This design was born because at one time I did not have access to those wonderful modern microcircuits that were specially designed for reading voltage from current sensors. I needed to create an analogue of such a microcircuit, as simple as possible, but no less accurate. In my opinion, the resulting scheme copes with its task quite well.

Automotive positive rail current sensor on discrete components.

The first current amplifier on transistor Q2 has a gain of 6.2 (Figure 1). A thermal compensation amplifier is assembled on Q1, controlled by an IC1B microcircuit and maintaining the Q1 collector voltage at a constant level, regardless of the temperature of the circuit. The circuit reference voltage is the 5V system power supply. The voltages shown in the circuit diagram were measured in a real device.

Figure 1. Q1 and Q2 convert the voltage drop across current sense resistor R3 into a common-mode voltage matched to the microcontrollers' ADC input levels.

IC1A amplifies the voltage difference across the collectors of transistors Q1 and Q2. The op amp gain of this is 4.9. R3 is formed by two surface mount resistors stacked on top of each other. With an output voltage of 5 V, the maximum current measured by the circuit is 25 A.

Two zener diodes protect the circuit from voltage surges in the vehicle's on-board network. As you know, voltage peaks in it can reach 90 V. If the circuit provokes you to criticize, select the values ​​of R6 and R7 with a minimum spread. If you consider this insufficient, coordinate R1 and R4.

I haven't done anything like that, but the operation of the circuit is quite satisfactory to me. The design uses surface mount resistors. With the exception of R3, all are size 0805 and have a 1% tolerance.

Don’t forget to choose fiberglass with foil of sufficient thickness for your printed circuit board and make a wide conductive path, and for R3 provide a two-wire connection according to the Kelvin circuit. At a maximum current of 25 A, this circuit heats up very little.

One of the simplest ways to measure current in an electrical circuit is to measure the voltage drop across a resistor in series with the load. But when current passes through this resistor, useless power is released in the form of heat, so it is selected to the minimum possible value, which in turn entails a subsequent amplification of the signal. It should be noted that the circuits given below make it possible to control not only direct, but also pulsed current, however, with corresponding distortions determined by the bandwidth of the amplifying elements.

Measurement of current in the negative pole of the load.

The circuit for measuring load current in the negative pole is shown in Figure 1.

This diagram and some of the information are borrowed from the magazine “Components and Technologies” No. 10 for 2006. Mikhail Pushkarev [email protected]
Advantages:
low input common mode voltage;
the input and output signals have a common ground;
Easy to implement with one power supply.
Flaws:
the load does not have a direct connection with the “ground”;
there is no possibility of switching the load with a key in the negative pole;
possibility of failure of the measuring circuit due to a short circuit in the load.

Measuring the current in the negative pole of the load is not difficult. Many op-amps designed to operate with a single-supply supply are suitable for this purpose. The circuit for measuring current using an operational amplifier is shown in Fig. 1. The choice of a specific type of amplifier is determined by the required accuracy, which is mainly affected by the amplifier's zero offset, its temperature drift and gain setting error, and the required circuit speed. At the beginning of the scale, a significant conversion error is inevitable, caused by a non-zero value of the amplifier's minimum output voltage, which is not significant for most practical applications. To eliminate this drawback, a bipolar amplifier power supply is required.

Measuring current in the positive pole of the load

Advantages:
load is grounded;
A short circuit in the load is detected.
Flaws:
high common mode input voltage (often very high);
the need to shift the output signal to a level acceptable for subsequent processing in the system (reference to ground).
Let's consider circuits for measuring current in the positive pole of the load using operational amplifiers.

In the diagram in Fig. 2, you can use any of the operational amplifiers suitable for the permissible supply voltage, designed to operate with a single-supply supply and a maximum input common-mode voltage reaching the supply voltage, for example AD8603. The maximum supply voltage of the circuit cannot exceed the maximum permissible supply voltage of the amplifier.

