Current limiting the new LED's

Posted: 2005-July-11 | Author: Jason Olshefsky | Filed under: Lighting, Projects, The Bike With 2 Brains |Leave a comment »

I originally bought resistors to use with the RGB LSDiodes.com but I realized the voltages and currents might be different. Indeed, the 3mm red/blue and 3mm green LED's require different values. I calculated the results with either a 4-volt or 5-volt supply (which I'll probably use.)

Color

Voltage

Current

Volts
across
resistor

Desired Resistor

red

2.2 V

20 mA

2.8 V
3.8 V

140 ohms
190 ohms

green

3.3 V

20 mA

1.7 V
2.7 V

85 ohms
135 ohms

blue

3.2 V

20 mA

1.8 V
2.8 V

90 ohms
140 ohms

Anyway, I tested using 150 ohm resistors for blue and green and a 180 ohm resistor on red. This balanced things so each LED was driven at about 30mA with a 7.5V source. The 150 ohm resistors dissipated 0.14 watts and the 180 ohm dissipated 0.17 watts, but with 20 mA, the figures are 0.06W and 0.08W respectively, allowing me to use 1/8W resistors. I also noted that I can use two layers of Scotch tape to diffuse the light nicely. I checked the circuit with 5 volts — green was 18mA, blue was 20mA, and red was 22mA — very close to ideal and easy to fix in software, so I went ahead and bought packs of 150-ohm and 180-ohm resistors.

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