CIRCUIT IDEAS ELECTRONICS FOR YOU FEBRUARY 2003 whereas no power is dissipated in a ca-pacitor. The value of capacitor is calcu-lated by using the following relationships: X C = 1/(2 π fC) ohms —————(a) X C = V RMS /I ohms ———— (b) where X C is capacitive reactance in ohms, C is capacitance in farads, I is the current through the LED in amperes, f is the mains frequency in Hz, and Vrms is the input mains voltage. The 100-ohm, 2W series resistor avoids heavy ‘inrush’ current during transients. MOV at the input prevents surges or spikes, protecting the circuit. The 390-kilo-ohm, ½-watt resistor acts as a bleeder to pro-vide discharge path for capacitor Cx when mains supply is disconnected. The zener diode at the output section prevents ex-cess reverse voltage levels appearing across the LEDs during negative half cycles. Dur-ing positive half cycle, the voltage across LEDs is limited to zener voltage. Use AC capacitors for Cx. Filter capaci-
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