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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PaulCo4 years ago

Put 5Volts across the LED and a series current limiting resistor of around 470 or 680 omhs, the value of the resistor is not vital provided that its low enough for the LED to light and high enough to stop it from burning out, the suggested values are good for a 5 volts supply.

The values specified will give us around 10ma of current through the LED.

I = V/R

.010 = 5/500

Now measure the voltage across the LED with your volt meter, thats the forward voltage drop. It will vary slightly across a batch of LEDs, It will vary a lot between different colour LEDs.

Here is a great resource that will tell you much more about LEDs than I can -
Patrik9 years ago
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...
pretty much always 20 mA
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
hassi (author) 9 years ago
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...
westfw9 years ago
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)
Patrik westfw9 years ago
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.
geetz9 years ago
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.
Patrik geetz9 years ago
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.
NachoMahma9 years ago
. 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.
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