Re: How can I measure correct polarity
- From: sam@xxxxxxxxxxxxx (Samuel M. Goldwasser)
- Date: 06 Apr 2009 20:23:11 -0400
letterman@xxxxxxxxxxx writes:
I bought a Seagate USB harddrive in a case. It was on the bargain
shelf because it was missing the power supply module (transformer).
The plug is labelled 12VDC 2.0 A.
I have lots of these transformer modules since I bought a whole box of
them at an auction, and I have several 12VDC ones. One is a 4A, the
others are smaller. I'd almost expect the 1.5A ones that I have to
work also, because I know that harddrives do not use that much power.
Either way, the problem is the polarity. It has one of those common
center pin and side connectors that look like a target in a hole.
I know these are normally wired so the center pin is positive, bu
there are exceptions. I can easily measure the polarity of the module
output with my meter, but how can I measure the input of the harddrive
to determine which polarity is required?
Originally I intended to open it and look at the circuit board for
polarity markings, but they have it sealed with some sort of oddball
screws that look like a star with a pin in the middle. I've
considered drilling them out, if I can use a meter to determine the
polarity, that should be fine.
The surest way would be provide the complete model number and then
someone who has one can check!
The next surest way would be to open and trace the input to the
disk drive power connector, of if is wired directly, to the
filter caps which will be labeled with polarity.
Those strange screws sound like "security Torx". You can get a
complete set of security screwdriver bits for those and many other
types of $5 or $10. Or, it is usually possible to simply break
off the center hib with a small screwdriver or other suitable tool.
However, if you check continuity between each side of the power socket
and ground of the USB connector, the one with 0 ohms would almsot
certainly be negative.
Normal disclaimers apply - I can't guarantee that Seagate didn't
do something totoally stupid and confusing!
--
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