Dangerous barrel connectors or
Why the cost of the Drawbot just jumped $200

Not all coaxial barrel connectors are created equal.  The male on the top is the one that was on the 30V drawbot stepper power supply before it blew up my new laptop.  The yellow one is what’s on that supply now.

I never thought much about these ubiquitous connectors until now.  In fact, I was slightly prejudiced against ones like the yellow one, with their wimpy internal wires/prongs for connection rather than the nice robust tube of the top one.  Now I understand.

The drawbot supply – a repurposed laptop switcher – has a 3 prong plug, and is ground referenced (which is the real problem).  I’d grabbed that male with the nice tube from the drawer and spliced it on in the common center-positive configuration to match the female pigtail from the drawbot, and have used that setup many times to drive the drawbot.

Unfortunately, that nice robust tube extends out and is in fact the frontmost surface of the plug.  Unless you’re very careful, that +30V-with-respect-to-earth-ground center conductor will bump into the front ring of the female.  That ring is ground on the drawbot – including to the USB serial port connected to the computer.  Unfortunately, that computer was my good new laptop, with its 3 prong power supply plugged in ensuring that it was grounded.

The path for the brief encounter with +30V thru the laptop is unclear, but it wasn’t just a short to earth ground.  The laptop went black, never to power on again.  (Yeah, I pulled the battery and held the power button down for a long time, and as many other power permutations as I could come up with.  It’s dead.)

Thinking about it, I’ve seen other cases where plugging in barrel connectors caused unexpected crashes, although this is the first time I can point to hardware damage (or understand what might have happened).

Taking a quick look thru the connectors I could find, it seems that the exposed center pin is pretty common.  In fact some look like they are made that way on purpose (red circle).  I found only one of the tube contact ones that apparently went out of its way to recess the tube so it was safely behind the front surface (green circle).

The takeaway is that males with the center pin exposed – particularly if it’s the outermost surface – can be dangerous, and should be shunned, or at least treated with special awareness and concern.

It’s an interesting, if sad observation that despite all this, packrat that I am I’m not throwing away the murderous end I cut off of the drawbot supply with its center pin exposed, and I’m not throwing away all its brothers, either.  But at least I won’t donate them to the space. 🙂  I will, however, try to remember about them.  (Yes, those are labels inside the bags.)  To ensure I won’t be tempted to use the dangerous ones, I just ordered another cheap bag of the “yellow” kind , and I’ll donate some of those to the space.

Update a couple of hours later:   I was troubled that the cable from the power supply brick had 3 wires disappearing into the heat shrink where I’d spliced in the (bad) male connector.  Was one earth ground, and I’d tied it to negative, and if I’d left it alone the supply would have been floating and none of this would have happened?  I’d spliced the yellow connector on leaving the original splice intact, so the only way to know was to cut the heat shrink to inspect the old splice.  (Well, wrong, but…)

The third wire had not been connected.  I rang it out, and it was clearly not directly connected to earth ground, so at least it wasn’t my fault. 🙂  But what was it connected to?

Looking at the label on the supply answered the question, and even provided a nice diagram of which conductor was which.  If I’d just RTFL, the question would have been answered, fears allayed, and I wouldn’t have had to have a dumb wrap of electrical tape covering the +16V wire exposed when I cut back the end of the heat shrink.  Oh well.

Update the next day: Thanks to a Slack post from Rick Stuart, I found an HP service manual for the blown ProBook 430 G1 and its same-model replacement.  Also a treasure trove of laptop manuals here.

And a further update on earth ground and the power supply:  I just rang out the ground pin on the supply.  While it was connected directly to the negative side of the barrel connector, it wasn’t connected to neutral.  I think that means if I somehow cut off or disconnect the 3rd prong, the supply will be floating, and any future ground loop problems will be avoided.  Interesting.

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