Followups set to sci.electronics.repair .
With a sharp knife, remove 2 inches (50 mm) or so of the outer
insulation of the cable, in the middle of the cable. Try hard not
to nick the insulation on the four individual wires inside the cable,
but if you do, it's not a total disaster.
Next, try to identify the two data wires and the two power wires. Often
the data wires will be twisted; the power wires may or may not be. The
power wires will also often be one or two wire gauges thicker than the
data wires, but not always, and this is kind of hard to tell. The power
wires *might* be red and black, but don't count on this.
Pick what you think is one of the power wires and cut it, right in the
middle of the stripped part. Strip the cut wire ends 1/4" (6 mm) or so.
Get your multimeter and test between all four contacts on the big (PC)
end of the cable, and the stripped wire that comes from that connector.
It may help to use a paper clip or other small piece of solid wire to
touch the contacts in the USB connector if the multimeter probe won't
fit.
If the stripped wire is continuous with only one of the two outside
contacts in the USB plug, you did indeed cut one of the power wires.
If the stripped wire is continuous with only one of the two inside
contacts (it doesn't matter which one), you picked a data wire; solder
the stripped ends back together, insulate with a small amount of tape
(electrical tape is ideal, Scotch tape will work fine for this), and
pick another wire.
If the stripped wire is continuous with more than one contact, then
you probably nicked the insulation on more than one wire when you were
initially stripping the cable - find the places that are touching and
separate them, and maybe insulate the bare spots individually with
tape.
Once you have figured out that you did indeed cut the power wire, you
can figure out how to hook the power wire up to your multimeter. Your
meter may have come with alligator clips that go on the probe ends; if
you have those, use them. What I use for things like this, because I
already own some, are clip leads - short stranded wires with alligator
clips on each end. Rat Shock 278-1157 or 278-1156 are typical; Fry's
probably has a better price on similar products.
http://www.radioshack.com/product/index.jsp?productId=2062661
http://www.radioshack.com/product/index.jsp?productId=2062660
If you have alligator clips that fit your meter probes, put them on the
probes. If you have a digital meter, clip one probe to one stripped
wire and the other probe to the other one - it does not matter which
way around the red and black wires are. If you get the probes
"backwards", you will get exactly the same current reading, just with
a minus sign in front of it. If you have an analog meter, you have to
get the probes the right way around, but IIRC you have a digital one.
If you have clip leads, clip a red clip lead to the red probe of your
meter and to one of the stripped wire ends. Clip a black clip lead
to the black probe of your meter and to the other stripped wire again.
As above, if you have a digital meter, it does not matter which way
around the red and black wires are.
Next, figure out how to make your multimeter measure current. With the
meters most people have, you need to rotate a dial to an "A DC" range,
and move the red meter lead over to a different jack on the meter. Do
whatever you need to for your particular meter.
Plug the mini-USB end into your peripheral (external hard drive or
whatever). Put the peripheral somewhere close to the meter, so you can
watch it and the meter at the same time. While watching both the
peripheral and the meter, plug the PC end of the USB cable into the PC.
If all is well, the peripheral should come on (hard drive spins up /
LED comes on / whatever), and the meter should show some amount of
current. If nothing is happening, or if you smell smoke, unplug the
USB cable from the PC immediately and investigate. If the peripheral
draws "too much" current, the PC may shut down that particular USB port;
this isn't permanent, but sometimes it takes a reboot to re-enable the
port.
Don't be surprised if the current jumps around some as the device
operates. Even "simple" stuff like keyboards will draw slightly
different amounts of current when you are typing vs not, or when (say)
the Caps Lock LED is on, or whatever. If your meter has a "max hold"
or "peak hold" function, that can be useful to capture the highest
current reading the meter sees. Some peripherals will have a current
rating printed on them, but this is usually the maximum current it
will ever draw - it won't usually draw that much continuously.
When you are done measuring current with your multimeter, disconnect
the leads from the circuit, and *IMMEDIATELY* move the red lead back
over to its regular socket - don't wait until later to move it back.
The reason is that in the amps range, the meter is nearly a dead short.
It's *easy* to measure current and then try to measure voltage without
moving the lead back, and blow the fuse in the meter. I've watched it
being done and I've even done it myself.
Standard disclaimers apply: I don't get money or other consideration
from any companies mentioned.
Matt Roberds