But there are op-amps that are capable of operating at an input common-mode voltage significantly higher than the supply voltage. In the circuit using the LT1637 op-amp shown in Fig. 3, the load supply voltage can reach 44 V with an op-amp supply voltage of 3 V. Instrumentation amplifiers such as LTC2053, LTC6800 from Linear Technology, INA337 from Texas Instruments are suitable for measuring current in the positive pole of the load with a very small error. There are also specialized microcircuits for measuring current in the positive pole, for example, INA138 and INA168.

INA138 and INA168

— high-voltage, unipolar current monitors. A wide range of input voltages, low current consumption and small dimensions - SOT23, allow this chip to be used in many circuits. Power supply voltage is from 2.7 V to 36 V for INA138 and from 2.7 V to 60 V for INA168. The input current is no more than 25 µA, which allows you to measure the voltage drop across the shunt with minimal error. Microcircuits are current-voltage converters with a conversion coefficient from 1 to 100 or more. INA138 and INA168 in SOT23-5 packages have an operating temperature range of -40°C to +125°C.
A typical connection diagram is taken from the documentation for these microcircuits and is shown in Figure 4.

OPA454

- a new low-cost high-voltage operational amplifier from Texas Instruments with an output current of more than 50 mA and a bandwidth of 2.5 MHz. One of the advantages is the high stability of the OPA454 at unity gain.

Protection against overtemperature and overcurrent is organized inside the op-amp. The IC operates over a wide range of supply voltages from ±5 to ±50 V or, in the case of a single-supply supply, from 10 to 100 V (maximum 120 V). The OPA454 has an additional “Status Flag” pin - an open-drain op-amp status output - which allows you to work with logic at any level. This high-voltage operational amplifier features high precision, wide output voltage range, and no phase inversion problems often encountered with simple amplifiers.
Technical features of OPA454:
Wide supply voltage range from ±5 V (10 V) to ±50 V (100 V)
(maximum up to 120 V)
Large maximum output current > ±50 mA
Wide range of operating temperatures from -40 to 85°C (maximum from -55 to 125°C)
SOIC or HSOP package design (PowerPADTM)
Data on the microcircuit are given in “Electronics News” No. 7 for 2008. Sergey Pichugin

Current shunt signal amplifier on the main power bus.

In amateur radio practice, for circuits whose parameters are not so stringent, cheap dual LM358 op-amps are suitable, allowing operation with input voltages up to 32V. Figure 5 shows one of many typical circuits for connecting the LM358 chip as a load current monitor. By the way, not all “datasheets” have diagrams for turning it on. In all likelihood, this circuit was the prototype of the circuit presented in the Radio magazine by I. Nechaev and which I mentioned in the article “ Current limit indicator».
The above circuits are very convenient to use in homemade power supplies for monitoring, telemetry and load current measurement, and for constructing short circuit protection circuits. The current sensor in these circuits can have a very small resistance and there is no need to adjust this resistor, as is done in the case of a conventional ammeter. For example, the voltage across resistor R3 in the circuit in Figure 5 is equal to: Vo = R3∙R1∙IL / R2 i.e. Vo = 1000∙0.1∙1A / 100 = 1V. One ampere of current flowing through the sensor corresponds to one volt of voltage drop across resistor R3. The value of this ratio depends on the value of all resistors included in the converter circuit. It follows that by making resistor R2 a trimmer, you can easily use it to compensate for the spread in the resistance of resistor R1. This also applies to the circuits shown in Figures 2 and 3. In the circuit shown in Fig. 4, the resistance of the load resistor RL can be changed. To reduce the dip in the output voltage of the power supply, it is generally better to take the resistance of the current sensor - resistor R1 in the circuit in Fig. 5 equal to 0.01 Ohm, while changing the value of resistor R2 to 10 Ohm or increasing the value of resistor R3 to 10 kOhm.

A current measuring transducer is a device that can replace the current transformers and shunts used today. Used for control and measurement, and is an excellent engineering solution. The design of the device is made in accordance with modern methods of technical implementation of equipment and methods for ensuring the versatility, convenience and reliability of the system. That is why measuring transducers developed by the Russian manufacturer are in great demand every year. The range of possible modifications pleases consumers, as this allows them to choose the most suitable solution without overpaying.

