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I have a RAID card where someone in their infinite wisdom decided that only the cathode side of the individual drives' fault/activity LEDs would be provided on the board, and the other side of the header unconnected; the "global" activity LED works normally.

My idea at this point was to simply connect all the anodes to the supply from the "global" LED; this works to some extent, but when the "global" LED lights up, all the others turn off.

Any idea what might be causing this?

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    \$\begingroup\$ Can you post a schematic? This will be required if we are to properly answer the question, otherwise we're just speculating. If lighting the global LED turns the others off, it seems that you haven't connected to the correct side of the LED. Common-anode designs are very common and considered good practice. \$\endgroup\$ – Kevin Vermeer Oct 4 '11 at 16:16
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There's probably a current limiting resistor on the "supply" side of the LED you are tapping from.

While you are unlikely to find an actual schematic for the board, perhaps you can trace it enough to figure out if there is a resistor on that lead, and tap power from the upstream side of it to supply to your led's through your own resistors.

Or tap power directly from the output of the board's voltage regulator.

Or from the 5v line to the disk drives.

You will need your own current limiting resistors - you can work out the value from the LED specs, or you can just start too large and make them smaller until the LEDs are "bright enough".

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