Figure 3-36.Full-wave rectifier with an LC choke-input filter.
It should be apparent that when the XC of a filter capacitor is decreased, it provides less opposition to
the flow of ac. The greater the ac flow through the capacitor, the lower the flow through the load.
Conversely, the larger the XL of the choke, the greater the amount of ac ripple developed across the
choke; consequently, less ripple is developed across the load. This condition provides better filtering.
Q24. In an LC choke-input filter, what prevents the rapid charging of the capacitor?
Q25. What is the value usually chosen for a filter choke?
Q26. If the inductance of a choke-input filter is increased, will the output ripple voltage amplitude (Er)
increase or decrease?
An LC choke-input filter is subject to several problems that can cause it to fail. The filter capacitors
are subject to open circuits, short circuits, and excessive leakage. The series inductor is subject to open
windings and, occasionally, shorted turns or a short circuit to the core.
The filter capacitor in the choke-input filter circuit is not subject to extreme voltage surges because
of the protection offered by the inductor; however, the capacitor can become open, leaky, or shorted.
Shorted turns in the choke may reduce the value of inductance below the critical value. This will
result in excessive peak-rectifier current, accompanied by an abnormally high output voltage, excessive
ripple amplitude, and poor voltage regulation. A choke winding that is open, or a choke winding that is
shorted to the core will result in a no-output condition. A choke winding that is shorted to the core may
cause overheating of the rectifier element(s), blown fuses, and so forth.
To check the capacitor, first remove the supply voltage from the input to the filter circuit. Then
disconnect one terminal of the capacitor from the circuit. Check the capacitor with a capacitance analyzer
to determine its capacitance and leakage resistance. When the capacitor is electrolytic, be sure to use the
correct polarity at all times. A decrease in capacitance or losses within the capacitor can decrease the
efficiency of the filter and produce excessive ripple amplitude. If a suitable capacitance analyzer is not
available, you can use an ohmmeter to check for leakage resistance. The test procedure is the same as that
described for the input capacitor filter.
So far, this section has discussed in detail the operation and troubleshooting of the basic inductive
and capacitive filter circuits. For the two remaining types of filters, we will discuss only the differences
between them and the other basic filters.