Describe the principles of a reservoir capacitor in basic power supplies. The effect of full wave rectifier ripple 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.
The higher the load current, the more the discharge, but provided that the current drawn is not excessive, the amount of the AC present in the output is much reduced. The DC output of the rectifier, without the reservoir capacitor, is either 0. 637 Vpk for full wave rectifiers, or 0. Adding the capacitor increases the DC level of the output wave to nearly the peak value of the input wave, as can be seen from Fig.
To obtain the least AC ripple and the highest DC level it would seem sensible to use the largest reservoir capacitor possible. This current partly discharges the capacitor, so all of the energy used by the load during most of the cycle must be made up in the very short remaining time during which the diode conducts in each cycle. Therefore the shorter the charging time, the larger current the diode must supply to charge it. Both the input transformer and the rectifier diodes must be capable of supplying this current. There is an advantage therefore in reducing the value of the reservoir capacitor, thereby allowing an increase in the ripple present, but this can be effectively removed by using a low pass filter and regulator stages between the reservoir capacitor and the load.