# Unknown LED spec's

Discussion in 'The Projects Forum' started by donjohnston, Aug 2, 2010.

1. ### donjohnston Thread Starter New Member

Nov 29, 2009
5
0
I have some LEDs that I have lost the spec's on. Is there a way to determine the spec's of an LED?

-Don

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
293
If they are single LED units and you have a meter, you can apply some voltage through a resistor. Assume a maximum allowable current of 20 ma through the LED, and a forward drop of 1.5 volts to play safe. When the LED is lit, you can measure the actual forward voltage across the LED and adjust the current accordingly. They will last longer if current is only the order of 10 ma.

3. ### tom66 Senior Member

May 9, 2009
2,613
218
Many multimeters can test LEDs on the diode range. That gives the forward drop at somewhere from 1mA to 5mA.

4. ### donjohnston Thread Starter New Member

Nov 29, 2009
5
0
Yes. They're single LED's. Do I have to put a resistor in the circuit for this test?

And what the test points for this. Either side of the LED? - battery and + LED?

Thanks,
Don

5. ### Papabravo Expert

Feb 24, 2006
11,678
2,454
Using a resistor and plotting a set of characteristic curves of current and voltage is the de rigueur way to do it; plus you'll have way more useful information than the datasheet.

6. ### donjohnston Thread Starter New Member

Nov 29, 2009
5
0
Good point.

But I'm unclear on the how part.

Apr 5, 2008
18,478
3,558
8. ### Papabravo Expert

Feb 24, 2006
11,678
2,454
You need an adjustable power supply, voltmeter, ammeter, a variable (fixed) resistor, and test LED.
Repeat with different resistance values the following until a suitable range of values is achieved.

1. Hook up power supply, variable resistor , LED, voltmeter(across the LED) and ammeter(in series with the LED).
2. Set the variable resistor at a known resistance
3. Set the power supply at zero volts or some reasonable value known to be less than the forward voltage of the LED. 2.2 V is a typical forward voltage for a RED LED
4. Turn the power supply on
5. Advance the power supply voltage making note of the voltage and current readings.
6. Use LED brightness as an indicator of the maximum current. 20 mA is a good rule of thumb for maximum current, and 10 mA is a typical value.