Power supply with automatic charger. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power Supplies
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In order not to discharge the battery of the receiver when using the security system, being at home, it is connected to a stationary network source.
Scheme, fig. 2.71, monitors the condition of the batteries and, if necessary, automatically recharges them. In addition, in the event of an alarm, transistor VT1 amplifies the volume of the alert sound signal, which ensures ease of use. The indicator that the power source is connected to the network is the glow of the green LED (HL1), and when the battery charging mode is on, the red LED (HL2) is on.
Rice. 2.71. Power supply circuit with charger (click to enlarge)
The device works as follows.
The D1 chip stabilizes the output voltage, the level of which can change in steps (6,6 or 5 V - it depends on whether the HL2 LED is lit or not).
The HL2 LED, in addition to the light indication of the charging process, is also a source of a 1,6 V reference voltage for the microcircuit.
On transistors VT2 and VT3, an analyzer of the output voltage level of the power supply is assembled. Due to the high gain of these transistors, they switch from off to on when the output voltage changes by 0,1 V.
The circuit, with the batteries connected, is adjusted by the resistor R8 so that the transistor opening threshold is approximately 3,9 ... 4 V (while the HL2 LED should not light up). The amount of current through the batteries depends on their condition, and as they charge, it gradually decreases. The maximum charge current is limited to approximately 20 mA (set by the value of resistor R5).
As the charge capacity increases, the voltage on the batteries gradually increases, and when it reaches 3,9 V, the output voltage of the stabilizer D1 will decrease from 6,6 V to 5 V. At the same time, charging will stop.
The circuit uses fixed resistors MLT, tuned R8 type SP5-2; electrolytic capacitors K50-35 for 25 V. Diodes can be replaced by any diodes with a current of at least 500 mA and a reverse voltage of 50 V.
LEDs are used in different colors from the series. KIP32 or similar with low current consumption when lit. The transformer is suitable from unified ones, for example, type TN1-220-50 or any other with a voltage in the secondary winding of 9 ... 12 V.
A variant of the printed circuit board topology and the arrangement of elements for the power supply circuit is shown in fig. 2.72.
Rice. 2.72. PCB layout and layout for power supply circuit
Structurally, the power supply housing is made in the form of a stand for the receiver, but in such a way that the vertically installed receiver is connected through the socket of the corresponding connector (X1) to the power supply circuits. Connector X1 used type MPH4.
Author: Shelestov I.P.
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