LEDs in Annunciators

1) Is there any way I can chek the proper voltage to be applied to a specific LED
Reason: I have bags of bulk LEDS without any spec sheet.
Or should I assume that yellow are 2.1V red 2,3V blue 3.5V ??

2) I found too that connecting 2 Yellows in series in each Annunciator could avoid the resistor and simplify things as my supply voltage is 5V across the whole system. But this would give 2.5v for each yellow LED. Would they fry after a while if they are the 2,1V type,or will they hack it?
Thanks for info
Jack

Hey Jack...

Anyone will always tell you to use a resistor always with an LED. I must say that I in the past have many times not followed this rule, however the proper way to control current going to the led is with a resistor.

That being said.. if you can regulate it properly without fluctuation most led's can tolerate under 3V. In order to maintain uniform briteness I suggest using resistors with all your led's regardless the voltage. 5V and a 330 Ohm resistor will keep you safe.

Trev

The resistor is there to limit the current through the LED, otherwise it will pop. Not using a resistor means that you have no control over the current flowing through the LED(s). Not recommended.

To find out what the voltage is (without a Datasheet) use the following method:

1) Use a voltage such as 5V or 12V which is easily obtained by a computer PSU.

2) Put a 10K (> 10K should also work) potentiometer in series with the led and set it to the maximum resistance before applying power.

3) Apply the power and slowly turn the potentiometer (variable resistor) until the LED is illuminated with the brightness you think is the maximum. (If it popped you went too far)

4) Take a voltmeter and measure the voltage over the LED. That is the voltage used by the LED (LED Voltage drop).

5) Power off and measure the resistance set on the potentiometer. That can be used to determine which resistor can be used with the LED and what the current is that the LED uses.

The formulae for the resistor is:

R = (Vcc - Vled) / I

Most LED voltages ranges between 1.5V and 3.3V. I normally use 20mA as a ball park figure for LED current. I wired Infra-red LEDs the other day that were rated at 100mA and 1.5V.

[Edit] See Mike Powell LED dimmer page: http://www.mikesflightdeck.com/led_dimmer.htm