Quasi-circuit--Miscellaneous talk on rectification (2)

In addition to very simple and rough loads such as DC motors, the rectifier circuits in electronic equipment need to be matched with filter circuits. The rectified voltage and current can be used by electronic equipment after being filtered.

The picture above is a typical circuit of full-wave rectifier capacitor filtering. In the figure, C is the filter capacitor, and ZL represents the load.

If we connect an oscilloscope to both ends of the filter capacitor C, we will see a waveform like the following.

The voltage across the capacitor C fluctuates, and this fluctuation is generally called ripple.

If we use a dual-trace oscilloscope, and the other channel is connected to point M in Figure 01, and the two channels use the same sensitivity, then the voltage waveforms of the two channels are as follows:

We can see that the voltage across winding A1 (black) has a section near the peak value that is close to coinciding with a "bulge" on the ripple (blue).

The blue curve is the voltage across capacitor C, and the voltage across C rises at the "bulge". A rise in the voltage across the capacitor indicates that the capacitor is being charged. Where does this charging current come from? Obviously, it is because the instantaneous value of the voltage across winding A1 exceeds the voltage across the capacitor, so winding A1 charges capacitor C through rectifier diode D1.

When the voltage across winding A1 passes through the maximum point and drops below the voltage across capacitor C, winding A1 will of course not charge the capacitor. At this stage, the capacitor discharges to the load, and the voltage at both ends gradually decreases.

From the time relationship between the voltage waveform at point M and the voltage waveform across the capacitor, we can imagine that the "bulge" on the voltage waveform at both ends of the capacitor when the voltage waveform at point M is at its valley value (maximum value in the opposite direction) is the N point of the other half of the winding A2. Diode D2 charges capacitor C.

If we use a more trace oscilloscope and can measure the current, then the relationship between the multi-current waveform at the midpoint K of the secondary winding of the transformer, the electromotive force of the winding, and the voltage across the capacitor is shown in Figure 04:

The blue curve in the figure is the voltage across the capacitor C, and the red curve is the current from the ground wire to the midpoint K of the transformer.

In order to analyze the work of the full-wave rectifier capacitor filter circuit, we add annotation symbols in Figure 04, as shown in Figure 05.

In the analysis of Figure 05, we assume that the diode is an ideal diode with zero forward voltage drop.

In Figure 05, the black curve is the electromotive force (open circuit voltage) of the two half secondary windings, not the terminal voltage of the two half windings. The part of the electromotive force curve below the horizontal axis is not drawn. The blue curve is the voltage across the capacitor C, and the red curve is the current in the two half windings.

Taking any half of the power frequency cycle, before time t1, the electromotive force of the secondary winding of the transformer is less than the voltage across the capacitor, there is no current in the two diodes, and the load relies on capacitor discharge to maintain the current in it. At time t1, the electromotive force of the winding begins to be greater than the voltage across the capacitor. Winding A2 charges capacitor C through diode D2, and the red curve begins to rise. As the electromotive force of the secondary winding increases, the current also increases rapidly, and the voltage across the capacitor increases. During the period from t1 to t2, the secondary winding not only charges the capacitor, but also supplies power to the load. When the electromotive force of the secondary winding begins to decrease, the charging current decreases rapidly. When the electromotive force of the secondary winding is lower than the voltage across the capacitor, charging stops and the current in the secondary winding is zero. After time t2, the capacitor discharges to the load to maintain the current in the load, and the voltage across the capacitor decreases.

In the figure, we can see that during the period from t1 to t2, the electromotive force of the winding and the voltage across the capacitor are slightly different, and the black curve is slightly higher than the blue curve. This is because the winding always has a certain resistance, which will drop part of the voltage, and the greater the current in the winding, the greater the voltage drop. The difference between the two curves is the voltage drop across the winding resistance.

In the second half of the power frequency cycle, the aforementioned process is repeated, except that the winding A1 and diode D1 start charging the capacitor at time t3 and end at time t4. From time t2 to time t3, there is no current in the winding, and the current in the load is maintained entirely by the capacitor discharging into the load.

The two half secondary windings A1 and A2 alternately charge the capacitor through the diode, and then the capacitor discharges the load. This is the working process of full-wave rectification capacitor filtering.

We see that in the capacitive filter circuit, the current in the transformer winding is intermittent, with relatively narrow pulses. The current is discontinuous and in the form of pulses, which is a characteristic of capacitive filtering.