Exercise 2-1. Battery Feed Power Supply

Transcription

1 Exercise 2-1 Battery Feed Power Supply When you have completed this exercise, you will be able to demonstrate how the central office supplies power to analog telephone sets. A telephone set requires electrical power to operate. However, an analog telephone set is not connected to an AC power outlet like many other electric appliances. It is powered via the telephone line by a battery feed circuit (non-interruptible DC power source) located in the central office of the local telephone company. This explains why analog telephone sets are still operative during electric power failures. Analog telephone sets with programmed function keys and an alphanumerical display are often powered by an AC power outlet in addition to the battery feed circuit in the local central office. This is because their power consumption is generally higher than that of basic analog telephone sets, and exceeds the maximum value allowed for one telephone set. This is also because a continuous source of power is required to keep data (such as telephone numbers) stored in the telephone memory. Figure 2-2 is a simplified diagram showing how the central office supplies DC power to an analog telephone set. The DC voltage from the battery feed circuit is applied to the subscriber loop interface circuit () of each line interface in the central office. The routes the DC voltage to the T and R terminals of the telephone line via current-limiting resistors (1 and 2 ). In case of an accidental short-circuit between the T and R terminals of the line interface, these resistors prevent damages to the interface circuitry by limiting the DC current flowing through the. The T terminal of the telephone line is connected to the positive terminal of the battery feed circuit through one of the current-limiting resistor and the. Notice that this terminal of the battery feed circuit is connected to ground. This results in a negative-polarity (with respect to ground) DC voltage on the R terminal of the telephone line. Positivepolarity voltages are generally avoided in outside telephone cables because if there is any moisture present, copper from the wires may be lost through electrolysis.

2 CENTRAL OFFICE LINE INTERFACE T TELEPHONE LINE (LOCAL LOOP) T CURRENT-LIMITING RESISTORS 1 ANALOG TELEPHONE SET R DC LOOP CURRENT (I L ) R 2 48 V BATTERY FEED CIRCUIT The value of the DC voltage provided by the battery feed circuit ranges between 24 and 104 V from one country to another. However, the DC voltage value is standardized to 48 V in North America and many other countries such as Brazil, Peru, Ireland, Spain, Sweden, China, Japan, Korea, and Australia. When the telephone handset is lifted off the cradle, the switchhook closes and DC current flows in the telephone set through the, current-limiting resistors, and the telephone line. The value of the DC loop current typically ranges from 20 to 80 ma under normal operating conditions, and is usually limited to 120 ma when the T and R terminals of the line interface are accidentally short-circuited. Figure 2-3 is an equivalent electrical circuit of the telephone set and central office shown in Figure 2-2, when the handset is lifted off the cradle. The DC loop current (I L ) depends on the battery feed circuit voltage ( ), the combined resistance of the two current-limiting resistors ( ), the telephone line resistance (R L ), and the equivalent resistance of the telephone set between the T and R terminals (R T ). Current I L can be calculated using the following equation. I L R L R T (1)

3 ANALOG TELEPHONE SET (OFF-HOOK) TELEPHONE LINE R L CENTRAL OFFICE R T I L I = L + R L + R T The combined resistance of the current-limiting resistors is usually The equivalent resistance R T of an analog telephone set is typically 400 6, although it can be as low as The resistance R L of the telephone line is proportional to the line length. R L is virtually zero in the case of a short line, and can hardly exceeds for the DC loop current to be at least 20 ma when the telephone switchhook closes (when = 48 V). This is an important parameter because the DC loop current must reach a certain minimum value so that it can be used reliably to determine the status (on-hook or off-hook) of the telephone handset. For instance, a maximum line resistance R L of limits the maximum length of a telephone line implemented with a pair of 22-AWG copper wires (0.635-mm diameter) to about 15 km. Figure 2-4 is an equivalent electrical circuit of the telephone set and central office shown in Figure 2-2, when the handset is on the cradle. Since the switchhook is open, resistance R T is infinite and no current flows in the telephone line. ANALOG TELEPHONE SET (ON-HOOK) TELEPHONE LINE R L CENTRAL OFFICE OPEN CIRCUIT (R T = æ) I L I = L + R L + R T = 0 ma

4 Figure 2-5 is another equivalent electrical circuit of the telephone set and central office shown in Figure 2-2, when the T and R terminals of the line interface are accidentally short-circuited. In this situation, the DC current flowing through the (I ) is limited only by the combined resistance of the current-limiting resistors. It is equal to 120 ma when and equal 48 V and 400 6, respectively. CENTRAL OFFICE SHORT-CIRCUIT BETWEEN THE T AND R TERMINALS OF THE LINE INTERFACE I I = In the first part of the exercise, you will set up a central office with the Telephony Training System (TTS). In the second part of the exercise, you will measure the DC voltage across the telephone line and the DC loop current, when the handset of the telephone set is lifted off the cradle. From these measurements, you will calculate the equivalent resistance of the telephone set as well as the total resistance of the telephone line and the current-limiting resistors of the line interface. In the last part of the exercise, you will increase the resistance (length) of the telephone line, while the handset is lifted off the cradle, to determine how this affects the DC voltage across the telephone line (at the telephone side end of the line) and the DC loop current. Refer to Appendix A of this manual to obtain the list of equipment required to perform this exercise.

