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Splicing a laptop power cable.
September 13, 2007 9:17 AM   Subscribe

How do I splice a laptop power cable?

My new puppy bit through the round section of power cable for my girlfriend's laptop and is lucky he didn't get electrocuted. The damage is similar to, but more serious than, the damage depicted in this image gallery.

When the adapter is plugged in and I wiggle the cable there are visible sparks, and I'm expecting the little nailpolish trick that sarah used (above link) isn't going to work. Many of the outer wires are severed on both sides of the wire, the insulation between the inner and outer wires is torn, and possibly severed a few of the inner wires.

My question is this: am I going to be able to approach this the same way I did when splicing a lamp cable in high school shop class (ie. strip the wire, twist the wires, apply wire nut and electrical tape)? Is this something I can even do at home? How can I avoid buying a new adapter?
posted by ThePants to Technology (9 answers total) 1 user marked this as a favorite
The lamp cable technique will work fine here.
posted by waxboy at 9:24 AM on September 13, 2007

First, IANAE, but if the puppy just bit through the cord and not the molded plastic, this should be easy to repair.

Keep in mind that there are two parts to a typical laptop power supply: the high voltage AC section that plugs into the wall, and the lower voltage DC section that runs from the power brick to the laptop. The high voltage part is typically lamp cord with two or three obvious wires of equal gage, while the low voltage section is likely a piece of coax.

Repair the high voltage side as you would any damaged lamp cord.

For the low voltage side, I don't like to use wire nuts, as the inner core of the coax is pretty damn thin.

This link suggests a better method to repair this type of cable.
posted by mosk at 9:37 AM on September 13, 2007

Wire nuts make for a bulky, snag-prone, and physically unreliable connection. They're suitable inside a wiring box where nothing moves, but not ideal for cord repair.

Do you have a soldering iron?

The best way to do this is with heat-shrink, solder, and more heat-shrink. The trick is to stagger your splices, so the joint in the positive wires is an inch or two away from the joint in the negative wires. That way, it would take a double failure of the insulation to result in a short.

I might have some pictures of myself employing this technique, I'll dig for them if you'd like.
posted by Myself at 11:33 AM on September 13, 2007

mosk: the split is, indeed, in the coax portion of the cable, between the brick and the laptop.
What is he using in that link to make the splice? I can't quite follow the text.

Myself: I don't have a soldering iron, and have never used one. I'd appreciate any help you can give toward illustrating a technique that will work better.
posted by ThePants at 12:13 PM on September 13, 2007

Crap. I just found the pictures I remember taking that day. None of the process itself, just one where I spliced two power adapters to each other, because I wasn't paying attention to which cable was which. Sorry 'bout that.

Mosk's link isn't making a splice, it's simply attaching a new plug further down the cable, discarding the section between the break and the end. Suitable if your cable frayed right at the plug, not so much for a chew in the middle.

I could take some pictures tomorrow. I've been meaning to post some instructables anyway...
posted by Myself at 12:42 PM on September 13, 2007

If the break is between the brick and the Laptop, then the voltages are at a safer level (5 and/or 12 volts DC), so there wouldn't have been a quite lethal shock for the dog. Note however that the currents that travel here can be 5 amps pr more which means it can generate a hefty, tool-gouging spark; also your repair has to be fairly well-made or it may later fail and burn up.

"Myself" described the right way to do it - a careful, soldered splice. If you haven't soldered, try to find a friend who does ans ask. It's about a 30 minute job to do carefully.

Another option is a small plastic or metal box, like an Altoids box. Cut the cable at the break, strip and prepare about 2 " of cable on each side, drill two wire-sized holes, stick the cables in the holes, and join each wire with a crimp cap. Use tie-weraps for strain relief on the cable where it pokes into the box. Close and tape up the box. Ugly, but easy and durable
posted by Artful Codger at 1:20 PM on September 13, 2007

Heck, stripping a quarter-inch of wire and simply hand-twisting the (probably finely stranded) ends together would probably work fine. I'd follow that with solder and heatshrink tubing, but that's because I have solder and heatshrink tubing handy.

Radio Shack used to sell little tiny flat squares of solder for doing splices: you'd fold one over the twisted-together splice, then hold a butane lighter under it until the solder wicked into the joint. They may still sell those. They probably sell heatshrink tubing, too (I find it a lot less messy and more reliable than electrical tape).

You might want to "splint" your splice with something, too. Otherwise, the section of bare wire that's not supported by insulation will flex a lot more often and more sharply than the rest of the cord, and it'll fatigue and break pretty soon.
posted by hattifattener at 2:13 PM on September 13, 2007

I'd use heat shink on the individual splices but go with a self vulcanizing tape to cover/join the splices. The SVT is a bit more moisture resistant and is more pliant.
posted by Mitheral at 3:40 PM on September 13, 2007

Butt Crimps. Choose the appropriate gauge for your job; the cable is probably a pretty small gauge.

When finished with the crimps for each lead (the hot and the common), electrical tape the whole thing just so it's covered. You could also heat shrink the whole thing too. Just remember to throw the heat shrink on the lines BEFORE you start to crimp them together. It always sucks to crimp something together and realize "DAMN! I forgot to put on the [insert part name here]."

Worst example of this ever: soldering a 19 conductor socapex cable, and not realizing that I forgot to add the locking collar until I was completely done.
posted by jimmyhutch at 9:12 AM on September 14, 2007

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