Describe the principles of a reservoir capacitor in basic power supply ripple rejection supplies. The effect of a reservoir capacitor on the DC component. The effect of a reservoir capacitor on the diode current. Describe the principles of a low pass filter used in basic power supplies.
Filter Components A typical power supply filter circuit can be best understood by dividing the circuit into two parts, the reservoir capacitor and the low pass filter. Each of these parts contributes to removing the remaining AC pulses, but in different ways. 1 shows an electrolytic capacitor used as a reservoir capacitor, so called because it acts as a temporary storage for the power supply output current. The rectifier diode supplies current to charge a reservoir capacitor on each cycle of the input wave. The reservoir capacitor is a large electrolytic, usually of several hundred or even a thousand or more microfarads, especially in mains frequency PSUs. The action of the reservoir capacitor on a half wave rectified sine wave is shown in Fig. During each cycle, the rectifier anode AC voltage increases towards Vpk.
At some point close to Vpk the anode voltage exceeds the cathode voltage, the rectifier conducts and a pulse of current flows, charging the reservoir capacitor to the value of Vpk. Once the input wave passes Vpk the rectifier anode falls below the capacitor voltage, the rectifier becomes reverse biased and conduction stops. Of course, even though the reservoir capacitor has large value, it discharges as it supplies the load, and its voltage falls, but not by very much. At some point during the next cycle of the mains input, the rectifier input voltage rises above the voltage on the partly discharged capacitor and the reservoir is re-charged to the peak value Vpk again. AC Ripple The amount by which the reservoir capacitor discharges on each half cycle is determined by the current drawn by the load.