Cutout in Rectifier Filters

When a rectifier, filter, and load are so related that filter operation is like that with condenser input, the capacitor (even though there may be an inductor between it and the rectifier) draws a high peak current and it charges almost to the peak of the rectified voltage wave; then cutout occurs. At this point, the rectifier current falls to zero, and the capacitor maintains the voltage across the filter and load. As a comparison, the voltages across the two types of filters are shown in Fig. 13. It is apparent that the average, or direct, voltage of Fig. 13 (a) with choke input is less than in Fig 13(6) for condenser input.

If for any condition of operation, cutout occurs, then the voltage across the load will rise, and the regulation¹ will be poor. If certain assumptions are made,¹² it can be shown that for cutout not to occur, the relation

Figure 13. Rectified waves that result when cutout does not occur (a) and when it does occur (6).

must apply. In this equation, L₁ is the minimum value of inductance in henrys that the first inductor must have, and R_L is the resistance in ohms of the load. It is common practice¹³ to use 1000 instead of 1130 in equation 3.

A resistor, called a bleeder, is often connected permanently across the filter output (Fig. 14). If this bleeder has the correct value given by equation 3, it will prevent cutout. Such a bleeder may be provided with taps, and then it serves as a voltage divider. A permanently connected bleeder ensures that the capacitors discharge when the rectifier is deenergized, and the bleeder may prevent injuries to operating personnel.

Figure 14. A typical full-wave rectifier unit. The inductance placed in the positive lead provides better filtering than if placed in the negative lead.