Or night duty (with reduced brightness) lighting, allowing at the same time significantly increase the lamp life, can be implemented in different ways. Among the simple and reliable method using a quenching capacitor in power supply circuit (Fig. 1). The capacitance C1 is calculated as reactance to obtain the desired lamp voltage ЕL1. Resistor R1 is necessary for capacitor discharge after disconnecting the circuitry from the network. Its value is in the range of 100-500 ohms.
Due to the nonlinearity of resistance of the filaments and the real scatter of the static parameters of the lamps themselves, the exact calculation of the capacitance of the capacitor C1 is quite difficult even for experienced professionals. However, the calculations can be significantly simplified, if we use the approximate formulas. In particular, for an electric circuit with low-voltage and low-power incandescent lamps (switch, lights, indicator and other 10-42 voltage V and current up to 0.5 A) is acceptable practice, the calibrated value:
where: C — capacity plus (damping) capacitor UF; I n — rated current of the lamps, And; Il — the voltage that you want to get the lamp; IP — mains voltage AC V.
When using standard lighting bulbs, on the cap or bulb which shall include the nominal operating voltage (usually 220-240V) and power (10-200 W), the coefficient in the numerator should be reduced to 1150, and I find using are known to everyone from school by Ohm’s law.