wkbdgeorge
Well-known member
Hey all, I havent been able to find a good answer to this yet. Say a there is a 12v bank of leds, If you feed it 6v, what happens? Do they light up dimly or not at all? -G
What happens when you short change an LED?
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wkbdgeorge
Well-known member
I guess also, what happens when it is presented with 20v?
They get brighter?
No seriously LEDs alone would start drawing serious current way before 6V so they are probably already current limited with some series resistance. Increasing the voltage could double the current but that would probably not blow them up if only applied for a short time.. longer time maybe, or could burn up the resistor.
JR
No seriously LEDs alone would start drawing serious current way before 6V so they are probably already current limited with some series resistance. Increasing the voltage could double the current but that would probably not blow them up if only applied for a short time.. longer time maybe, or could burn up the resistor.
JR
An LED has a cut-off voltage of 1.6-4V. If you power a red LED (1.6 cut-off) with 1.5V, it will be completely off (well, maybe not, but veeeery dim). You need a current limiting device for an LED. The simplest is a ballast resistor. Let's say you have a 5V DC source and this red LED. You want 10mA in the LED, you put a 340 ohms res in series (5v - 1.6v = 3.4V; 10mA is 100 ohms/Volt; 3.4*100=340). You don't have 340ohms, you have 330; current wil be 3.4/340 = 10.3mA. Close enough. Now for some reason you don't have 5V, you have a fresh PP3 battery, 9.6Volts. The current will be: 9.6 - 1.6 > 8V/330ohms = 24mA. The LED will be brighter and last only 57 years instead of 114 (1 000 000 hours); big deal!
Now if you power the LED out of one AAA cell which delivers 1.6V when fresh, you may run out of luck and never exceed the cut-off voltage and just a slight temperature variation may change the battery voltage enough to enable lighting or turning it off.
In applications where a string of LED's is involved (push-button illumination or LED "bargraphs"), it is usual to implement a CCS (constant-current source) - typically one transistor and a couple of diodes, so the variation of the number of LED's does not change their luminosity.
Now there are LED drivers based on SMPS or capacitor charge-discharge, that handle the current regulation aspect.
Now if you power the LED out of one AAA cell which delivers 1.6V when fresh, you may run out of luck and never exceed the cut-off voltage and just a slight temperature variation may change the battery voltage enough to enable lighting or turning it off.
In applications where a string of LED's is involved (push-button illumination or LED "bargraphs"), it is usual to implement a CCS (constant-current source) - typically one transistor and a couple of diodes, so the variation of the number of LED's does not change their luminosity.
Now there are LED drivers based on SMPS or capacitor charge-discharge, that handle the current regulation aspect.
