my 2.x laser got zapped while it was off!

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my 2.x laser got zapped while it was off!

Postby oldbot » Mon Sep 09, 2013 3:02 am

My 2.x laser was off when it started sparking.

By off, I mean that the hot leg was off. the neutral was still connected and so was the ground.
There was some kind of power problem on the power circuit.
This kind of looked like a brownout--all the other computers on the circuit went off or locked up and had to be restarted.
But I think there may have been a spike as well.
How else can I explain what happened.

Sparks stated coming out of the printer cable.
The computer on the printer cable never worked again.
The cable had opens on most of the wires.
I was using a db25 switch to control the 2.x from two different computers.
This switch was ruined as well.
The 2.x was dead.

I tested the control board and found many components fried.
I tried replacing them, but ruined the board in the process.
I soldered together an new board.
The laser power supply was fried.
I replaced it.

It is working fine now.
I now plug the 2.X into an isolation transformer.
And I plug the isolation transformer into an EMI filter.

This may not be enough to prevent this from happening again.
What if the computer (which was on) was the cause of the problem?
Perhap the computer sent 115 volts through the printer cable.
The sparking certainly looked like about this voltage.
The sparks that broke through the cable were only about one foot from the computer.
The 2.X 5v power regulator was intact, but everything that was connected to the signal cable was fried.

I frequently use the printer cable on a computer for control and data acquisition.
I often use optoisolators on every printer line to protect the computer.
This takes two 5v supplies, one for inputs and one for outputs.
The worst that can happen is that the optoisolators for one direction and their power supply will burn up.
I never thought about protecting the external circuits from the computer, but the optoisolators would also do this.

This may have been just a fluke.
Perhaps it will never happen again.
But I am thinking about optoisolators on pins 2,3,4,5,6,7,10,11,12,14,16,17
These are the only pins I use.
I put the air pump, water pump, laser and relay for blower on one power strip.
I just turn this on and off when I want to use the laser.
The grounds should not need isolation.
I think the burned up (just the switch was fried) db25 box might be a good place to do this.

Anybody have any ideas?
fuses? zeners?
Another explanation of how my 2.X got fried?
Any other ideas on protection?

Thanks

Oldbot
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Re: my 2.x laser got zapped while it was off!

Postby cvoinescu » Mon Sep 09, 2013 9:00 pm

Because most power supplies are designed to also work in Japan and Europe, where many outlets are not polarized (e.g. the German "shuko" plug can be inserted either way), they don't treat neutral differently from phase, so I don't think something happening to the neutral could have done that. I suspect that the problem involved the ground (earth) connection, quite possibly a short between phase and ground somewhere (not necessarily the computer, but something in the same outlet or power strip, or maybe in the device at the other end of the cable). Normally, that should pop the breaker within a few milliseconds, but not if the ground connection was iffy. The case (and signal GND) of the computer would have been close to phase voltage, and the connected device, with a better ground connection, would have been close to zero, allowing lots of current to flow through the printer cable (and the delicate circuitry at either end). An RCD, aka GFCI, should have detected this situation, possibly cutting power before anything got fried. I am a great fan of those.

Check that the outlets and power strips involved have a good ground connection. Maybe a good connection would have saved your gear, and maybe it'll save your life one day.
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