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

 

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.

 

Many multimeters can test LEDs on the diode range. That gives the forward drop at somewhere from 1mA to 5mA.

 

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.

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

 

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.

 

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.

Good point.

But I'm unclear on the how part.

 

Hello,

On this page you will find some basic led information:

Light Emitting Diodes (LEDs)


This is a link from the EDUCYPEDIA page on leds:
LED's and laser diodes

Bertus

 

Good point.

But I'm unclear on the how part.

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.