In a pi- section inductive filter, there is a shunt input capacitor, C1 a series inductor (choke), L1 and a shunt output capacitor, C2. The two types of pi-section filters are inductive and resistive.Ī pi-section filter consists of three elements. Pi-section filters get their name from the Greek letter pi (π) because the filter configuration resembles the symbol for pi. Pi-Section FiltersĪ pi-section filter is a filter made with two capacitors and an inductor or resistor to smooth out the AC ripple in a rectified waveform. The pulses not blocked by the inductor are bypassed by the capacitor. The DC portion of the signal is allowed to pass through the inductor. Therefore, the inductor tends to block the pulsating DC voltage. When the pulsating DC voltage is applied to the inductor, the changing voltage produces a high inductive reactance. The operation of an L-section inductive filter can also be seen through the effect that inductive reactance has on the circuit. An L-section inductive filter circuit diagram and output waveform The inductor also aids the filtering effect of the capacitor since the CEMF of the inductor tends to cancel out the effects of the ripple voltage.įigure 3. Surge current is greatly reduced and the capacitor charges slowly. See Figure 3.Īn inductor (L1) in series opposes a change in current by creating a counter electromotive force (CEMF) or counter voltage. An L-section resistive filter circuit diagram and output waveform L-section Inductive FiltersĪn L-section inductive filter is a filter that reduces surge currents by using a current-limiting inductor and a capacitor. The filtering of the resistor is not as good as other filters, but it is less expensive.įigure 2. This protects the rectifier from the high surge of charging current that flows through the rectifier from the input capacitor C1 when the circuit is first energized.Ī low-value resistor of about 50 Ω or less is typically used in the application. R1 should always be used in series with the rectifier and the input capacitor of the filter system. R1 controls surge currents by limiting the current flow to slow the charging of the capacitor. L-section Resistive FiltersĪn L-section resistive filter is a filter that reduces or eliminates the amount of DC ripple at the output of a circuit by using a resistor and capacitor as an RC time constant.Īn L-section resistive filter reduces surge currents by using a current-limiting resistor (R1). Ripple voltage increases when the load increases on the capacitive filter. If the load resistance is large and the capacitance is large, the ripple voltage will be small, resulting in a smooth output. The amount of discharge between voltage peaks is controlled by the resistor-capacitor (RC) time constant of the capacitor and the load resistance. In a capacitive filter, the ripple voltage is the voltage drop before the capacitor begins to charge again. Ripple voltage is the amount of varying voltage present in a DC power supply. A capacitive filter circuit diagram and output waveform.īetween peaks, the capacitor discharges through the resistive load RL, and the voltage gradually drops. As pulsating DC voltage from a rectifier is applied across capacitor C1, it charges to the peak voltage. Since a large capacitor is needed, an electrolytic capacitor is typically used. The capacitive filter provides a maximum voltage output to a load. A capacitive filter is a circuit consisting of a capacitor and a resistor connected in parallel.
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