Analysis of GP88S walkie-talkie charger circuit

GP88S walkie-talkie is one of the main models currently in use. Its charger is uniquely designed and has the characteristics of a switching power supply. It uses pulse surge charging to increase the battery life and reduce the "memory" effect that often occurs in nickel-cadmium batteries and nickel-metal hydride batteries. It is safe, stable and has powerful protection functions. It can identify the battery type and provide a suitable charging current. The indicator light shows the status clearly, giving people a refreshing feeling.

The charger for GP88S walkie-talkie is designed and manufactured by MOTOROLA, with the voltage-driven pulse width modulation control integrated circuit TL494 as the core component, plus EPROM5180539Y02 and some peripheral components. To understand the working principle of this charger, you must first understand the pin configuration and function of TL494 and the circuit structure of the battery.

TL494 pin configuration and its function

The internal circuit of TL494 consists of a reference voltage generation circuit, an oscillation circuit, an intermittent adjustment circuit, two error amplifiers, a pulse width modulation comparator, and an output circuit. Figure 1 is a schematic diagram of its pins, where pins 1 and 2 are the in-phase and inverting input terminals of the error amplifier I; pin 3 is for phase correction and gain control; pin 4 is for intermittent adjustment, and when a voltage of 0 to 3.3V is added to it, the cutoff time can be linearly changed from 2% to 100%; pins 5 and 6 are used for external oscillation resistance and oscillation capacitance respectively; pin 7 is the ground terminal; pins 8, 9 and 11, 10 are the collector and emitter of the two final output transistors inside TL494 respectively; pin 12 is the power supply terminal; pin 13 is the output control terminal, which is a parallel single-ended output mode when grounded, and a push-pull output mode when connected to pin 14; pin 14 is a 5V reference voltage output terminal, with a maximum output current of 10mA; pins 15 and 16 are the inverting and in-phase input terminals of the error amplifier II.

Battery circuit diagram

From the battery circuit structure (see Figure 2), it can be seen that multiple switches are used between the batteries to prevent battery short circuits; and a coding register is set at the charging pin 2 to identify the battery type and provide a suitable charging current. It can also be seen here that if the battery is a counterfeit product, it cannot be charged.

Figure 3 is a simplified circuit diagram of the charger base of the GP88S walkie-talkie. The charging voltage of the charger base is adjusted according to the battery type and its requirements, and is modulated using pulse width modulation. As the power of the rechargeable battery continues to increase, the potential of the TL49416 pin also continues to rise, while the pulse width duty cycle of the TL4948 and 11 pins continues to decrease. The on or off of the adjustment tube is adjusted by the duty cycle of the pulse output from the 8th and 11th pins of the TL494 (the output is a pulsed charging voltage). The continuous reduction in the duty cycle causes the charging voltage to continue to decrease, and the charging current becomes smaller and smaller. When the battery charge reaches the predetermined value, there is no output voltage, and the green light is always on, indicating that the battery is fully charged.

How the charger works

When a battery to be charged is inserted into the charger, first the 6th pin of the EPROM component receives a coded signal, and then two situations may occur:

1) When the serial number is incorrect (i.e. the battery is not a regular MOTOROLA product), the EPROM 2 pin outputs a high level, and the level input to the TL494 2 pin is high, so that the TL494 8th and 11th pins cannot output square wave signals, that is, there is no output charging voltage, and the red light flashes, which means that charging is not possible;

2) When the numbering is correct, pin 7 receives a signal, pin 2 outputs a low level, and is input to pin 2 of TL494, which is the inverting input terminal IN- of differential amplifier I. Then both pin 1 of EPROM and pin 3 of TL494 input a low level, and pin 16 of TL494 performs voltage division, and the potential is lower than 4V, which makes pin 8 and pin 11 of TL494 output a square wave signal with a certain width, modulating the adjustment tube. At this time, the red light is always on, indicating that charging is possible.

Problems and solutions

Although the GP88S walkie-talkie charger has many advantages mentioned above, we found that its damage rate is very high in our actual work, which is related to our working environment and usage methods. The charger requires an input voltage of 16±20%V, and is also specially equipped with a charger head with rectified AC input. However, when working in the field, because we often cannot return to the camp and there is no 220V AC voltage, we often do not use a charger head, but directly connect the charger to the battery for charging. Obviously, the battery voltage will exceed its specified range. In addition, if the operator is not careful and connects the power supply in reverse, it will burn out its circuit and components. Because the charger is composed of SMD components, most of the components are often burned to the point where they cannot be repaired, resulting in missing parts, especially EPROM components.

In response to this situation, we had to use the original circuit board and charging stand to modify the charger that could not be repaired according to the environmental characteristics of field work.

1. Renovation Plan 1

The first solution is simple and easy to implement. It only needs to add three resistors. At this time, the charging voltage is adjusted by the resistor (5W50Ω) according to the size of the charging current. However, when the input voltage of this solution is relatively large, the battery

The charging current is relatively large, which has a great impact on the battery life, and the battery consumes a lot of power. Figure 4 is the circuit diagram of the modified charger seat.

2. Renovation Plan 2

This solution is an improvement of the first solution. During installation, the PNP adjustment tube in the original charger is replaced with an NPN transistor (2M6121). Since the adjustment transistor is used, in this solution, no matter how the input voltage changes, the charging voltage to the battery is constant, and the output voltage is always 9-10V. The impact on the battery life is smaller than that of the first solution, and the power consumption is also smaller than that of the first solution. Figure 5 is the circuit diagram of the charger after the modification according to the second solution.

Conclusion

Although the two emergency modification schemes introduced above sacrifice the unique and excellent characteristics of the original charger, they are effective methods for emergency response in the field.