 TWO LED TROUBLE LIGHT

INSTRUCTION SHEET    The 18000 mcd LED

Specifications

Emitted colour: White
Lens colour: Water Clear
Wave length: 0.315(nm) max
Pd W: 120mW
If mA: 30
If mA (peak): 150
Vf Min: -
Vf (V) Typ: 3.3
Vf Max: 3.6

Iv Min: 17000
mcd Min:17000
mcd Typ: 18000
Viewing Angle: 15

As with all LED's, the current through the LED is the critical piece of information - If mA: 30.  If this is exceeded it is likely the LED will be destroyed.

It is also interesting to note the normal operating voltage limits - Vf (V) Typ: 3.3 and Vf Max: 3.6.  This indicates that to power the diode without a resistor the voltage would Typically be 3.3 V to a Maximum of 3.6 V.

CALCULATIONS

{1} Simplify the circuit and look at one branch: To calculate the total resistance required to limit the current to 30 mA,

we need OHM's Law.

V = 12

I = 30 mA

therefore R = 400 Ohms

This is the total resistance of the unknown resistor value + the inherant resistance of the LED.

To calculate the resistance of the LED

If we note that at a maximum voltage of 3.6V the LED needs no resistor - therefore we can assume that the LED will draw 30mA when connected to a 3.6 V source.  Again we use ohms law

V = 3.6

I = 0.030 A

Therefore R = 120 Ohms

If we now think of the diode as aResistor in series with the unknown resistor, by deducting the resistance of the LED from the total resistance we learn the unknown resistance to operate the LED without destroying it.  So a 280 Ohm resistor should be used with a 12 V source:

400 - 120 = 280 Ohms

You may now ask why a 390 Ohm resistor was used instead of a 280 Ohm resistor - because the voltage on a vehicle varies and can be as high as 14.5 Volts - to eliminate the possiblity of damage to the LED a resistance buffer was included in the value of the chosen resistor.