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conjunction with a full-wave or bridge rectifier, improved filtering is provided because the increased
ripple frequency decreases the capacitive reactance of the filter capacitor. This allows the ac component
to be passed through the capacitor more easily. Therefore, the output of a full-wave rectifier is much
easier to filter than that of a half-wave rectifier.
It should be obvious that the smaller the XC of the filter capacitor in respect to the load resistance, the
better the filtering action. By using the largest possible capacitor, we achieve the best filtering. The load
resistance is also an important consideration. If load resistance is made small, the load current increases,
and the average value of output voltage (Eavg) decreases. The RC discharge time constant is a direct
function of the value of the load resistance; therefore, the rate of capacitor voltage discharge is a direct
function of the current through the load. The greater the load current, the more rapid the discharge of the
capacitor, and the lower the average value of output voltage. For this reason, the simple capacitor filter is
seldom used with rectifier circuits that must supply a relatively large load current.
Q17. What is the most basic type of filter?
Q18. In a capacitor filter, is the capacitor in series or parallel with the load?
Q19. Is better filtering achieved at a high frequency or at a low frequency at the input of the filter?
Q20. Does a filter circuit increase or decrease the average output voltage?
Q21. What determines the rate of discharge of the capacitor in a filter circuit?
Q22. Does low ripple voltage indicate good or bad filtering?
Q23. Is a full-wave rectifier output easier to filter than that of a half-wave rectifier?
In general, with the supply voltage removed from the input to the filter circuit, one terminal of the
filter capacitor can be disconnected from the circuit.
CAUTION
REMEMBER-AN UNDISCHARGED CAPACITOR RETAINS ITS
POLARITY AND HOLDS ITS CHARGE FOR LONG PERIODS OF TIME. TO
BE SAFE, USE A PROPER SAFETY SHORTING PROBE TO DISCHARGE THE
CAPACITOR TO BE TESTED WITH THE POWER OFF BEFORE
CONNECTING TEST EQUIPMENT OR DISCONNECTING THE CAPACITOR.
You can check the capacitor by using a capacitance analyzer to determine its effective capacitance
and leakage resistance. During these checks it is very important that you observe correct polarity if the
capacitor is an electrolytic. A decrease in capacitance or losses within the capacitor can cause the output
to be below normal and also cause excessive ripple amplitude.
If a suitable capacitance analyzer is not available, you can get an indication of leakage resistance by
using an ohmmeter. You can make resistance measurements across the terminals of the capacitor to
determine whether it is shorted, leaky, or open. When you test electrolytic capacitors, set the ohmmeter to
the high range, and connect the test probes across the capacitor. Be careful to observe polarity. This is
important because current flows through an electrolytic capacitor with less opposition in one direction
than in the other. If you do not observe the correct polarity, you will get an incorrect reading and you may
damage the meter. When you first connect the test probes, a large deflection of the meter should take
place, and then the pointer should return slowly toward the infinite-ohms position as the capacitor
charges. For a good capacitor with a rated working voltage of 450 volts dc, the final reading on the
