Power supply 12 volts for security systems (module 24). Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Power supply 12 volts for security systems (module 24). Encyclopedia of radio electronics and electrical engineering

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The power supply of the security system must be such as to maintain the charge of the batteries and accumulators and at the same time provide a fairly large current necessary for the functioning of the alarm equipment (siren, flashing beacon, etc.). The proposed power supply has a high power and is designed to charge batteries connected directly to its 13,5 V output. The most commonly used battery is a car battery, which guarantees complete autonomy of the security system. In addition, at very high power, these batteries are inexpensive, and therefore they are widely used.

scheme

Voltage regulator

On fig. 10.1 shows a circuit diagram of a stabilized power supply based on the LM723 chip. This regulator, using an internal zener diode and a temperature-compensated current generator, generates a stable reference voltage. The output current is limited by an internal transistor that responds to a voltage drop across a resistor connected in series with the output, through which the output current flows. The maximum current of the output stage of the LM723 microcircuit does not exceed 150 mA. Nevertheless, it is quite enough to drive the load transistor T1, which provides a gain of 500 at a collector current of 10 A.

Power supply 12 volts for security systems (module 24)
Rice. 10.1. Circuit diagram

Tab. 10.1. The list of elements for the scheme of fig. 10.1

current limit

Resistor R5 limits the current at a level of 5 ... 6 A. The voltage drop across this resistor biases the base-emitter junction of the transistor for detecting and limiting the output current built into the microcircuit. Therefore, when the voltage between pins 2 and 3 of DA1 reaches the threshold value. Around 0,6 V, the output voltage (pin 10 of the LM723 chip) decreases. The maximum value of the output current corresponds to the expression:

I=0,6/R5(A).

where R5 is measured in ohms.

Voltage stabilization

The operational amplifier included in the LM723 ensures high output voltage stability. The reference voltage of 7,15 V from output 6 of DA1 is fed to the non-inverting input of the amplifier. As for the inverting input, it is supplied with a voltage from the R1RP1R2 divider proportional to the output voltage. Therefore, any change in the stabilized voltage is immediately compensated by the correction of the output voltage of the operational amplifier, the main function of which is to maintain a zero voltage difference at the differential input. The stabilized output voltage depends on the 7,15 V reference voltage source. The final adjustment of the output voltage is carried out using the variable resistor RP1. Capacitors C1, C2, C3 and C4 serve as a filter and power isolation.

Production

On fig. 10.2 shows a drawing of the conductive tracks of the printed circuit board. The width of the tracks is of great importance, and the widest ones need to be tinned by melting the solder with a soldering iron tip.

Power supply 12 volts for security systems (module 24)
Rice. 10.2. Drawing of the printed circuit board from the side of the conductors

All circuit elements listed in Table. 10.1, with the exception of the transformer, are soldered into the printed circuit board (see fig. 10.3). Diodes D1, D2 are either soldered with a gap into the printed circuit board, or replaced with a diode bridge if the secondary winding of the transformer does not have a tap from the middle. In this case, the diode bridge is either fixed on the metal case of the power supply module, or soldered to the transformer petals.

Power supply 12 volts for security systems (module 24)
Rice. 10.3. Location of elements on the printed circuit board

Transistor T1 is mounted on a sufficiently powerful heatsink, which guarantees a thermal resistance of 1,2°C / W when directly mounted on a heatsink and 1°C / W if the transistor is isolated from the heatsink by a mica gasket. In all cases, silicone grease improves heat transfer conditions.

When using a stabilized power supply in "floating" mode with a 12 V battery connected in parallel to the output, the output voltage is set to 13,5 V. The floating voltage is in the range of 2,2 ... 2,25 V per section, therefore, for an automotive battery of six sections, it is equal to 13,2 ... 13,5 V, which guarantees a full charge of the batteries, if necessary, connected to the "12 V" inputs of various modules.

Author: Kadino E.

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