The role of memory to maintain the circuit in the electronic telephone line status, to send code to provide memory to maintain power manifold, so that people's memory through the telephone keypad to enter numbers and last dialed numbers, the signal is lost when you hang up.
First, the common memory to maintain the circuit has the following three
1. Capacitor energy storage to maintain the memory circuits
Capacitor energy storage memory to maintain the circuit shown in Figure 1. Parallel capacitor C code in the hair of the DC input circuit manifold. In the off-hook state, the handset by the resistance R, voltage regulator diode VD1, filter capacitor C1 voltage source consisting of Uz, the first through the diode VD2 for charging the capacitor C, and then send code to the manifold supply. After hanging up the call is completed, the regulator sources Uz disappear, reverse bias diode D deadline. The role of the energy storage capacitor, its voltage can not be mutated at both ends, it will slowly discharge through the hair code Manifold, Manifold to send code numbers stored in internal memory can be saved. Energy storage capacitor C discharge capacity and made the time code with its own manifold is proportional to the internal resistance of the input power supply, usually through the selected capacitor C to determine the memory capacity of the maintenance time. 220µF if C is the case, you can ensure that memory duration 7 to 11 hours.
2. Hang loop memory of high-value resistor in series to maintain the circuit
Hook loop memory of high-value resistor in series to maintain the circuit generally have the following two forms (see Figure 2, Figure 3). They are the power supply via the telephone office to maintain the hair code monolithic block of memory function, Figure 2, Figure 3 is the difference between keeping the memory resistor in series with the polarity switching circuit, respectively, before or after. In Figure 2, the phone off hook, the hook switch contact closure of 1,2 HK, memory resistor R is shorted to maintain, does not affect the normal dial-up phone and call. After hanging up the phone, hook switch contacts 1,2 HK disconnect the power supply through the telephone office R, polarity conversion circuit, receiving the issued by the relevant components of the supply-side code Manifold VDD. Since the resistance R is large (general requirements for R = 10MO), made the power input code block is also a great internal resistance (megohm level), so the current from the telephone office to a very small share, to maintain the sleep state hair code block supply. In fact, the hair on the code block in the design, memory to maintain voltage of 1 V, the memory just to maintain current zero a few microamps. Figure 3 Basic usage is similar to Figure 2, but there is no memory to maintain resistance in parallel with the hook switch contacts, but the code block directly with the hair connected to VDD side, the resistance R-value than in Figure 2 is larger.
3. External power supply to maintain memory circuits
External power supply to maintain the memory circuit shown in Figure 4, in the phone hook state, outside the internal voltage regulator circuit through the telephone, got the voltage regulator is Uz, this voltage is sent through the code block VD1 to supply, when Uz greater than the external battery voltage, VD2 will reverse bias cut-off, the battery does not provide power. In the phone hook state, Uz disappeared, VD1 end, VD2 conduction, the battery voltage E VD2 to send code through the manifold to provide memory maintenance of power supply terminal VDD. At this point, VDD is the voltage on the E-0.7 volts.
Typically, the first combination circuit and the second circuit, the first circuit and third circuit used in conjunction with a third circuit to cancel the second circuit.
Second, the memory circuit is a failure to maintain the situation
Maintain the circuit in the capacitor energy storage memory, if the energy storage capacitor failure or leakage, will not redial the phone or memory duration was significantly reduced, if the energy storage capacitor breakdown short, the hair code Manifold does not work.
Memory to maintain the circuit in the resistance, resistance if the memory remain open, then the code monolithic block of memory hair disappeared, can not redial or store telephone numbers, but still normal phone calls and dial-up; if the memory circuit to maintain resistance, then the phone always in the hook.
Memory to maintain the external power supply circuit, if the blocking diode VD2 open, or the battery voltage is lost, then the hook, the LCD screen can not display, memory function alone made the maintenance of the VDD side code block in parallel with energy storage capacitor to maintain memory was significantly shorter, once the energy storage capacitor is damaged, the phone will redial function completely disappear.