Measuring LED Voltage
Greetings...
I have quite a few "salvaged" LED's, none of which have I the slightest clue as to their voltage. I have attempted to measure them using the diode setting on my voltmeter with dubious/mixed/questionable results.
Although the measurements are momentary (the reading is barely of sufficient length to note), I can usually arrive at a value. Unfortunately, when I repeat the measurement on the same LED the reading is invariably different.
For example, I just measured the value of a random LED (green) and came up with the following: 1.3. 1.4, 1.8, 1.6. I've rounded the values to the nearest decimal point.
Are these values through voltage?
Are the differences between the readings significant or should I use the minimum when calculating resistor values?
I'll, no doubt, have a few more question later on.
I appreciate your elucidation on this matter...
Best to all,
hassi
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Feb 4, 2008. 3:28 PMPatrik
says:
Does anyone know of a good way to figure out roughly what current/power a LED is rated for? Other than going by size and/or doing some destructive testing, of course...
Feb 4, 2008. 4:42 PMPatrik
says:
That used to be the case yes. And if they're scavenged from a defunct piece of equipment it's a good bet.
However the more recent types of high-flux and power LEDs can take significantly more current. Naturally, they won't be harmed by a lower current - you just wouldn't be using them to their potential.
If you get a real power LED, they're easy enough to recognize because of the heatsink. But there are also a few which seem fairly indistinguishable from a normal LED:
5mm InfraRed LED @ 50mA
100mA 5mm leds
However the more recent types of high-flux and power LEDs can take significantly more current. Naturally, they won't be harmed by a lower current - you just wouldn't be using them to their potential.
If you get a real power LED, they're easy enough to recognize because of the heatsink. But there are also a few which seem fairly indistinguishable from a normal LED:
5mm InfraRed LED @ 50mA
100mA 5mm leds
Feb 4, 2008. 2:50 PMhassi (author)
says:
Whew! The suggestions are nearly overwhelming...I think I'll try the single/multiple resistor methods...I have a bunch of resistors but no potentiometers of the suggested range (yes, I'm a cheapskate; ergo the salvaged LED's)... Many large and obsequious thanks...
But one of the characteristics of LEDs (and diodes in general) is that the voltage is relatively constant, rather than depending on current. Just chuck a 1k resistor in series with your 9V battery (9mA max current) and LED, and measure the voltage across the LED. This will get you plenty close for any real application, and will certainly put your LED into one of the common bins (1.8 - 2.5V: old fashioned red/yellow/yellowgreen, 3-4V: newfangled green, blue, white)
Feb 4, 2008. 1:38 AMPatrik
says:
True - that's about as simple as it gets. Use a 390 Ohm resistor, and you'll get even closer, while still staying safe in case you stumble into a low-voltage IR LED: a 1.2V IR LED would give you exactly 20mA, whereas for a 4.5V blue LED you'd only get 11mA.
Feb 3, 2008. 7:00 PMgeetz
says:
I wired up an adjustable voltage regulator (LM317) with a resistor (see LM317 datasheet) to make a 20ma current source and hooked it to a 12V power supply. This circuit will adjust it's voltage to try and push 20ma through whatever load you put on it. I hooked it to a couple of small brass plates that I could just press an LED to for a few seconds and I hooked my voltmeter to the plates. When you press the LED to the plates, it will light and the voltage that puts 20ma through the LED shows up on the meter.
Feb 3, 2008. 11:40 PMPatrik
says:
Even easier - get a 9V battery, a 100 Ohm resistor, and a potentiometer somewhere between 270 and 1K Ohm. Make sure the 100 Ohm resistor has a tolerance of 5% (gold band). Crank the potentiometer all the way up (max resistance - check using your multimeter if you're not sure), and put the LED, 100 Ohm resistor and potentiometer in series with the battery. Measure the voltage across the 100 Ohm resistor, and slowly lower the potentiometer until you get exactly 2V across the 100 Ohm - that's 20mA. Now measure the voltage across the LED.
. I _think_ what you are reading are ohms, not volts, when using the diode check setting on your VOM. . I wouldn't worry too much about the inconsistent readings - that's just normal measurement error in a DIY environment. I'd call it one-and-a-half ohms (or whatever units they turn out to be) when doing calculations.
Feb 2, 2008. 10:11 AMtech-king
says:
that could cost several leds. green, red and yellow leds (non-clear lenses) generally use 2 volts at 20 milliamps. start with that.
Feb 2, 2008. 9:21 AMcvxdes
says:
I'd say get a bunch of resistors, and a power supply. Then wire up the resistors onto the +5v output, and add enough of them for it to go down to 1 volt. Test the LED to see if it works, and if it doesn't, keep removing resistors till it does. At that point, measure it.
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