Led and resistor circuit?

Question

I'm using 2 circuits, one with 3 LEDs and one with 4 LED's. Led's are rated at 2.6v, 70ma. LED's will be wired in series. Power source is ~13.8v.

Circuit 1
3LEDs in series connected to a 82ohm 1w resistor and a 6.8ohm 1w resistor (resistors in series).

Circuit 2
4LEDs in series connected to a 51ohm 1/2w resistor and a 1.3ohm 1/2w resistor (resistors in series).

Will these give me nominal voltage and current?

These are the Phillips superflux LEDs. They are for a turn signal application on a car. Running voltage is ~13.8-14 depending on the vehicle. These are rather tolerant to overvoltages because I ran them at twice the voltage and they got hot but didn't fail.

Answer 1

First find resistance of the LEDs using the rated voltage and current.
R = V/I

Circuit 1) Add up the resistances

Rtotal = 3 Rled + R1 + R2

You have a voltage divider, so each LED has voltage:
Vtotal * Rled / Rtotal

Circuit 2: same thing with 4 LEDs and two other resistors

Check the powers on the resistors
(P = IV = V^2 / R, where V = Vtotal R/Rtot
to make sure none of the resistors are over rated power

Answer 2

Just because the LEDs are rated for 70 mA doesn't necessarily mean you *need* to run 70 mA through them (unless you are making an LED flashlight, or something -- the more current, the better). The ratings on these LEDs seem awfully high, to me, but I'll trust you read the datasheet, OK.

If you only have 68 mA, or 75 mA it won't make much difference in the amount of light output from the LEDs, so you don't need 85.714285 Ohms of resistance -- any single value of resistor between 81 Ohms and 100 Ohms will do just fine for circuit 1. Any resistor between 45 and 68 Ohms will do just fine for circuit 2.

You will probably see light even if you run 50 mA through them. Also... that 13.8 V power supply sounds suspect. 13.8 Volt supplies are usually unregulated, so check it out with a voltmeter. It's just another reason you don't need to be so precise with your resistance calculations.

Make sure the resistors are rated for 1/2 Watt or greater (I see that you are doing that).

.

Answer 3

typical forward voltage drop across a diode = 0.7v (the led forward current you have listed is NOT how much the diode will take, they are the typical and max currents that should be allowed through the diode. Look at the voltage/current graph for a diode, a diode dont "pull" a current) Connect it to a 1v supply the diode tries to drop the 1v to 0.7v - If there is no resistor the diode will try to sink as much current as the supply can source - it cant do this so it will blow, (cos its taking more than the max forward current/wattage) put a resistor in series the 0.3v can be dropped across the resistor. Or looking at it another way - the resistor limits the current through the diode to be less than the max forward current.

Answer 4

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