This little circuit allows to connect a LED to line voltage. Two issues have to be solved for that: LEDs cannot cope with such a high AC voltage, and the current has to be limited to ca. 15mA. The first issue is simply solved with a diode that eliminates the voltage in reverse-biasing. To limit the current of the LED a capacitive resistor is used.
In AC, capacitors work as frequency dependent resistors. With the formula shown below you can calculate the resistance inserting frequency (f) and capacity of the capacitor (C). Based on a frequency of 50Hz and a capacity of 220nF you arrive at a resistance (reactance) of 16kΩ which is just about right for one LED. For the US grid (110V, 60Hz) the values have to be adjusted accordingly.
It is also possible to adjust this circuit to connect several LEDs in parallel. This increases the required current, hence the capacitor has to be adjusted with the help of the formula.
- C1: foil capacitor 220nF, 250V~ (stick to the values!)
- R1: 2,2kΩ
- R2: 220Ω
- D1: 1N4007 (or comparable; must be suitable for 220V/230V)
- LED1: standard LED
You will notice the additional R2 in the circuit diagram. It prevents that the capacitor charges too quickly when empty which would cause a short circuit.