LED current limiting

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LED color If nominal Vss
5 V 12 V 15 V 24 V
Red 10 mA 270 Ω
1/8 W
1 kΩ
1/8 W
1.2 kΩ
1/4 W
2.2 kΩ
1/4 W
Green
Blue
Yellow
Amber
20 mA 120 Ω
1/8 W
470 Ω
1/4 W
560 Ω
1/2 W
1 kΩ
1/2 W

series

White LEDs have a forward voltage drop of about 3.3V. The supply voltage should be about 1.5 times the forward voltage drop (total series voltage drop should be 2/3 supply voltage). So 18 white LEDs in series would need a 90V power supply.

Capacitive current limiting for LED lighting powered with AC

For more detail see information on Capacitive power supplies.

Using resistance to limit current from an AC power source requires that the resistor dissipate power. Using capacitive reactance may also be used to limit current in AC circuits with the advantage that the capacitor dissipates less power. The disadvantage is that capacitive reactance varies depending on power frequency and load.

Xc may substitute for R provided f and load remain constant.

P = V2 / R

Xc = 1 / 2pi * f * C ... at 60Hz this is approximately, Xc = 1 / 377 * C

Note that the capacitor should be an X2 or Y2 class Line-Filter capacitor. Line-Filter capacitors are designed to safely handle transient spikes that might cause other capacitors to fail. Class Y2 is preferred because it is safer, but X2 are more common. Y class capacitors are built more ruggedly than standard capacitors and are designed to fail open rather than fail short.

    0.47uF 200V
<-------||--------*----------*
120VAC            |          |
                 _|_         |
                  ^ 1N914   _v_ LED
120VAC            |          |
<----/\/\/\-------*----------*
       1K


General purpose 5V power supply

     1.5uF 200V         1N4002
<-------||--------*------->|--------*--------------> +5V
120VAC            |                 |
                 _z_                |+
                  ^ 5.6V Zener     === 1000uF 16V
120VAC            |   1W            |
<----/\/\/\-------*-----------------*--------------> Gnd
    270 Ohm 2W