To perform these types of measurements, a current sensor must be used. Physical quantities cannot be managed unless they can be measured. Let's delve into the behavior of current sensors.
Current sensor
A current sensor is a device that converts a current signal into another analyzable signal. The signal to be measured is called the "primary current," while the output signal is called the "secondary current or voltage." The latter signal is used by electronic boards, ADCs, and other analog instruments. Because different measurement techniques exist, and the primary current can vary depending on waveform, pulse type, isolation, and current intensity, a variety of current sensors are available on the market. As shown in Figure 1, the most common current sensors fall into two categories:
Based on the working principle of the "shunt", the first type applies Ohm's law (V = R × I).
The second type uses Ampere's law (I = ∮ H × ds) and uses a magnetic field to measure the current.
Figure 1: Different current measurement methods
Ohm's law applies to shunt measurements, with the formula V = R × I. In practice, a shunt is a robust resistor with a known ohmic value. When current flows through a shunt, the resulting voltage is proportional to that current. Using this principle, we can accurately obtain both AC and DC currents for relatively low currents. On the other hand, when the current rises and exceeds 100 A, excessive heat is generated, potentially rendering the measurement system ineffective and critical.
Hall effect current sensors can be used to overcome these limitations. Powering a Hall probe applies a magnetic field perpendicular to the surface and generates a voltage proportional to the magnetic field strength. The amount of current flowing through the conductor can then be calculated using Ampere's law. Hall current sensors use a magnetic core to concentrate the magnetic field in the air gap where the probe is located. The output voltage is proportional to the magnetic field, which in turn is proportional to the primary current. The performance of the current sensor depends on the performance of the open-loop Hall probe. To improve linearity and reduce drift due to temperature shifts, a closed-loop principle is implemented; another type of current sensor is represented by a magnetoresistor, where the value of the resistor varies proportionally to the magnetic field. These current sensors are generally more accurate than the Hall effect, but are limited in sensitivity due to the air gap. However, the device must ensure efficient and accurate measurements with very high detection quality, an extremely flat frequency response, and excellent DC stability. All of these characteristics can be found in Danisense's current sensors, a company that provides customized solutions to meet customers' exact needs.
Current measurement method
Current measurement can be performed using the LT6106 integrated circuit, a multi-functional amplifier for current sensing (see application diagram in Figure 2). Its features are highly regarded:
Maximum voltage offset: 250 µV
Maximum input bias current: 40 nA
The gain of the device can be set using only two resistors.
Accuracy: 1%
Maximum output current: 1 mA
PSRR: 106 dB
Its primary applications include automotive and industrial applications, battery monitoring, energy management, engine control, lighting monitoring, and overcurrent and fault detection. The device measures current by detecting the voltage across an external resistor (shunt resistor). Internal circuitry converts this voltage into an output current. The LT6106's extremely low supply current also makes it suitable for battery applications.
Figure 2: Example of load current using LT6106 device
Once you understand how it works, you can easily configure the design to meet all your needs using Ohm's law and the power equation. In the example wiring diagram configuration, the power supply is 30 V, supplying a 19-Ω load with a current of approximately 1.57 A. The 0.01-Ω shunt resistor does not affect the system's operation, and its power dissipation is approximately 25 mW. Configure the configuration resistor to make the amplifier's gain in units. In this case, the output voltage (absolute value) equals the current flowing through the load. The overall efficiency is very high, certainly exceeding 99.8%. Obviously, this solution cannot be used for very high currents.
Use a current sensor instead of a shunt.
Using a current sensor instead of a shunt has significant advantages. A shunt, while not an ideal component, contains an inductor (in series) and a capacitor (in parallel), as shown in the previous wiring diagram. Therefore, current measurements are needed not only in DC but also in AC, requiring more efficient methods to make better estimates across different frequencies and current ranges, thus achieving higher accuracy.
Current sensors offer the advantage of electrically isolating the primary and secondary circuits. This eliminates common-mode voltage ripple interference and significantly reduces noise on the primary current. Compared to shunts, current sensors provide a higher output signal and lower noise. The much lower insertion impedance reduces power consumption, undoubtedly improving the short-term and long-term stability of the system.
A simple example illustrates this idea: a shunt resistor providing 50 mV at 1.500 A consumes 75 W. In effect, its impedance is 0.000033 Ω. For high measurement stability and repeatability, the shunt needs a very low temperature coefficient. Sensitivity, measured in volts per ampere, will differ depending on whether the system is used at low or high currents. For example, the DL2000UB-10V current sensor has an effective insertion impedance of less than 0.5 µΩ and an effective power dissipation in the primary circuit of less than 1 W at 1.500 A, 100 times smaller than the previous shunt. Even with very low primary currents, the signal-to-noise ratio is very high. The table in Figure 3 shows an excerpt of the Danisense current sensors from the DS, DM, DL, and DQ series. These sensors are ultra-stable and highly accurate, and based on a closed-loop operating principle, they provide excellent linearity and stability over time.
Figure 3: DS, DM, DL and DQ series transducer models
Broadly speaking, certain models of transducer assemblies can be categorized as follows:
Sensors from 0 A to 600 A
DP50IP-B (72A)
DC200IF (300A)
DS400ID (600 A)
DS50UB-1V (150 A)
Sensors ranging from 600 A to 3,000 A
DM1200ID (1.800 A)
DL2000ID (3.000 A)
DL2000ID-CB100 (3.000 A)
DL2000UB-10V (2.200 A)
Sensors with an amplitude greater than 3.000 A
DR5000IM (8.000 A)
DR10000IM (11,000 A)
DR5000UX-10V/7.500A (8.000A)
Some of these provide current output, while others provide voltage. One of the many interesting models is the DM1200UB-1V (Figure 4), an ultra-stable and high-precision sensor for measuring non-invasive and isolated DC and AC currents up to 1,800 A. Its 45 mm aperture diameter allows for the measurement of large insulated cables and high-precision leakage current measurements. With 15 ppm linearity and 10 ppm offset, its closed-loop technology enables highly accurate measurements. The housing is constructed entirely of aluminum to improve EMI shielding and allow for a wide operating temperature range. Its applications range from power measurement and analysis to the construction of stable power supplies, from particle accelerators to precision drives, from battery testing and evaluation systems to power system calibration. Here are some of the device's electrical characteristics:
Rated primary AC current: Maximum. 1.200 Weapon
Nominal primary DC current: Maximum 1,200 amps.
Measurement range: from –1.800 A to 1.800 A
Nominal voltage output: –1 V to 1 V
Primary/secondary ratio: 0.8333 V/kA
Linearity error: from –15 ppm to 15 ppm
Bandwidth (3 dB): 400 kHz
Amplitude error (10 Hz to 3 kHz): 0.01
Amplitude error (3 kHz to 50 kHz): 1
Amplitude error (50 kHz to 300 kHz): 20
Power supply voltage: ±14.25 V to ±15.75 V
Positive current consumption: 140 mA
Negative current consumption: 130 mA
Operating temperature range: –40˚C to 65˚C
Figure 4: DM1200UB-1V sensor
in conclusion
Today, all industrial systems have current sensors, which are irreplaceable in certain situations. Their main advantages are that they can be used and easily interpreted by industrial control systems, and are completely isolated from the circuitry and load. In fact, directly connecting a measurement system to the relevant circuitry is not always convenient. Their footprint is non-invasive; they are typically square or rectangular in shape, perhaps resembling a small speaker. However, their functionality is paramount. The engineering behind transducer operation is quite complex, but the operation and analysis of the output data are remarkably simple, which is the most important aspect.
