A friend gave my sister a used hard-drive without the power supply.
The case says the voltage, millamperage, and the fact it is dc but
fails to say whether it is center positive or center negative.
Is there a funny symbol next to the connector, like a dot surrounded
by the latter C, something like this (dot not shown due to ASCII
limitations)?
- -------C------- +
The line on the right goes to the dot in the center, which indicates
the center contact, and the C represents the surrounding hole. In the
above example, the outer contact is negative, the inner one positive,
which is the most common connection.
If you have a multimeter (even a $3-5 digital one is fine, but avoid
cheap analog meters), you can measure the ohms between the case and
each contact, and the contact that shows the lowest resistance is the
negative one. Measure both ways, one way with the black meter lead
touching the case and the red one the power contact, then reverse the
leads. If the case is plastic, measure between the contact and the
outer metal shell of the USB or firewire connector
Can you open up the drive case? It may be held together by screws
hidden under the rubber feet, or it may have snaps that can be pressed
where the two halves meet. The negative connection of the power
socket should be attached, either directly or maybe with a black wire,
to a copper trace on the circuit board that goes to the negative lead
of a large filter capacitor, which will be a plastic-sleeved cylinder
with minus signs printed on one side. Also the outer shell of the USB
or firewire connector will be attached to the same copper area.
