What Is a Current Generator?
A current generator is a device that generates a constant current for electrical measurements, such as in electronic equipment.
Many products are marketed as voltage-current generators. Since a constant current must flow continuously, the magnitude of the current is controlled by an on-board operational amplifier or reference voltage IC so that a constant current can flow even if the load resistance changes.
Some current generators can be used to measure large-scale electrical equipment by applying a large current.
Uses of Current Sensors
Current sensors are mainly used in electrical circuits. The following are examples of applications of current sensors:
1. Power Control
Current is monitored and controlled in power suppliers and power converters. In photovoltaic and wind power generation systems, current sensors monitor the amount and characteristics of the generated current to ensure a stable power supply.
They are also used to protect circuits and equipment from overcurrents. In power supply equipment and power circuits, currents exceeding the rated current can be detected by current sensors to activate protective circuits. Overcurrent protection is important to ensure safety from equipment failure due to short circuits, etc.
2. Battery Control
Used in battery management systems to monitor the charging and discharging current of batteries. By measuring the current, it is possible to evaluate the condition of the battery and estimate the remaining capacity. Battery monitoring with current sensors is important in various applications such as electric vehicles and mobile devices.
3. Motor Control
Current sensors are important devices in motor drive control. It measures the motor current and feeds it back to the control algorithm to control the torque and speed of the motor. It is widely used in drives, mainly inverters.
Principle of Current Generators
The principle of a current generator is that a negative feedback circuit using an operational amplifier and reference voltage circuit is utilized to configure a constant current generator circuit that is independent of the impedance value of the load, and the current value to be generated is determined by the reference voltage (Ref voltage) and the internal resistance value.
A circuit often used to generate a constant current is a circuit that establishes a virtual short circuit, in which 0V is generated at the input terminal of an operational amplifier in a negative feedback circuit. There are usually two types of circuits to establish a virtual short: the suction-type and the discharge-type.
1. Suction-Type
The suction-type is a method of creating a virtual short circuit by applying a current from the outside to a circuit that generates a constant current so that it is sucked into a transistor, and then using an operational amplifier and ground.
2. Discharge-Type
The discharge-type is a method of establishing a virtual short by passing current externally from the circuit of the current generator to the transistor so that it is amplified and discharged using the transistor.
In both cases, the current value is determined by the value obtained by dividing the ref voltage applied to the operational amplifier by the internal resistance, so the current value is independent of the load impedance and its value can be adjusted by the resistance.
Types of Current Sensors
Different types of current sensors exist, depending on the measurement principle. The following are examples of current sensor types.
1. Shunt Resistor Type Current Sensor
This type of current sensor measures the current value by connecting a shunt resistor in series with the circuit in which the current flows. The current flowing through the shunt resistor is calculated by the resistance value and the measured voltage drop according to Ohm’s law. The current can be measured by measuring the voltage at both ends of the shunt resistor whose resistance is known.
The shunt resistor type can measure current with high accuracy. In addition, its simple structure allows it to be manufactured at low cost. They are used on boards and in rectifier circuits for large currents.
2. Hall Effect Current Sensor
A Hall effect current sensor uses a Hall element to detect current. The Hall element is placed near a conductor through which a current flows, and a Hall voltage is generated by the magnetic field. By measuring this Hall voltage, the current is detected.
DC currents can be measured without contact. This type of sensor is used in portable clamp ammeters for DC.
3. Current Transformer Type Current Sensor
This is a current sensor that transforms and measures alternating current. It consists of a conductor that serves as the secondary winding and measures the secondary winding current, which varies according to the primary winding current. This allows the AC current to be calculated.
Because of its low cost and high measurement accuracy, it is widely used in industrial equipment. However, their heavy weight and large area for use are drawbacks.
Other Information on Current Generators
1. Current Generators and Instrumentation
Standardly used in the field of instrumentation, 4-20 mA and 1-5 V are analog signals that are widely used as output signals from sensors (transducers) or as control signals for regulators, sequencers, etc.
For example, in the case of a valve opening, the output signal from a degree of opening meter is as follows:
- Valve opening 0%: 4mA or 1V
- Valve opening 100%: 20mA or 5V
In other words, 4mA or 1V is output when the measured value is 0, and 20mA or 5V is output when the measured value is 100. By standardizing and unifying signals, it is possible to pass signals between instrumentation devices.
The reason for outputting 4mA when the measured value is 0 is to determine if the wire is broken. In other words, the purpose is to determine whether a 4mA current flows and indicates 0, or whether the wire is disconnected and indicates 0. The wide-angle indicator is designed to indicate 0 at 4mA and less than 0 when the wire is disconnected and no current is flowing.
2. Noise Suppression During Evaluation of Instrumentation Equipment
When signals are sent in voltage, a voltage drop occurs, causing measurement errors.
Another feature of current signals is that if the input of another instrumentation device is 1-5V, it can be easily converted to a voltage signal by inserting a 250ohm resistor. Conversely, a disadvantage is that it is easily affected by noise, which can cause errors in measurement values.
Effective noise countermeasures include the use of shielded cables, installation of noise filters, and grounding to minimize noise effects. Another point is that if a loop circuit is formed with 4-20mA signals, the entire loop will be affected if a wire is disconnected. This is because it is a series circuit. One countermeasure is to use an isolator.