T3D Wiring LEDs

[Tom Hubin <thubin@xxxxxxxxx>]

Alan and I have had some discussion about wiring LEDs, particularly
white LEDs. I thought I might as well send a copy of my explanation to
T3d.

*********************************

(from Alan Lewis)

> What would I need to buy for
> LED's and power supply (circuit).  
> Is it as direct as connecting LED's
> to batteries through a switch?

(from Tom Hubin)

Just about. You need to provide more than the forward voltage (Vf) to
get them to turn on and less than the maximum rated forward current (If)
to not damage them. These numbers should be listed in the LED specs. The
white LEDs I am using list Vf=3.6v and If=20ma.

Start with a battery or DC supply with more than Vf. A 5vdc or 6vdc is
plenty but you can use 9v or 12v if it is more convenient. You can
string LEDs in series if you prefer. This will waste less power but if
one dies they all go out and you will have to figure out which one is
bad. You can also wire a 10k ohm to 100k ohm resistor in parallel with
each series diode. Then when one dies the good LEDs glow faintly. Only
the bad one is completely dark.

String as many LEDs in series as you want. The total forward voltages
must be less than the power supply. You can string 2 in series and use a
9vdc battery. The LEDs will add up to 7.2v. This leaves 1.8v surplus
from the 9v supply. Divide this 1.8v surplus by the desired current to
determine a resistor value. 1.8v divided by 0.020 amps is 90 ohms. So
put a 90 ohm or slightly larger resistor in series with the LEDs,
batttery, and switch. The LEDs will be running at or near max power with
20ma of current.

Now you may want to dim the light. Perhaps to 2ma for 10% power. Then
divide the 1.8v surplus by 0.002 amps to get 900 ohms. So you would want
an 810 ohm (900 - 90) potentiometer to control the current down to 2ma. 

Common values near these are a 100 ohm fixed resistor and a 1000 ohm
potentiometer. You can now vary the resistance from 100 ohms to 1100
ohms. This will allow 18ma to 1.6ma current to the LEDs with the 1.8v
surplus voltage.

The power requirement on the fixed resistor is the square of the voltage
across it divided by the resistor value. That is 1.8v * 1.8v / 100 ohms
= 0.032w. So any resistor rated for over 1/30 watt will work. That is
just about any resistor you can get. Most are rated for over 1/4 watt
and just about all for over 1/10 watt.   
I'll skip the math for the potentiometer power rating since it is even
less than the fixed resistor. 

You can mix LED colors wired in series if you wish. Just add up the
forward voltages to get the total forward voltage. Then subtract that
sum from the power supply voltage to get the surplus voltage. Then
divide teh surplus voltage by the current to get the series resistor
value. All LEDs in this series string will have the same current.
 
Tom Hubin
thubin@xxxxxxxxx