot into the air, and we didn’t actually have any idea what was going on, so we were all AAAAAHHHHHHH!
        Afterwards, we thought it was pretty cool.

  2. Speaking of things that go boom unexpectedly, I nominate the humble lead-acid storage cell.

    I’ll never forget the time I finished a tune up on my first real car, cranked the engine for a test run only to be greeted by the sound of an under-hood grenade. Opened things up to find a splintered battery, and a 1/2″ bulge in the steel reinforcement beam that ran along the hood over the battery area.

    I have a healthy respect for hydrogen now :-)

    1. They’re supposed to have two differently sized terminals, but it’s amazing what a distracted mechanic can do with an open-end wrench and a bit of determination… [grin]

      Or was in a British car with an electrical system by Lucas, The Prince of Darkness?

      1. >Or was in a British car with an electrical system by Lucas, The Prince of Darkness?

        You guessed it. A Triumph TR6. I’m looking for a bumper sticker for it; you know, the one that says “Why Do The British Drink Warm Beer? Because They Have Lucas Refrigerators!”

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