Squirrels have fuzzy tails.
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Or how about one of those sockets like walkmans use for headphones. It disconnects the other thing (robot) when the plug is inserted.
[...]The reason is because all these devices then become connected (ground loop), but they *might* not use the same voltage for ground (ground does NOT need to be 0v). In effect, you'll create a short and something will fry.
the Earth is allways connected (or you get in trouble with the Man).
Ground is allways 0V, unless you have very different set of legal regulations in the US, as the Ground (Earth) is connected to Neutral (the 0V reference) at the power station.
[...] a plug or something on the outside to plug in a charger,
I guess you could just use two switches to cut off the power and ground,
Sure there can be a difference, but then either Earth or Neutral is defective and needs a fix (it's usually the Earth resistance that is getting too high. I don't recall the exact max. resistance value of the Earth (and it might differ between US and Europe), but it's quite low and there are regulated test procedures.
Metal cabinet PCs running from a non-earthed outlet will be at 115V potential
IMHO, modern switching power supplies and electronics are less likely to have a ground loop with each other. I have been tying AC powered DC equipment together for a decade and never had an issue, but it is a possibility. I'm ok with frying something every once in a while, so to stop and check every time is a waste of time for me.
Think about it, just as bad as charging your robot while it's connected to your computer , so is powering your robot from a DC bench supply and have your computer connected to it. So, the only safe answer is to ALWAYS run off battery when it's connected to your computer. This isn't acceptable for me. If you wanted to be able to work on your robot while it charged but still be safe, maybe fuse both VCC and GND wires from your charger? Admin seems to be a less reckless engineer as I, so he would be a better source for safety advice.
What if the user is your grandma who happens to lean on her radiator while fiddling with your design? Never mind if a bit of hardware dies, that can be cured, but unless you have a defibrillator ready, granny will be a little harder to de-zombiefy
If it's imperative to work on the powered robot while charging, the charger circuit just have to be protected against putting out too much voltage (or too much current for that matter). A soft start (and end) won't go amiss either.
Soeren, correct me if i'm wrong, but the scenario I can think of is an old/cheap DC power supply, with no ground wire, plugged into an outlet with a poor (floating or high resistance) neutral, could end up causing trouble if it finds a better path through the DC side.
[...] at the nearest transformer station.
The output ground of the battery charger isn't guaranteed to have a 0V difference with Earth ground. If its 1V higher, and you short it with Earth ground, you have a bad ground loop.
Ummm.... I'm just gunna switch off both the power and GND whenever I'm charging the batteries...
And I don't really want to put a charger on board for two reasons... space, and I already have a really nice battery charger that I'd like to use.
What about simple power connectors? (You know, the ones that are in the back of almost every router/switch/external hard drive. The ones that have a positive center and negative outside.) [Snip pic]What do you guys have against those? I know they don't lock in, but I've never seen one fall out accidentally.