A resonant type electronic ballast consist of an input circuit which rectifies the AC mains voltage into a DC voltage. The input circuit also contains an EMI filter inductor L1. The resistor R1 is a fusible resistor which is a combination of a resistor and a fuse. The resistance is needed to limit the current spike created by the charging electrolytic capacitor C2. A half bridge T1 and T2 driven by an IC (or integrated into an IC) makes a 40-100 kHz square wave with amplitude Vin. The resonant tank is formed by the lamp, C5 and L2 in the circuit diagram. L2 is the impedance that determines the lamp current. The inductor L2 has a value of 1-4 mH depending on the lamp power and mains voltage. The higher harmonics of the half bridge square wave are filtered by the resonant tank inductor L2, giving close to a sine wave at the resonant tank L2, C5. There are also one or two DC blocking capacitors marked as C3 and C4. They take the DC component of the square wave which is half the Vin bus voltage.