Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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This article describes a simple stabilizer for powering a radio station with a voltage of 3,6 V from the vehicle's on-board network. The device can also be used to power cell phones.

Recently, low-power radio stations operating in the 433 ... 434 MHz band have appeared on sale. Among them is a miniature (a little more than a pack of cigarettes) Apollo radio station. In terms of its capabilities and ease of use (69 channels, personal call in the CTCSS system, calls, "locks", etc.), it can be attributed to the most modern communication equipment. The small "range" of Apollo (according to the passport - 2 miles) in many cases turns out to be quite sufficient.

Among the possible applications of this radio station is operational communication in a group of vehicles. However, a regular power supply - three AA elements - cannot provide a sufficiently long operation in the mode of continuous monitoring of the air, which is mandatory in such cases. Even when working only for reception, the radio station will use up a galvanic battery with a capacity of 400 mAh in -16 ... 60 hours, and batteries (750 mAh) will require recharging after 30 ... 100 hours. , - 18 ... 25 mA, in the air control mode - 7 ... 15 mA, and during transmission - 110 ... 120 mA. Reduced power consumption of the station in the air control mode is achieved by the fact that the active operation of the receiver is interspersed with pauses. But this does not affect the efficiency of communication: the duration of the pause is less than the duration of the correspondent's ringing signal, and his appearance on the air is detected immediately.

On fig. 1 shows a diagram of a device that generates the voltage needed to power the Apollo radio station from a car battery. The device is based on an emitter follower made on a composite transistor VT1. A voltage of 5,6 V at its base sets the reference zener diode VD1. The voltage at the emitter VT1 will be lower than the voltage at the base and will be in the range of 3,6 ... 4,5 V.

The most important requirement for a stabilizer that feeds an expensive device is reliability. Here it is achieved by the fact that each element is placed in a mode that is far from the maximum allowable. So, the voltage on the collector of the transistor VT1 is about 8 times lower than the maximum, the current in the zener diode VD1 is 5 times less, and the collector current of the transistor even in the transmission mode does not reach 0,051Kmax. There is also a sufficient margin for power dissipated on VT1.

But that's not all. In the event of an unlikely breakdown of the transistor VT1 or a break in the zener diode VD1 (in both of these cases, the voltage at the output of the converter would increase to completely unacceptable 11 ... These elements form a structure that functions like a high-current zener diode. If the resistor R12 is set on the base of the transistor VT2 to a voltage close to opening the base-emitter junction, then in the event of an emergency increase in voltage at the output in the collector VT2, a current will occur that burns the fuse FU3. So, even with a breakdown of the transistor VT3 or a break in the reference zener diode VD2, the supply voltage of the radio station will increase by only a few tenths of a volt, and even then for a short time.

Almost any zener diode with a stabilization voltage close to 1 V can be taken as VD5,6. But since the spread in this parameter is quite large (see table), it is recommended to select a zener diode.

You can use two zener diodes connected in series, for example, KS133A and KS119A. To slightly increase the stabilization voltage in series with the zener diode, you can turn on a germanium or silicon diode (germanium will increase the voltage at the output of the stabilizer by 0,3 ... 0,4 V, silicon - by 0,6 ... 0,7 V). A compound zener diode may have better thermal stability, since the negative voltage temperature coefficient (VTC) of one of its components can be compensated by the positive SVC of another.

Resistor R1, which limits the emergency current, is MLT-1. True, in emergency mode, it will dissipate power significantly exceeding 1 W, but for small fractions of a second before the fuse blows, it will not even have time to heat up. The resistor can be made from a piece of PENKh (nichrome) wire with a diameter of 0,15 and a length of 10 ... 15 cm, wound on a suitable blank - a burnt fuse or a high-resistance resistor.

A small-sized fuse FU1 type VP1-2 is soldered directly into the board. There is no need to quickly replace it, since this should be preceded by clarification of the causes of what happened.

The device is mounted on a board made of one-sided foil fiberglass with a thickness of 1,5 mm (Fig. 2).

The MZ holes in the board are used for fastening transistors (KT972A is attached "face" to the board), and holes 0 2,1 are for fixing the board itself in a case measuring 48x44x13 mm, glued from sheet polystyrene 2 mm thick. Such a design can be inserted directly into the radio station in the place freed from the standard battery.

The overvoltage protection threshold is set by resistor R3 when the load is off. They begin with the fact that the engine of this resistor is set to the lower (according to the diagram) position. After making sure that the output voltage is within 3,6 ... 4,5 V, and the current consumed from the +12 V source does not exceed 17 ... 20 mA (total current in the zener diodes VD1 and VD2), move the engine resistor R3 until the current consumption begins to rise (transistor VT2 opens). Slightly returning the engine R3 back (thus closing the transistor VT2), leave it in this position.

In all operating modes of the radio station and in the entire operating temperature range, the voltage at the output of the stabilizer must remain within 3,2 ... 4,5 V, and the turn-on voltage of the transistor VT2 should not exceed 5,5 V. must exceed 20...22 mA.

Author: Yu.Vinogradov, Moscow

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