What's special about current transducers?

The main feature of the current measuring transducer is its versatility. Direct current, pulsed current, and alternating current can be supplied to the input of the device. To make this versatility possible, manufacturers have developed a device based on the Hall principle. The converter uses a small semiconductor circuit. With its help, the magnitude and direction of the magnetic field of the current supplied to the input of the device is determined. Thus, the Hall effect current converter is a unique device with high performance and functionality.

The device is made in the form of a housing with a hole through which a current-carrying conductor is passed. The electronic circuit of the converter is powered from the mains with a DC voltage of 15 volts. A current appears at the output of the device, which changes in value, direction and time in direct proportion to the current at the input. In this case, a current measuring transducer based on the Hall effect can be made not only with an opening for the output of current-carrying conductors, but also in the form of a device intended for installation in an open circuit.

Design features of current measuring transducers

The non-contact current measuring transducer is made with galvanic isolation between the control circuit and the power circuit. The converter consists of a magnetic core, a compensation winding and a Hall device. When current flows through the bus bars, induction is induced in the magnetic circuit, and the Hall device produces a voltage that changes as the induced induction changes. The output signal is fed to the input of the electronic amplifier, and then goes to the compensation winding. As a result, a current flows through the compensation winding, which is directly proportional to the input current, while the shape of the primary current is completely repeated. Essentially, it is a current and voltage converter.

Non-contact AC Current Transducer

Most often, consumers purchase current and voltage sensors for three-phase AC power networks. Therefore, manufacturers have specially developed PIT-___-T measuring transducers with simpler electronics and, accordingly, a lower price. The devices can operate at different temperatures, in the frequency range from 20 to 10 kHz. At the same time, consumers have the opportunity to select the type of output signal from the converter - voltage or current. Non-contact current measuring transducers are manufactured for installation on a round or flat busbar. This significantly expands the scope of application of this equipment and makes it relevant for the reconstruction of substations of various capacities.

In order to successfully automate various technological processes and effectively manage instruments, devices, machines and mechanisms, it is necessary to constantly measure and control many parameters and physical quantities. Therefore, sensors that provide information about the state of controlled devices have become an integral part of automatic systems.

At its core, each sensor is an integral part of regulatory, signaling, measuring and control devices. With its help, one or another controlled quantity is converted into a certain type of signal, which allows one to measure, process, register, transmit and store the received information. In some cases, the sensor may affect controlled processes. The current sensor used in many devices and microcircuits fully possesses all these qualities. It converts the effects of electric current into signals convenient for further use.

Sensor classification

Sensors used in various devices are classified according to certain characteristics. If it is possible to measure input quantities, they can be: electrical, pneumatic, sensors of speed, mechanical movements, pressure, acceleration, force, temperatures and other parameters. Among them, the measurement of electrical and magnetic quantities takes approximately 4%.

Each sensor converts an input value into some output parameter. Depending on this, control devices can be non-electrical or electrical.

Among the latter, the most common are:

• DC sensors
• AC amplitude sensors
• Resistance sensors and other similar devices.

The main advantage of electrical sensors is the ability to transmit information over certain distances at high speed. The use of a digital code ensures high accuracy, speed and increased sensitivity of measuring instruments.

Operating principle

According to the principle of operation, all sensors are divided into two main types. They can be generators - directly converting input quantities into an electrical signal. Parametric sensors include devices that convert input quantities into changed electrical parameters of the sensor itself. In addition, they can be rheostatic, ohmic, photoelectric or optoelectronic, capacitive, inductive, etc.

All sensors have certain requirements for their operation. In each device, the input and output quantities must be directly dependent on each other. All characteristics must be stable over time. As a rule, these devices are characterized by high sensitivity, small size and weight. They can operate in a wide variety of environments and be installed in a variety of ways.

Modern current sensors

Current sensors are devices that are used to determine the strength of direct or alternating current in electrical circuits. Their design includes a magnetic core with a gap and a compensation winding, as well as an electronic board that processes electrical signals. The main sensitive element is a Hall sensor, fixed in the gap of the magnetic circuit and connected to the input of the amplifier.

