Imported radiotelephone power supply. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Imported radiotelephone power supply. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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Consider a typical power supply circuit of an imported radiotelephone (see fig.).

Imported radiotelephone power supply

Resistor R1 attenuates the current pulses through the rectifier bridge at the moment of switching on, limits the current through the primary winding T1 when the mains voltage is too high, and burns out in cases of very high mains voltage or an interturn short circuit in the transformer. Zener diode VD2 determines the value of the output voltage (if necessary, select a copy of the zener diode with the load off). The HL1 incandescent lamp serves to limit the power released on the VT1 transistor in nominal mode and limit the short-circuit current. If under load the voltage decreases by more than 1 V, a more powerful lamp should be used (one or two lamps from a 1 V garland can be soldered in parallel to HL13,5).

The radiator that cools the VT1 transistor is cut out of tin-plated sheet. For better heat dissipation, the tin of the radiator should be pressed against the metal of the transistor from both sides, the shape and dimensions of the radiator should cover more space in the existing box. Collector output bite off, current

to the collector of the transistor is brought through the radiator. It is possible to supply current to the collector both through the tab on the fastening screw, and from the platform on the printed circuit board through the fastening screw. The ventilation holes must ensure the removal of heat from the lamp so that the rectifier bridge and the transistor are cold in the operating mode, and warm up a little during a short circuit. Due to the specifics of a telephone with a handset-transceiver (the presence of a battery), it is impossible to load the output of the power supply with a resistor so as not to discharge the battery when the mains voltage is turned off. The principle of a reliable power supply does not allow you to turn on the discharge resistor, even if you know that the radiotelephone circuit has its own diodes and blocking! If, after heating the unit with a table lamp with the load turned off, it turns out that the output voltage begins to increase, it is necessary to shunt the B-e transistor junction with a resistor with a resistance of 5 kOhm ... 500 Ohm.

The operating voltage of the HL1 lamp in this circuit is chosen without a margin, so that long-term short circuits lead to the burnout of the incandescent lamp and the de-energization of the circuit, and in the absence of the owners of the phone, emergency operation does not last for months. To reliably turn off the circuit during an interturn circuit in the power transformer, make sure that during normal operation under load for 1 hour, the resistor R1 is warm to the touch (turn off the plug from the network at the time of checking!). And the general rule is to put the power supply not on a soft stand, which impairs ventilation, but on a hard platform.

One more note: due to the specifics of the operation of the radiotelephone, the load on the power supply is maximum at the moment of waiting - the handset is laid down, the battery is being charged. In this regard, when developing the circuit, the goal was not to strongly suppress the ripple of the supply voltage; it was more important to reduce the dimensions of the device. When repeating this circuit to power other devices, you may have to increase the capacitance of the capacitor C1, and also connect the capacitor to the output of the stabilizer.

It is impossible to shunt a zener diode with a high-capacity capacitor (more than thousands of picofarads): if the stabilizer output is short-circuited, a breakdown of the e-b transition of the regulating transistor is possible!

See other articles Section Power Supplies.

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