Current Measurement Components

Current measurement is used to control various parameters, one of which is the power at the load. There are many reading elements for measuring the current through the load. Their choice is dictated by the needs of each specific device, as well as the magnitude of the measured current. We will discuss in this article three different types of reading components for measuring current.

1. Shunt Resistors
Shunts and shunt resistors are the simplest version of current-sensitive elements. It is only necessary to remember about the temperature coefficient of resistance (TKS) of the resistor and to avoid its heating. Recall the rule of thumb for choosing a current-sense resistor: its maximum allowable power must be at least twice the working dissipation power.

The change in temperature of the resistor depending on the amount of current flowing through it is directly proportional to the ratio of the nominal power to the dissipated.

When choosing a current-sensitive resistor, it is necessary to take into account the thermal resistance of its body. This parameter, which is the thermal resistance between the resistor and its external surface, is the main indicator that determines the temperature rise of the resistor. The table lists the thermal resistances of standard surface mount enclosures.
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Conductor width
When designing a printed circuit board, it is necessary that its copper conductors withstand the maximum current required for the device.
For each device, it is necessary to find a reasonable compromise between thickness, width of conductors and cost.

Topology
The length of the conductors between the current-measuring resistor and the measuring circuit should be as small as possible in order to reduce not only the resistance of the conductor, but also its parasitic capacitance and inductance, which can introduce an error in the readings of the rapidly changing current.

Connection of signal conductors to a current-sensing resistor
It is recommended to use a current sensing resistor with four
conclusions. If for any reason two-pin resistors are used, the signal bus should be under the current-sensing resistor at the point where it is connected to the pad of the printed circuit board.

In many cases, the width of current-sensing resistors is less than the width of current-carrying buses. The conductors are connected to these tires at an angle of 45 ° to ensure a uniform flow of current.

Magnetic interference
The magnitude of the magnetic field generated by the conductor is directly proportional to the current through the conductor and inversely proportional to the distance to the measurement point. It must be remembered that signal conductors with high impedance should not be parallel to conductors with high current. Avoid the intersection of conductors with large currents. If this is not possible for any reason, it is recommended that these conductors be perpendicular to each other and cross the layer that is farthest from the signal conductor in order to limit the effect of interference.

2. Schemes with resistances

Consider the design of current-sensitive circuits with active resistances (direct current resistance, DCR), which do not cause additional losses on the measuring chain.



As a rule, circuits with active resistances are used in low-voltage devices, in which the voltage drop across the current sense resistor is a significant fraction of the value of the supply voltage applied to the load.
The current measurement circuit with active resistance is an alternative to current-sensing resistors. It uses the parasitic resistance of the inductor to measure the load current. This circuit remotely measures the current through the choke of the regulator's impulse circuit. Due to the absence of components installed in series with the regulator on the load, the circuit operates without loss.
A properly matched DCR circuit has an effective impedance on the ADC side equal to the inductor resistance. The figure shows a simple circuit with active resistance for measuring the load current of a step-down pulse converter.

Designing a DCR circuit that is not equipped with an adjustment function increases the measurement error by up to 35%, which is associated with the spread of inductance and capacitance values ​​in this circuit. In some cases, the measurement error may increase up to 50%. But using a simple leveling circuit with non-volatile digital potentiometers (digital potentiometers, DCP) significantly improves the accuracy of current measurement.

So, DCR-schemes do not introduce losses and take up little space on the printed circuit board. Since these solutions require adjustment for proper functioning, additional measures are needed in the manufacture of devices based on them. Large tolerances for deviations of reactive component values ​​can lead to a large scatter of values ​​between the effective resistances of the circuits. High temperature coefficients of inductors and capacitors increase the error of the circuit. In general, the architecture of a circuit with active resistance can be considered good for measuring large currents.

3. Hall Sensors
Consider the Hall sensors. Typically, these sensors, designed for high-current devices, determine the current through a conductor by measuring the induction of its magnetic field. Since current measurement is carried out remotely, it is believed that Hall sensors work without loss. These devices are intended for systems with a current higher than 200 A, since the power dissipated by the current sense resistor is large enough.



The figure illustrates the basic concept of the Hall-based current measurement method. In this scheme, the current through the conductor is determined by measuring the induction of the magnetic field B generated by it. The magnitude of the field is directly proportional to the flowing current and is determined by its direction.
Linear Hall Effect Sensors are active circuits that consume 3-10 mA of current. The noise level of these sensors is about 25 mV, or 5 Gs. These devices are not suitable for devices with low currents or large distances between the conductor and the sensor due to the large noise and current consumption.

The conditions under which the signal conductor and the sensor are operated should be taken into account when measuring weak magnetic fields. Linear Hall sensors measure the total magnetic field at the location of the sensor itself. Conductors with a current located near the sensor change the magnitude of the measured magnetic field, worsening the accuracy of the readings. The sensor also reacts to other external magnetic fields that occur when the engine or any other energy-generating device is switched.

To limit the influence of external magnetic fields on the sensors, a magnetic screen is used, which surrounds the conductor with current. The figure shows an example of using a metal casing (Faraday cage), shielding a conductor and a sensor.

Recently, Hall sensors with an integrated conductive channel, a compensation circuit and a protective screen appeared on the market. Integration of the conductive channel into the sensor facilitates the calculation of the output signal as a function of the current through the conductor. The single-chip solution simplifies the design of the device and the development of a current measurement application using a Hall sensor.

Despite the fact that lately the design of the Hall effect sensors has been improved, their accuracy and protection from interference have increased, the application of this technology is limited to high-current devices. Hall sensors dissipate less power than shunt resistors.

findings

Shunt resistors are the most common current-sensing elements due to the simplicity of the circuit design and its cost, as well as measurement accuracy. DCR circuits are designed for devices with pulse controllers and low regulated output voltages due to remote current measurement. Finally, Hall sensors are designed for high-current devices, since the power dissipated by them is less than that of solutions based on shunt resistors.

Each of the three solutions considered has its own advantages and disadvantages. Due to the fact that shunt resistors dissipate power, the energy efficiency of solutions based on these components is relatively small. In addition, in devices with low voltage, the magnitude of the voltage drop across the current-sense resistor may be commensurate with the operating voltage, which is unacceptable. The operation of the circuit using active resistance (DCR) depends on the matching of the capacitor and the inductor. Both components have high tolerances and high temperature coefficients. The Hall sensor is susceptible to ambient noise, and its use is complicated by the drawbacks of the circuit. Despite the improvement of this technology, still the limiting factor in the way of its application remains the accuracy of measurements.