The principle of operation is generally the same for all such devices. Under the influence of the measured current, a magnetic field arises, then, using a Hall sensor, the corresponding voltage is generated. This voltage is then amplified at the output and applied to the output winding.

Main types of current sensors:

Direct Gain Sensors (O/L). They are small in size and weight, and have low energy consumption. The range of signal conversions has been significantly expanded. Allows you to avoid losses in the primary circuit. The operation of the device is based on a magnetic field that creates a primary current IP. Next, the magnetic field is concentrated in the magnetic circuit and its further transformation by the Hall element in the air gap. The signal received from the Hall element is amplified and a proportional copy of the primary current is formed at the output.

Current sensors (Eta). They are characterized by a wide frequency range and an extended range of conversions. The advantages of these devices are low power consumption and low latency. The operation of the device is supported by a unipolar power supply from 0 to +5 volts. The operation of the device is based on a combined technology that uses compensation type and direct amplification. This results in significantly improved sensor performance and more balanced operation.

Compensating current sensors (C/L). They are distinguished by a wide frequency range, high accuracy and low latency. Devices of this type have no loss of primary signal, they have excellent linearity characteristics and low temperature drift. Compensation of the magnetic field created by the primary current IP, occurs due to the same field generated in the secondary winding. The generation of secondary compensating current is carried out by the Hall element and the electronics of the sensor itself. Ultimately, the secondary current is a proportional copy of the primary current.

Compensating current sensors (type C). The undoubted advantages of these devices are a wide frequency range, high accuracy of information, excellent linearity and reduced temperature drift. In addition, these instruments can measure residual currents (CD). They have high isolation levels and reduced interference with the primary signal. The design consists of two toroidal magnetic cores and two secondary windings. The operation of the sensors is based on ampere-turn compensation. Small current from the primary circuit passes through the primary resistor and the primary winding.

PRIME current sensors. A wide dynamic range is used to convert AC current. The device is characterized by good linearity, insignificant temperature losses and the absence of magnetic saturation. The advantage of the design is its small dimensions and weight, high resistance to various types of overloads. The accuracy of the readings does not depend on how the cable is positioned in the hole and is not influenced by external fields. This sensor does not use a traditional open-loop coil, but rather a sensor head with sensor printed circuit boards. Each board consists of two separate coils with air cores. All of them are mounted on a single base printed circuit board. Two concentric circuits are formed from the sensor boards, at the outputs of which the induced voltage is summed. As a result, information is obtained about the parameters of the amplitude and phase of the measured current.

Current sensors (type IT). Features high accuracy, wide frequency range, low output noise, high temperature stability and low crosstalk. The design of these sensors does not contain Hall elements. The primary current creates a magnetic field, which is subsequently compensated by the secondary current. At the output, the secondary current is a proportional copy of the primary current.

Advantages of current sensors in modern circuits

Current sensor chips play a big role in energy conservation. This is facilitated by low power and energy consumption. Integrated circuits combine all the necessary electronic components. The characteristics of the devices are significantly improved due to the joint operation of magnetic field sensors and all other active electronics.

Modern current sensors enable further reduction in size because all electronics are integrated into a single common chip. This has led to new innovative compact design solutions, including the primary busbar. Each new current sensor has increased insulation and successfully interacts with other types of electronic components.

The latest sensor designs allow them to be installed in existing installations without disconnecting the primary conductor. They consist of two parts and are detachable, allowing these parts to be easily installed on the primary conductor without any disconnection.

Each sensor has technical documentation, which reflects all the necessary information that allows preliminary calculations to be made and the location of the most optimal use to be determined.

In the practice of measuring current, there is a standard technique - connect a low-resistance resistor in series to the circuit under test and measure the voltage drop across it. If you divide the voltage (b^ism) resistance (/?meas)’ ^^ according to Ohm’s law, you get the desired current (/meas) - The resistor must be low-resistance and high-precision so as not to introduce additional power losses in the load and not to worsen the instrumental measurement error.

