Car battery low alarm. Free Technical Library article

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Car battery low alarm

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Encyclopedia of radio electronics and electrical engineering / Automobile. Electronic devices

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To prolong the life of a car battery, effective control over its charging mode is necessary. The described device signals the driver when the voltage on the battery is increased and when it is lowered, and the generator is not working. In the event of increased current consumption in the on-board network at a low frequency of rotation of the generator rotor, the signaling device does not work.

When developing the device, the goal was to place it in the housing of the PC702 signal relay available in the car, which determined the design features of the signaling device and the types of transistors used.

A schematic diagram of an electronic signaling device, together with its connection circuits with elements of the on-board network, is shown in fig. one.

Car battery low alarm. Schematic diagram of the electronic signaling device

On transistors VT2, VT3, a Schmitt trigger is made, on VT1 - a node for prohibiting its operation. The collector circuit of the transistor VT3 includes an indicator lamp HL1, located on the instrument panel. When hot, the filament has a resistance of about 50 ohms. Cold thread resistance is 7...10 times lower. In this regard, the VT3 transistor must withstand a surge current in the collector circuit up to 2,5 A. This requirement is met by the KT814 transistor.

Similar transistors are also used as VT1 and VT2. But here the reason for their choice was the desire to obtain small geometric dimensions of the device - three transistors are installed one below the other and fixed with a common screw with a nut.

The voltage of the on-board network, minus the voltage at the zener diode VD2, is fed through the divider R5-R6 to the base of the transistor VT2. If it is higher than 13,5 V, the Schmitt trigger switches to a state in which the output transistor VT3 is closed and the HL1 lamp is off.

The base of the transistor VT2 through the zener diode VD1 and the divider R1R2 is also connected to the midpoint of the generator winding. With a working generator, a pulsating voltage is created in it relative to its positive output with an amplitude equal to half the generated voltage. Therefore, even if due to a large current load in the on-board network, the voltage drops below 13,5 V, the current from the divider R1R2 enters the base of the transistor VT2 and does not allow the lamp to burn. To exclude the prohibition of turning on the alarm when there is no current in the excitation winding of the generator, a circuit is used, consisting of a divider R1R2 and a zener diode VD1. It prevents leakage current through the rectifier diodes of the generator (up to 10 mA in the worst case) to the base of the transistor VT2.

The voltage of the on-board network minus the voltage at the zener diode VD2 through the divider R3R4 is also supplied to the base of the transistor VT1, the collector-emitter section of which shunts the base circuit of the transistor VT2. When the mains voltage is above 15 V, the transistor VT1 goes into saturation mode. In this case, the Schmitt trigger switches to a state in which the transistor VT3 is open and, therefore, the lamp HL1 lights up.

Thus, the red light on the instrument panel lights up when there is no charging current and the mains voltage is below 13,5 V, and also when it is above 15 V.

When using an electronic voltage regulator in a car that does not have a separate wire to the battery terminal, due to a voltage drop (about 0,1 ... 0,2 V) in the circuit to the input terminal of the regulator (most often in idle mode) when switched off current consumers, there is a short-term periodic loss of the charging current from the generator. The duration and period of this effect are determined by the time the voltage on the battery drops by 0,1 ... 0,2 V and the time it takes to increase it by the same value and, depending on the state of the battery, are about 0,3 ... 0,6 s and 1...3 s, respectively. At the same time, the signal relay PC702 is activated with the same cycle, lighting the lamp. Such an effect is undesirable. The described electronic signaling device excludes it, since during short-term loss of the charging current, the voltage in the on-board network does not reach the lower threshold of 13,5 V.

The electronic signaling device is made on the basis of the PC702 signaling relay available in the car. The relay itself was removed from the getinax board (after the rivet was eliminated). In addition, the rivet from the contact tab "87" and the L-shaped post at its base were removed.

