My auto-disconnect hack

[Bob in a quantum-state-of-faith]

Lately, I have installed my venerable Dell Streak 7 up into the headliner of my work van, using velcro.   It sits there neatly behind the sun visor, hidden from casual view.

I also have routed a 12v power box over to that spot (the headliner has a nice 4" space between it and the actual sheet metal, at that point).    The power box I used was one I'd had, but it has a lovely little auto-disconnect feature in it, if you disconnect the power, it does not leave accessories inter-connected to each other.   So I powered it via a convenient switch, naturally.

But into that 12v box, I have a 12v to 5v adapter, high power (2.1 amps, actually) that then goes through a proprietary cable to the Dell, keeping it's battery topped up.

Now, I have no idea how well that 12v adapter is constructed, reverse-voltage-wise.  Does it leak volts backwards through itself, trickle-draining my Streak's internal battery?   I do not know, so I've been pulling the 5v plug anytime I'd powered off the 12v box.  Kinda defeating the point of a handy switch, if I also manually pull the plug too.

So....

... a trip to Mouser (a lovely house of everything electronic-parts wise, thousands of electronic products) and a look-through their 5v relay offerings and I selected me a nice 5v coil, DPST relay:  double-pole, single-throw.   That is, there are two switches that only go on/off-- on when energized, off when not. 

And I took one of my spare Dell cables, and .... sliced it open with a razor knife.   Inside, I found what I expected:  4 wires, neatly color-coded:  red, black, white and green.  According to the online specs, they are +5v, -ground volts, -Data (about 1v) and +Data (approximately 3v or so).   I reasoned that switching the +5v and the +D lines should be sufficient to keep from draining my internal battery through the adapter's electronics.   I confirmed all this, by plugging the now open USB plug into a charger, and measuring the volts coming out of each wire with respect to the others. 

So I wired the now-split cable (which I also conveniently shortened to a more manageable length).

The coil has 6 pins:  2 to the coil (which does not care about polarity), and 4 more, 2 per switch.   So it was from the USB plug end, red/+5v to one coil terminal, including a short jumper over to one side of one switch, and on the remaining switch terminal, the red/+5v from the Dell plug.   Then the black/ground to the other coil pin, and also the black wire from the Dell-plug end, a convenient tie point.   For the final two pins, I put one green wire from the USB plug, and the other green wire from the Dell plug.  Lastly, I twisted the two white wires together.  Careful soldering of all points, including the twisted pair.

Now, to test it's functional operation:  re-plug into the charger, and measure volts at the various pins:  all is as it should be, with the relay switching the 5v and D+ wires anytime it's plugged in, and disconnecting them when it's not.

A careful wrapping of electrical tape, to also mechanically secure the ends of the cables to the relay body, and a nice, neat little package, with a nice shortened USB to Dell via a relay cable for my Streak.

Now?

Whenever I hit the switch to kill power to the 12v ports in the upper left corner of my van's headliner, it does kill 5v power to the Dell's cord, allowing the relay to open, eliminating a possible battery-leak backwards through the converter's electrics.   No need to manually disconnect.   

Nice. 

[Sibling DavidH]

?

[Bob in a quantum-state-of-faith]

All diodes leak current, even in reverse. 

A positive disconnect eliminates any possibility of leakage.

I wanted to avoid even a slight leakage.