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Combining sources

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There's a 1k potentiometer limiting the current. I'll be putting three of them in series with the potentiometer on a 5V supply to test them out.

Do you have a suggestion on how to protect the board?
 
It might be better to hook 2 in series instead of 3, just to ensure you have enough voltage. Assuming they drop an average of 2V each, 2 in series would drop around 4V, meaning you want your resistor to drop 1V and limit current to, let's say 2 mA. That would be 500 ohms, or the closest common value would be 470 ohms.

I would test them with the 470 ohm resistor, try 2 and 3 in series with your 5V supply to see how well they work. If you run 3 in series, lower the resistor to 220 ohms, or even 100, but you might find that 5V can't drive 3 in series.

Experiment and see what it takes to get the brightness you require. Then measure the current and ensure that all the LEDs put together don't draw more than 100 mA to be safe for your motherboard. The resistors will ensure that the current draw doesn't exceed the maximum the motherboard can handle, if you size them correctly (as mentioned, 100mA is a good upper limit to shoot for).
 
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