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Old 9th January 2015, 03:24 AM   #1
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Default Confused about DC supply board

I had the idea to use a dc power supply board out of an old non-functioning Toshiba cd/dvd player for a bench dc power supply. It has a 14 wire ribbon cable out with a 19.5, several 10, and a couple of 5 volt dc lines. Great! Well line 1 was white and the rest grey so I figured it was ground. I hook my voltmeter and unless I have the positive clip on what I thought was ground I get neg. voltages??? The thing is with the way I tested I get 1-19.5, 10-9.8V, and 2-4.97 DC volt connections but with one positive leg. I'm not an EE and I don't understand how this works? Can anyone explain?

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Old 9th January 2015, 10:06 AM   #2
DF96 is offline DF96  England
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Ribbon cables often have one edge marked with black or white so that people know which edge is which. It means precisely nothing at all about the use of the wire at that edge.

It is likely that there are several ground wires in the ribbon, probably each associated with the adjacent power lines. Use a pencil and paper to work it out from voltage measurements, or look at how it is connected to the PSU PCB.
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Old 13th January 2015, 02:10 PM   #3
Stuey is offline Stuey  Australia
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I've found with these types of power boards the voltage is sometimes marked in tiny text figures at the origin of the wires on the PCB, where the ribbon cable plugs in. I also noticed what DF said - that there are a number of ground wires, seemingly one per power wire.
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Old 15th January 2015, 12:16 AM   #4
gibbman is offline gibbman  United States
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As an example, a red edge on the ribbon cable going to a disk drive indicated Pin 1 on the connector. No indication of what was riding on that wire.
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