GB2108732A - Electric current monitoring and control circuit - Google Patents

Abstract

An electric circuit having a current indicating ammeter (AM) and shunt (S), includes a potentiometer (VR1) of relatively high resistance in parallel with the shunt (S). The potential between the slider and one end of the potentiometer (VR1) is connected to the inputs of a differential amplifier (A) to generate a control output (CO) dependent upon the current flowing through the electrical circuit, for limiting that current, eg. by controlling a transistor in a charging circuit (CH) connected to a battery (B) through the shunt (S). <IMAGE>

Description

SPECIFICATION Electrical monitoring circuit This invention relates to an electrical circuit for monitoring the current flowing in an electrical circuit and for devising a control output which may be used to control the operation of the electrical circuit.

It is an object of the present invention to provide a relatively simple electrical monitoring circuit which may be readily and easily applied to existing electrical circuits.

The present invention consists in an electrical monitoring circuit for an electrical circuit having current indicating means, wherein the monitoring circuit includes a potentiometer of relatively high resistance connectable in parallel with the current indicating means, the potential between the slider and one end of the potentiometer being connected to the inputs of a differential amplifier arranged to generate a control output dependent upon the current flowing through the electrical circuit for controlling that current.

Preferably the current indicating means consists of an ammeter with a shunt.

In carrying the invention into effect according to one convenient mode by way of example, the accompanying drawing shows the circuit diagram of a supply circuit SC and a current monitoring circuit CM connected to a conventional battery charging circuit consists of a charger CH connected to a bank of battery cells B with an ammeter AM connected in series in the negative supply line to measure the charging current, a resistance shunt S being connected in parallel with the ammeter AM.

The supply current includes a rectifying circuit having zener diodes ZD1 and ZD2 arranged to provide a +iOVd.c. supply from a 12-0-12V isolated a.c. supply. The d.c. supply is fed to a transistorTR1 connected in emitter follower configuration and to a differential amplifier A.

A potentiometerVR1 of high resistance compared with the shunt S is connected in parallel with the shunt S and the voltage between the more negative side of the potentiometer VR1 and its slider is fed to the inputs of the differential amplifier A. As this voltage increases, the amplifier A will apply a positive charge to the base of the transistor TR 1 and thus a positive control output voltage at terminal CO which can be used to control a control transistor in the charger CH to limit the charging current. Conveniently a connection may be provided via diode D4 for the base of the control transistor.

In one typical arrangement, the maximum charging current is intended to be 10 amps and at this current the shunt has a voltage of 75mV across it. The differential amplifier only requires an input of 1 5mV to produce an output of 5V which in turn will produce an output of 4V at the control output CO. If this value of 4V is used to prevent a further current increase then when the slider of VR1 is moved to the positive end of the potentiometer the maximum current will be limited to 1/5 i.e. 2 amps. As the slider is moved towards the negative end of the potentiometer, current limiting will take place at a higher current. A resistor may be added in series with the potentiometer on the negative side so that the slider of the potentiometer can be adjusted to give only a maximum limited current value.

Conveniently the supply current SC and current monitoring circuit CM may be constructed on small printed circuit boards housed in a box mounted upon an octal pin base.

Since the a.c. supply is isolated the system can be used to monitor shunts at any potential, and the system may be used to monitor currents from a few milliamps to hundreds of volts.

With small modification the system may be used to monitor and control the output voltages of battery chargers and power supplies.

The system may also be used as a control unit in applications other than battery charging, such as temperature control, light intensity control and so forth.

Claims (Filed on 4.10.82) 1. An electrical monitoring circuit for an electrical circuit having current indicating means, wherein the monitoring circuit includes a potentiometer of relatively high resistance connectable in parallel with the current indicating means, the potential between the slider and one end of the potentiometer being connected to the inputs of a differential amplifier arranged to generate a control output dependent upon the current flowing through the electrical circuit for controlling that current.

2. An electrical monitoring circuit substantially as described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (2)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Electrical monitoring circuit This invention relates to an electrical circuit for monitoring the current flowing in an electrical circuit and for devising a control output which may be used to control the operation of the electrical circuit. It is an object of the present invention to provide a relatively simple electrical monitoring circuit which may be readily and easily applied to existing electrical circuits. The present invention consists in an electrical monitoring circuit for an electrical circuit having current indicating means, wherein the monitoring circuit includes a potentiometer of relatively high resistance connectable in parallel with the current indicating means, the potential between the slider and one end of the potentiometer being connected to the inputs of a differential amplifier arranged to generate a control output dependent upon the current flowing through the electrical circuit for controlling that current. Preferably the current indicating means consists of an ammeter with a shunt. In carrying the invention into effect according to one convenient mode by way of example, the accompanying drawing shows the circuit diagram of a supply circuit SC and a current monitoring circuit CM connected to a conventional battery charging circuit consists of a charger CH connected to a bank of battery cells B with an ammeter AM connected in series in the negative supply line to measure the charging current, a resistance shunt S being connected in parallel with the ammeter AM. The supply current includes a rectifying circuit having zener diodes ZD1 and ZD2 arranged to provide a +iOVd.c. supply from a 12-0-12V isolated a.c. supply. The d.c. supply is fed to a transistorTR1 connected in emitter follower configuration and to a differential amplifier A. A potentiometerVR1 of high resistance compared with the shunt S is connected in parallel with the shunt S and the voltage between the more negative side of the potentiometer VR1 and its slider is fed to the inputs of the differential amplifier A. As this voltage increases, the amplifier A will apply a positive charge to the base of the transistor TR 1 and thus a positive control output voltage at terminal CO which can be used to control a control transistor in the charger CH to limit the charging current. Conveniently a connection may be provided via diode D4 for the base of the control transistor. In one typical arrangement, the maximum charging current is intended to be 10 amps and at this current the shunt has a voltage of 75mV across it. The differential amplifier only requires an input of 1 5mV to produce an output of 5V which in turn will produce an output of 4V at the control output CO. If this value of 4V is used to prevent a further current increase then when the slider of VR1 is moved to the positive end of the potentiometer the maximum current will be limited to 1/5 i.e. 2 amps. As the slider is moved towards the negative end of the potentiometer, current limiting will take place at a higher current. A resistor may be added in series with the potentiometer on the negative side so that the slider of the potentiometer can be adjusted to give only a maximum limited current value. Conveniently the supply current SC and current monitoring circuit CM may be constructed on small printed circuit boards housed in a box mounted upon an octal pin base. Since the a.c. supply is isolated the system can be used to monitor shunts at any potential, and the system may be used to monitor currents from a few milliamps to hundreds of volts. With small modification the system may be used to monitor and control the output voltages of battery chargers and power supplies. The system may also be used as a control unit in applications other than battery charging, such as temperature control, light intensity control and so forth. Claims (Filed on 4.10.82)

1. An electrical monitoring circuit for an electrical circuit having current indicating means, wherein the monitoring circuit includes a potentiometer of relatively high resistance connectable in parallel with the current indicating means, the potential between the slider and one end of the potentiometer being connected to the inputs of a differential amplifier arranged to generate a control output dependent upon the current flowing through the electrical circuit for controlling that current.

2. An electrical monitoring circuit substantially as described with reference to the accompanying drawing.