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I have a 3-digit, 7-segment+decimal display I bought from an ebay store. I believe it is a hi-eff red one, and the seller provided this link to the data sheet. A search found this data sheet which is more complete and seems to describe the part better, though bottom line - I don't know exactly which part I have. The part has "C403E (1) T0831 RoHS" printed on one edge.

I checked each segment as I wired them on a breadboard, and when I inadvertently powered one without its resistor, surprise! It glowed green - not overly bright, and didn't burn out. I repeated this with, and without its resistor; with a resistor it's red, without one, it's green.

Anyone know why / how this could happen? Every bi-color, common cathode part I've been able to find has two anodes per segment. This one has only one.

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    \$\begingroup\$ LEDs can shift in color slightly as a function of current and temperature. The part you listed is normally orange, so it'd only have to shift 20-30 nm in wavelength to look green. \$\endgroup\$ – Theran Jan 23 '12 at 18:48
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Many red and orange leds looks green when powered beyond their current limit, and some of them lasts for minutes. But every one of them finally change from a LED to a SED (where the S stands for SMOKE :-) )

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    \$\begingroup\$ If you keep applying too much power, it eventually turns from a SED to a DED-- or Dark Emitting Diode. \$\endgroup\$ – user3624 Jan 23 '12 at 22:40
  • \$\begingroup\$ @David: With enough voltage it can continue to be a SED, right up until the point that it becomes a NED (non-existent diode). If there's not enough power to make the transition from SED to NED, it will become a DED. Sounds like Dr. Seuss, actually. \$\endgroup\$ – darron Jan 24 '12 at 3:12
  • \$\begingroup\$ Then this is the likely explanation for what I saw - thank you. \$\endgroup\$ – JRobert Jan 24 '12 at 20:25
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  • It is almost certain that you are murdering your LEDs by running them in the manner described.

    There is every chance that LED lifetime will be much to very very much reduced as a result.

You do not tell us your voltage source, what current it can provide or what current it does provide in "murdering mode". We need this data to comment more usefully about operating conditions, but it is very very very very likely that what you are doing is NOT intended by the manufacturer

There is no reason to think that the LED is different from what is in the data sheets. While there is a small possibility that the datasheets that you have do not apply, this is very very very very unlikely.

The datasheets give a very straight forwards and usual description of the display as being a 3 gigit display with 24 LEDS total (3x8). Unless the datasheets are the wrong ones you need to decide between 'interesting but very short lifetime' and 'normal operation'.

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  • \$\begingroup\$ I'd expect so; but thanks for the heads-up. It was a brief, and accidental (except for me repeating it to confirm what I saw), energizing without the resistor and I don't intend using them that way. But I didn't expect the color shift, but rather a very bright LED. That's not what I saw, so I asked. BTW, the source is a USB to Serial converter cable powering a breadboarded circuit I was ringing out. \$\endgroup\$ – JRobert Jan 24 '12 at 20:29
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Could it be that there are actually 2 LEDs wired in there per segment with cathodes and anodes both tied together? At one current (the resistor limited one) only one of them turns on (the "red" one) and at the higher current (no resistor) both turn on giving a different aggregate color ("green")?

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  • \$\begingroup\$ This sounds like a reasonable enough explanation -- do you know of any like that? I had just never heard of one, nor can I find reference to one. \$\endgroup\$ – JRobert Jan 23 '12 at 18:47
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    \$\begingroup\$ Pure speculation: There may be a parasitic path in the LED device that emits in the green, which would have the same effect as having two LED's in parallel. Actually operating the part with enough current to make the parasitic path light up brighter than the designed path is probably not good for reliability. \$\endgroup\$ – The Photon Jan 23 '12 at 19:24
  • \$\begingroup\$ @ThePhoton's explanation seems correct. It's notable that the human eye is far, far more sensitive to green than to red. A bright green LED next to a normal red one will look completely green. \$\endgroup\$ – Kevin Vermeer Jan 24 '12 at 13:29

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