LC Choke cont

Now that you have read how the LC choke input filter functions in the previous tutorial, 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 the following illustration, 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) × (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:

Half-wave rectifier with an LC choke-input filter.

This means that the capacitor (C1) offers 265 ohms of opposition to the ripple current. Note, however, that the capacitor offers an infinite impedance to direct current. The inductive reactance of L1 is:

XL = 2 pi fL

XL = (2) (3.14) (60) (10)

XL = 3.8 Kilohms

This shows that L1 offers a relatively high opposition (3.8 kilohms) to the ripple in comparison to the opposition offered by C1 (265 ohms). Thus, more ripple voltage will be dropped across L1 than across C1. In addition, the impedance of C1 (265 ohms) is relatively low in respect to the resistance of the load (10 kilohms). Therefore, more ripple current flows through C1 than the load. In other words, C1 shunts most of the ac component around the load.

Let's go a step further and redraw the filter circuit so that you can see the voltage divider action. (Refer to the next illustration below.) Remember, the 165 volts peak-to-peak 60 hertz provided by the rectifier consist of both an ac and a dc component. The first discussion will be about the ac component.

Looking at the picture below, you see that the capacitor (C1) offers the least opposition (265 ohms) to the ac component; therefore, the greatest amount of ac will flow through C1. (The heavy line indicates current flow through the capacitor.) Thus the capacitor bypasses, or shunts, most of the ac around the load. By combining the XC of C1 and the resistance of RL into an equivalent circuit, you will have an equivalent impedance of 265 ohms.

AC component in LC choke input filter.

You now have a voltage divider as illustrated in the next picture below. You should see that because of the impedance ratios, a large amount of ripple voltage is dropped across L1, and a substantially smaller amount is dropped across C1 and RL. You can further increase the ripple voltage across L1 by increasing the inductance:

XL = 2 pi fL

Actual and equivalent circuits.

Now let's discuss the dc component of the applied voltage. Remember, a capacitor offers an infinite (") impedance to the flow of direct current. The dc component, therefore, must flow through RL and L1. As far as the dc is concerned, the capacitor does not exist. The coil and the load are, therefore, in series with each other. The dc resistance of a filter choke is very low (50 ohms average). Therefore, most of the dc component is developed across the load and a very small amount of the dc voltage is dropped across the coil, as shown in the next illustration.

DC component in an LC choke-input filter.

As you may have noticed, both the ac and the dc components flow through L1, and because the coil is frequency sensitive, it provides a large resistance to ac and a small resistance to dc. In other words, the coil opposes any change in current. This property makes the coil a highly desirable filter component. Note that the filtering action of the LC capacitor input filter is improved when the filter is used in conjunction with a full-wave rectifier as shown in the next picture below.

This is due to the decrease in the XC of the filter capacitor and the increase in the XL of the choke. Remember, the ripple frequency of a full-wave rectifier is twice that of a half-wave rectifier. For a 60-hertz input, the ripple will be 120 Hertz. Let's briefly calculate the X C of C1 and the XL of L1:

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.

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.