I am testing a board powered by 12V and 5V supplies. I see the voltage drop across my led is 12V. Can this happen?
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\$\begingroup\$ if. is wired incorrectly or blown, yes \$\endgroup\$– Tony Stewart EE75Oct 4, 2016 at 17:08
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1\$\begingroup\$ Possible duplicate of LED Voltage Drop Confusion \$\endgroup\$– Artūras JonkusOct 4, 2016 at 20:26
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3 Answers
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12 volts across a bare LED should destroy it instantly.
However, there are LED indicator assemblies that include appropriate current limiting circuits, so the assembly can be operated directly from 5 or 12 volts.
answered Oct 4, 2016 at 17:08
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Unless your LED is one of those big multi-watt jobbers, then no, it cannot happen (assuming the LED is just an LED). Somewhere on your board is a series resistor which is dropping about 9-10 volts, leaving 2-3 volts on the LED itself. There are some LEDs which have an internal IC which regulates current. These used to be available 30 years ago, but I don't know if they're still around. I suspect they went out of style due to heating of the LED.
If the LED is a panel mount (rather than PC mount) then it's entirely posssible that there is a resistor built in, in which case 12 volts is applied to the unit, but only 2 or 3 volts per LED inside.
answered Oct 4, 2016 at 17:12
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Couple of possible options here. One is that the LED contains a current limiting resistor or is built into an assembly that contains a current limiting resistor, and you're measuring across the LED and resistor in series. Another option is that the LED has failed open and no current is flowing, hence you will measure the supply voltage across the LED as the resistor is not dropping any voltage. Finally, you may be connecting the probes incorrectly and you aren't measuring what you think you're measuring.
answered Oct 4, 2016 at 19:57