5 * 1. Make sure that the Reconfigurable Training Module, Model 9431, is connected to the TTS Power Supply, Model Make sure that there is a network connection between the Reconfigurable Training Module and the host computer. Install the Dual Analog Line Interface, Model 9475, into one of the analog/digital (A/D) slots of the Reconfigurable Training Module. Connect two analog telephone sets to the Dual Analog Line Interface. Make sure that the tone dialing mode is selected on the analog telephone sets. Connect the AC/DC power converter supplied with each analog telephone set to one of the AC power outlets on the TTS Power Supply. Connect the DC power output jack of each AC/DC power converter to the DC power input connector on either one of the analog telephone sets. The analog telephone set requires an auxiliary DC power source for the digital display to be operative. * 2. Turn on the host computer. Turn on the TTS Power Supply then the Reconfigurable Training Module. * 3. On the host computer, start the Telephony Training System software, then download the CO program to the Reconfigurable Training Module. The CO program configures the Reconfigurable Training Module so that it operates as a central office. If the host computer is unable to download the CO program to the Reconfigurable Training Module, it may not be using the proper IP address. Have your instructor or the LAN administrator check if the host computer uses the proper IP address to communicate with the Reconfigurable Training Module.

6 * 4. On the host computer, zoom in on ANALOG LINE INTERFACE A, connect Oscilloscope Probe 1 to TP1 (voltage across the telephone line connected to ANALOG LINE INTERFACE A), and start the Oscilloscope. Probe 1 is associated with channel 1 of the Oscilloscope. Make sure that the resistance of the telephone line connected to ANALOG LINE INTERFACE A is set to * 5. Make the following settings on the Oscilloscope: Channel 1 Mode Normal Sensitivity V/div Input Coupling DC Time Base ms/div Display Refresh Manual * 6. Make sure the handset of telephone set A is correctly placed on the cradle. The DC loop current indicated in ANALOG LINE INTERFACE A should be equal to 0 ma because the handset of telephone set A is in the on-hook state. Refresh the Oscilloscope display. Observe that a DC voltage is applied to the telephone line. Record this DC voltage in the following blank space. DC Voltage Across the Telephone Line: V (handset on the cradle) What does this voltage correspond to? * 7. Lift off the handset of telephone set A. Observe that DC current is flowing through the telephone line. Record this current in the following blank space. DC Loop Current: ma (handset off the cradle) * 8. On the host computer, set the sensitivity of channel 1 on the Oscilloscope to 2 V/div.

7 Refresh the Oscilloscope display. Observe that the DC voltage across the telephone line has decreased significantly. Record this DC voltage in the following blank space. DC Voltage Across the Telephone Line: V (handset off the cradle) Replace the handset of telephone set A on the cradle. Briefly explain why the DC voltage across the telephone line decreases when the handset of a telephone set is lifted off the cradle. * 9. Calculate the equivalent resistance (R T ) of telephone set A using the DC loop current and the DC voltage across the telephone line measured when the handset is off the cradle. Record the value in the following blank space. LINE MONITOR 1, which measures the voltage across the telephone line, is connected to the telephone side of the telephone line. Thus, the signal observed at TP1 is the voltage across the Tip and Ring terminals of telephone set A. R T = 6 * 10. Using the DC loop current and the DC voltage across the telephone line that you measured so far, calculate the total resistance of the telephone line and the current-limiting resistors of ANALOG LINE INTERFACE A. Record the value in the following blank space. = 6 * 11. Make the following settings on the Oscilloscope: Display Refresh Continuous * 12. Lift off the handset of telephone set A. On the host computer, increase the resistance of the telephone line connected to ANALOG LINE INTERFACE A by steps until it is equal to 2.0 k6. While doing this, observe the DC loop current as well as the DC voltage across the telephone line (TP1).

8 Replace the handset of telephone set A on the cradle. Describe what happens when the telephone line resistance (length) is increased. * 13. On the host computer, close the Telephony Training System software. Turn off the TTS Power Supply as well as the host computer (if it is no longer required). Disconnect the AC/DC power converters from the TTS Power Supply and the analog telephone sets. Disconnect the analog telephone sets from the Dual Analog Line Interface. Remove the Dual Analog Line Interface from the Reconfigurable Training Module. In this exercise, you learned that the central office applies a DC voltage (48 V in many countries) across the telephone line to supply electrical power to an analog telephone set. You saw that this DC voltage is applied across the telephone line through the and current-limiting resistors of the line interface. You observed that the DC current flowing through the local loop (DC loop current) depends on the value of the current-limiting resistors, the telephone line resistance, and the equivalent resistance of the telephone set. 1. Briefly explain why basic analog telephone sets are still operative during electric power failures.

9 2. In many countries, what is the standard value of the DC voltage provided by the battery feed circuit? 3. Briefly explain why the telephone line resistance cannot exceed a certain maximum value. 4. Briefly explain why positive-polarity voltages (with respect to ground) are generally avoided on outside telephone lines. 5. Describe how DC power is applied to analog telephone sets.