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Normally I think of LEDs being polar and having a definite current direction, the "D" in LED standing for "diode".

However, in working with with a computer case recently, the front part of the case has several LEDs, like the power-on indicator, and these connect via a Dupont two-way housing to a pin header on the motherboard. These connectors are non-polar--I can plug them in either way and it works.

So, how is it that these computer case LEDs appear to be non-polar?

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  • \$\begingroup\$ @Finbarr I missed the two-way part. I'll remove my comment. \$\endgroup\$ – Oldfart Nov 22 '18 at 12:42
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They are bi-directional LEDs.

Most bi-directional LED have different colour, red, green being the majority, but you can get red,red: enter image description here

It would be more expensive then a standard LED but still be a fraction of the cost of the cable plus connector and savings in assembly.

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Indeed LEDs are diodes and only light up if the current flows in the right direction.

To make the 2-wire connection work both ways I see 3 solutions:

  • the case LEDs have have a build in bridge rectifier

  • the case LEDs are actually 2 LEDs (or more) connected in anti-parallel so in one direction one LED light up, in the other direction the other LED light up (see Oldfart's answer). I think this is the most likely scenario as it is the cheapest.

If neither of the above is true it could be that the motherboard does a trick:

  • the motherboard doesn't output DC like is usual but it outputs AC (not a sinewave but a square wave, that's easy to do in digital circuits) that way whatever way the LED is connected it will light up (half of the time but if that time is short enough, we humans cannot see that).
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It's a bi-directional led -- two led's in a common package in back to back configuration.

It looks like a single led with equal lead size.

enter image description here

I don't know why they used it for computer application even though DC power is readily available. Actually this led is not required for this application, they can go with traditional led's.

I came across these kind of leds in AC application. In power panel indicator lamps.

enter image description here

There will be no DC supply available so they will use a capacitive/resistive power supply and connect a bi-directional led to it. One led will be in forward bias for 1st half cycle other led will be forward biased for 2nd half cycle. So there is no need of bridge rectifier, filtering capacitor here.

So they can save some money in high volume production.

For capacitive and resistive power supply please see schematic. The component values we are going to select based on LED properties

schematic

simulate this circuit – Schematic created using CircuitLab

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