This device is designed to monitor the vehicle battery current. It allows you to visually monitor the magnitude of the charge and discharge current and even the current that the starter consumes when starting the engine. In principle, this scheme will make it possible to monitor the state of all the "electrical equipment" of the car - the battery, starter, generator and all electrical wiring by the amount of current consumed.
The principle of operation is to measure the amount of voltage drop on the wire that connects the car body to the negative terminal of the battery.
Resistors R1-R5 form the so-called "resistive measuring bridge", from the output of which a signal of different polarity is taken, which is then amplified by an operational amplifier (OA) of the K153UD2 type. In the circuit, you can apply other op-amps of widespread use (K140UD6, K140UD7, K140UD8 or similar imported). You should only take into account the possibility of their different pinout. Capacitor C1 in this case is needed to exclude the possibility of self-excitation, for other types of op-amp it may not be required (see the typical wiring diagram for the applied microcircuit in reference books).
Diodes VD1-VD4 constitute a "logarithmic node", which is needed to expand the range of the measured current. This makes it possible to measure high currents, up to 100 amperes, which makes it possible to monitor even the starting current consumed by the starter.
As a dial indicator, you can use any device that has a zero mark in the middle of the scale (for example, M733 or its modern analogue, a dial-up microammeter). The total deflection current of the arrow should be 50 μA in the minus and plus areas of measurement. The scale of this indicator should be graduated. For this, to the right and to the left of the "zero", three marks are evenly made (at an equal distance from the "zero" and from each other), which will correspond to the current values "5", "50" and "100" amperes. All resistors in the circuit with a power of 0.125 watts, except for the resistor R8 (it is from 0.5 watts). Trimmer resistors of any type.
This circuit does not require any separate power supply. Power is taken from the vehicle's electrical system and stabilized at 6.6 volts with the simplest parametric stabilizer on two zener diodes VD5 and VD6 of the KS133A type.
The device is configured as follows.
The circuit is supplied with power directly from the battery (directly from its “+” terminal) and the indicator arrow is set to “zero” using the trimmer R4.
Then the car is completely de-energized (the ignition key is “turned off”) and the “plus” of the battery is connected to the car body through a 2.4 Ohm resistor. In this case, you should use a resistor with a power of at least 60 watts (you can use, for example, a piece of nichrome wire with an appropriate resistance).
The current consumption in this case will be 5 amperes, and with the help of the trimmer resistor R7, the arrow of the device is set to the corresponding mark on its scale ("5A"). This completes the setup and calibration of the device. The positive terminal of the circuit (the right terminal of the resistor R8) is disconnected from the positive terminal of the battery and connected to the positive wire of the vehicle's on-board network.
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When writing the article, information was used taken from the source: magazine "Radio", No. 4, 1989, "Battery current indicator".