Mathematical calculations of the current formula can be entrusted to MK. His program will include the voltage measured across a reference resistor through the built-in ADC. The resistance of the resistor is known a priori, so all that remains is to choose the right circuit for pairing it with the MK (Fig. 3.71, a...c).

Rice. 3.71. Diagrams for connecting resistor current sensors to the MK (beginning):

a) the transmitter signal /?iz is scaled by the amplifier DAL1 v\ buffered by the repeater DA1.2. Resistor /?2 regulates the gain of the op-amp, and therefore the sensitivity of the sensor. A DA 1.2 signal repeater may not be available in many cases;

b) a divider on resistors /?/, R2 weakens the signal from the sensor /?meas by about 10 times. Capacitor C J reduces RF interference. The resistance of resistor R2 is selected according to the MK datasheet (in this case for AVR controllers) in terms of the optimal operating mode of the ADC. Resistors RJ, /?meas ^^ sum should have a resistance an order of magnitude greater than resistor R2;

c) resistor R3 regulates the sensitivity of the current sensor, made on a powerful wirewound resistor /?meas - Chain R4, C J reduces interference and protects the MK from voltage surges;

d) an example of a symmetrical connection of the measuring circuit to the MK using identical resistors /?/, R2. Diodes VDJ, VD2 limit the input signal in amplitude. The voltage difference is measured by a two-channel ADC MK in differential mode;

Fig, 3.71. Schemes for connecting resistor current sensors to the MK (continued):

e) transistor VT1 opens at a certain current flowing through the resistor /?iz’ after which a HIGH level is formed at the input MK. If the voltage in the measured circuit does not exceed +5 V, then the limiting resistor R2 can be replaced with a jumper;

e) sensor for excess current through a resistor /?izm with an indicator on the LED NI\

g) The MK checks whether the motor L// is currently running by the presence of voltage on the low-resistance resistor RL. The circuit has a lower threshold determined by voltage (/^e VT1\

h) current pulses flow through the motor Ml, which periodically open the transistor VT1. Due to the large capacity of capacitor C2, a LOW level is maintained at the MK input, which turns to a HIGH level when the engine stops;

i) bipolar current sensor. Transistor VTL1 works as a diode, VTL2 as a switch. Both transistors are included in the same assembly and have identical parameters, hence the high temperature stability. Optional diodes VD1, KSh protect transistors from overloads;

Rice. 3.71. Schemes for connecting resistor current sensors to the MK (continued): j) symmetrical reading of information from the current sensor /?meas - Voltage can be supplied from the MK output of the same name. Resistor /?J serves for initial calibration of readings;

l) the voltage at the MC input is proportional to the current in the measured circuit with the coefficient “1 V/1 A”. The supply voltage at pin 8 of the D/1/chip should be +5…+30 V;

m) DAI is a weak signal amplifier with sensitivity adjustment by resistor R4. Resistors /?/, /?2 must be the same in resistance;

n) resistor R2 sets the response threshold of the current sensor. The VDI zener diode protects comparator DA1 from voltage surges;

o) the signal and protective “grounds” are electrically connected by long wires, so filter capacitors C/…CJ are introduced into the input circuits of the amplifier?14/. The MK is connected to the signal ground, and a resistor /?iz’ ® is connected to the protective ground

Rice. 3.71. Connection diagrams for resistor current sensors MK. (end): p) the DA J microcircuit (Zetex Semiconductors) allows you to measure the absolute value of the current (UiT pin) and its direction (FLAG pin). The voltage in the measured circuit at any of the resistor terminals /?measurement relative to the common wire of the MK should not exceed +20 V;

p) current measurement using a specialized DA chip! from Texas Instruments. The voltage in the measured circuit relative to the common wire of the MK should not exceed +36 V. The resistance of the resistor /? is selected so that the voltage drops across it at full current load is 50... 100 mV. Replacing the DA1 chip - INA193, INAt95, in this case it is necessary to adjust the conversion coefficient in the MK control program;

c) current measurement using instrumentation amplifier DA1 from Analog Devices. Capacitors C1...SZ eliminate high-frequency interference and, together with resistors R1, R2, balance the circuit.