How can I measure correct polarity

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.

Anyone know how?

Thanks

LM

Reply to
letterman
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On Mon, 06 Apr 2009 18:01:24 -0400, snipped-for-privacy@invalid.com put finger to keyboard and composed:

Use a multimeter to test for continuity between the hard drive's +12V power pin and the enclosure's power socket. Do the same for the ground pin.

If it is a PATA drive, then the yellow power wire goes to +12V and the black one goes to ground. Presumably the +5V supply is then generated within the USB enclosure via a switchmode regulator.

If it is a SATA drive, then use this pinout diagram:

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Be sure that your +12V supply is regulated. An unregulated supply may damage the hard drive's Transient Voltage Suppression (TVS) diode.

- Franc Zabkar

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Reply to
Franc Zabkar

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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Reply to
Samuel M. Goldwasser

Jeez, you can probably find a set of security bits at your local hardware store, or Harbor Freight, Northern Tools, ... Also online at

and many others.

Besides, if you're into this sort of thing (no pun intended), you might enjoy browsing around the AllElectronics site.

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Rich Webb     Norfolk, VA
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Rich Webb

On Mon, 06 Apr 2009 20:31:38 -0400, snipped-for-privacy@invalid.com put finger to keyboard and composed:

Use the Maximus Decim NUSB generic mass storage driver set:

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That's the way I did it, but from within a Windows DOS box, IIRC.

- Franc Zabkar

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Reply to
Franc Zabkar

I've got one of them and the PSU label says centre +ve.

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Reply to
Rodney Pont

I just had an idea...

Try sticking your ohmeter -- specifically, with the diode test selected -- across the power-jack terminals, and measure the resistance in both directions. I'm guessing that the lower resistance represents the correct polarity. (Your ohmeter is probably + on the red lead, but this isn't always true.)

Reply to
William Sommerwerck

I don't know about hard drives, but every device I have that runs from a wall wart, has a molded-in logo that identifies polarity. Hard to see at my age without strong light and strong magnification, but it's always been there.

Besides voltage and polarity, the other thing that's critical is the center pin diameter. Easy to overlook if the outside contact is snug, because you don't notice that the inside is loose.

Reply to
Smitty Two

It does it's job but drives aren't something I get attached to. I wouldn't call it fast, USB is a lot slower than internal SATA II :-)

I'd try it with the 4A one first and time how long it takes to come ready. Then do the same with the 1.5A one. They can take a fair bit of power to spin up and if 1.5A isn't enough it may not come ready if it pulls the voltage down too much.

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Rodney Pont

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