The elements of the signaling device are mounted on a printed circuit board (Fig. 2) made of foil fiberglass 1,5 ... 2 mm thick. Transistors VT1 -VT3 are located along the axis of the central hole of the board: VT3 from the side of the printed wiring with a collector plate from the board, and VT2, VT1 (in that order) - from the opposite side of the board with collector plates towards the board. Before soldering, all three transistors must be tightened with an M3 screw with a nut. Their conclusions are connected by children of the board with tinned copper conductors soldered into the necessary holes in the board. Resistors R5 and R1 are soldered not to conductive tracks, but to wire pins. This will make it easier to replace them when setting up the device. Elements VD2 and VD1 are installed vertically with a hard lead to the board. Capacitor CXNUMX is also vertically located, placed in a PVC tube along the diameter of the capacitor.

Car battery low alarm. Printed circuit board

Resistors (except R8) - OMLT (MLT) should be used in the signaling device with ratings and dissipation power indicated in the diagram. Rating tolerance ±10%. Resistor R8 is made from a high-resistance wire wound (1-2 turns) on an MLT-0,5 resistor. Capacitor C1 - K50-12. Transistors VT1-VT3-any of the KT814 or KT816 series. The VD1 element is a D814 zener diode with any letter index, VD2-D814B or D814V.

After the installation of the printed circuit board is completed, the electronic signaling device is assembled in the following sequence:

remove the nut and screw tightening the transistors;
a PVC tube with a diameter of 1 mm is placed in the through holes of transistors VT2, VT3;
petals (conclusions) "702/30" (in the center) and "51" are inserted into the board released from the PC87 relay; the latter is fixed with an M3 screw (head on the output side) with a nut XNUMX mm high;
an M2,7 screw 15 ... 20 mm long is passed through a hole in the board from the PC702 relay (from the "30/51" output side), then the mounted board with transistors is mounted on the ends of the screws;
provide contact output "30/51" with the collector plate of the transistor VT3 (by its snug fit to the flat part of the output);
check the connection of output "87" with the printed circuit board through a nut with a screw;
short pins of terminals "85" and "86" are bent so that they fit into the holes intended for them on the printed circuit board;
using nuts M2,7 and MXNUMX with washers fasten both boards;
solder the pins of the conclusions "85" and "86" to the conductive tracks.

When setting up the signaling device, a power supply with an adjustable voltage from 12 to 16 V and a lamp with a power of 3 W at 12 V are required. First, with the resistor R5 disconnected, the resistor R3 is selected. It is necessary to ensure that when the voltage increases, the lamp lights up at the moment it reaches 14,5 ... 15 V. Then the resistor R5 is selected so that the lamp lights up when the voltage drops to 13,2 ... 13,5 V.
The well-adjusted signaling device is installed in place of the PC702 relay, while the output "86" is connected to the "ground" of the car with a short wire under the screw for fastening the signaling device itself. The wires of the electrical equipment are connected to the rest of the terminals, as provided for by the standard circuit of the car with the RS702 relay, i.e., to terminal "85" - the wire from the midpoint of the generator (yellow), to "30/51" - the wire from the indicator lamp (black) , to "87" - wire "±12 V" (orange).

Tests of the signaling device showed the following result. With a short circuit of the regulator, the glow of the lamp is observed with an increase in the generator speed and depends on it. When the fuse in the regulator circuit is removed, the lamp lights up after about a minute, regardless of the speed. This information is enough to establish the cause and type of malfunction of the generator-voltage regulator system.

When the ignition is turned on an hour or more after the engine is stopped, the indication works, as with a relay signaling device. If it turns on after a short time (less than 5 minutes), the charging indicator lamp does not light up, but when the engine is started by the starter, it flashes and goes out, indicating that the indicator is working.

Installing the described regulator instead of the standard PC702 in Zhiguli cars (VAZ-2101, VAZ-2102, VAZ-2103, VAZ-2106, etc.) will clearly warn the driver about all deviations in the battery operation mode and save it from destructive overcharging.

Author: A. Korobkov. To help the radio amateur, issue 99; Publication: cxem.net

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