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.

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 ohmmeter should be over 500,000 ohms. (A rough rule of thumb for high-voltage capacitors is at least 1000 ohms per volt.) Low-voltage electrolytic capacitors (below 100 volts rating) should indicate approximately 100,000 ohms.

If the ohmmeter does not deflect when you make the resistance check explained above, you have found an open-circuit capacitor.

A steady full-scale deflection of the pointer at zero ohms indicates that the capacitor being tested is shorted.

An indication of a leaky capacitor is a steady reading on the scale somewhere between zero and the minimum acceptable value. (Be certain this reading is not caused by an in-circuit shunting part.) To be valid, these capacitor checks should be made with the capacitor completely disconnected from the circuit in which it operates.

In high-voltage filter capacitor applications, paper and oil-filled capacitors are used in addition to mica and ceramic capacitors (for low-capacitance values). In this case, polarity is of no importance unless the capacitor terminals are marked plus or minus. It is, however, good maintenance practice to use the output polarity of the circuit as a guide, connecting positive to positive, and negative to negative. Thus, any adverse effects of polarity on circuit tests are minimized and the possibility of damage to components or to test equipment is eliminated.

The LC Choke-Input Filter

The LC choke-input filter is used primarily in power supplies where good voltage regulation is important and where the output current is relatively high and subject to varying load conditions. This filter is used in high-power applications such as those found in radar and communication transmitter power supplies.

In figure 3-26 you can see that this filter consists of an input inductor or filter-choke (L1) and an output filter capacitor (C1).

Figure 3-26. - Full-wave rectifier LC choke-input filter.

Inductor L1 is placed at the input to the filter and is in series with the output of the rectifier circuit. Since the action of an inductor is to oppose any change in current flow, the inductor tends to keep a constant current flowing to the load throughout the complete cycle of the applied voltage. As a result, the output voltage never reaches the peak value of the applied voltage; instead, the output voltage approximates the average value of the rectified input to the filter, as shown in figure 3-27.

Figure 3-27. - Waveforms for a LC choke-input filter.

The reactance of the inductor (X_L) reduces the amplitude of ripple voltage without reducing the dc output voltage by an appreciable amount. (The dc resistance of the inductor is just a few ohms.)

The shunt capacitor (C1) charges and discharges at the ripple frequency rate, but the amplitude of the ripple voltage (E_r) is relatively small because the inductor (L1) tends to keep a constant current flowing from the rectifier circuit to the load. In addition, the reactance of the shunt capacitor (X_C) presents a low impedance to the ripple component existing at the output of the filter, and thus shunts the ripple component around the load. The capacitor attempts to hold the output voltage relatively constant at the average value of the voltage.

The value of the filter capacitor (C1) must be relatively large to present a low opposition (X_C) to the pulsating current and to store a substantial charge. The rate of the charge for the capacitor is limited by the low impedance of the ac source (transformer), the small resistance of the diode, and the counter emf developed by the coil. Therefore, the RC charge time constant (fig. 3-28) is short compared to its discharge time.

Figure 3-28. - LC choke-input filter (circuit resistance).

As a result, when the pulsating voltage is first applied to the LC choke-input filter, the inductor or filter choke (L1) produces a counter emf that opposes the constantly increasing input voltage. The net result is to effectively prevent the rapid charging of the filter capacitor (C1). Thus, instead of reaching the peak value of the input voltage, C1 only charges to the average value of the input voltage. After the input voltage reaches its peak and decreases sufficiently, the capacitor (C1) attempts to discharge through the load resistance (R_L). C1 will attempt to discharge as indicated in figure 3-29. Because of its relatively long discharge time constant, C1 can only partially discharge.

Figure 3-29. - LC choke-input filter (discharge path).

The larger the value of the filter capacitor, the better the filtering action. However, due to the physical size, there is a practical limitation to the maximum value of the capacitor.

The inductor or filter choke (L1) maintains the current flow to the filter output (capacitor C1 and load resistance R_L) at a nearly constant level during the charge and discharge periods of the filter capacitor.

The series inductor (L1) and the capacitor (C1) form a voltage divider for the ac component (ripple) of the applied input voltage. This is shown in figure 3-30. As far as the ripple component is concerned, the inductor offers a high impedance (Z) and the capacitor offers a low impedance. As a result, the ripple component (E_r ) appearing across the load resistance is greatly attenuated (reduced). Since the inductance of the filter choke opposes changes in the value of the current flowing through it, the average value of the voltage produced across the capacitor contains a much smaller value of ripple component (E_r), as compared with the value of ripple produced across the coil.

Figure 3-30. - LC choke-input filter (as voltage divider).

Now look at figure 3-31, which illustrates a complete cycle of operation where a full-wave rectifier circuit is used to supply the input voltage to the filter. The rectifier voltage is developed across capacitor C1. The ripple voltage in the output of the filter is the alternating component of the input voltage reduced in amplitude by the filter section.

Figure 3-31. - Filtering action of an LC choke-input filter.

Each time the plate of a diode goes positive with respect to the cathode, the diode conducts and C1 charges. Conduction occurs twice during each cycle for a full-wave rectifier. For a 60-hertz supply, this produces a ripple frequency of 120 hertz. Although the diodes alternate (one conducts while the other is nonconducting), the filter input voltage is not steady. As the plate voltage of the conducting diode increases (on the positive half of the cycle), capacitor C1 charges-the charge being limited by the impedance of the secondary transformer winding, the diode's forward (cathode-to-plate) resistance, and the counter emf developed by the choke. During the nonconducting interval, (when the plate voltage drops below the capacitor charge voltage), C1 discharges through the load resistance R_L. The components in the discharge path cause a long time constant; thus C1 discharges slower than it charges.

The choke (L1) is usually of a large value, on the order of 1 to 20 henries, and offers a large inductive reactance to the 120-hertz ripple component produced by the rectifier. Therefore, the effect that L1 has on the charging of the capacitor (C1) must be considered. Since L1 is connected in series with the parallel branch consisting of C1 and R_L, a division of the ripple ac voltage and the output dc voltage occurs. The greater the impedance of the choke, the less the ripple voltage that appears across C1 and the output. The dc output voltage is fixed mainly by the dc resistance of the choke.

Now that you have read how the LC choke-input filter functions, let's take a look at it using actual component values. For simplicity, the input frequency at the primary of the transformer will be 117 volts 60 hertz. We will use both half-wave and full-wave rectifier circuits to provide the input to the filter.

Starting with the half-wave configuration as shown in figure 3-32, the basic parameters are: with 117 volts ac rms applied to the T1 primary, 165 volts ac peak-to-peak is available at the secondary [(117 V) X (1.414) = 165 V]. You should recall that the ripple frequency of this half-wave rectifier is 60 hertz. Therefore, the capacitive reactance of C1 is: