Tag Archives: ohm’s law

Measuring Very High Currents

High currents, such as 500 Amps and above, are common nowadays. Industries regularly use equipment consuming high currents, and vehicle batteries experience very high currents for a short time during starting. With vehicles increasingly trending towards their electric versions, it is becoming necessary to be able to measure high currents with precision. Increasingly, it is increasingly becoming critical to monitor current consumed accurately for ensuring performance and long-term reliability.

Current sensing is necessary for essential operations such as battery monitoring, DC to DC converters, motor control, and so on. Device specification mainly defines the performance of any current sensor solution. This includes the efficiency, precision, linearity, bandwidth, or accuracy. However, for designers designing a system to satisfy all the requirements of the specifications can be a challenging task. One way to do this is to use a precision shunt, a shunt monitor, and a signal conditioner.

The ±500 A precision shunt-based current sensor design from Texas Instruments (TI) has an accuracy of 0.2% of full scale reading (FSR) over a huge temperature range of -40 to +125°C. Several applications such as motors and battery management systems require such precision current sensing. In general, these applications suffer from poor accuracy caused by shunt tolerances, temperature drift, and non-linearity. Shunt monitors such as INA240, and signal conditioners such as PGA400-Q1 from TI help to solve these problems.

The design from TI works on 48 V or 12 V battery management systems and is suitable for measuring ±500 A, with both high- and low-side current sensing. It accurately compensates for temperature and non-linearity to the second order with an algorithm. Furthermore, it offers protection against harness faults such as input/out signal protection, reverse polarity, and overvoltage. TI has protected its design from electromagnetic interference.

Several ways of measuring currents are available, and these include using magnetic saturation, magneto-resistance methods, Lorentz force law, Ohm’s law, Faraday’s induction law, and more. While each technology presents its own advantages and disadvantages, every customer has his or her own preference and place of use for the specific topology they prefer to choose.

The simplest and most common technology makes use of Ohm’s law, which this TI design also uses. When designing the system for measuring currents, essentially the designer must choose where the current is to be measured—high side or low side, the range of measurement, and whether the current is uni-polar or bi-directional. These parameters define the suitable topology and the design the designer must use. Most vehicle systems now prefer to use 48 V and this new trend implies the current sensor will have to measure a large span of range.

The method of measurement follows a simple process. The ±500 A current passes through the shunt whose resistance measure 100 µΩ. This causes a noticeable amount of voltage drop across the shunt. The current sense monitor INA240 measures this small amount of voltage and passes it on to the signal conditioner, PGA400-Q1. The delta-sigma ADC micro-controller inside PGA400-Q1 creates a ratio-metric voltage between 0.5 and 4.5 V using its linearity and compensation algorithms.

How do AC Current Sensors Work?

You can sense current using a series resistor and measuring the voltage drop across it. According to Ohm’s law, the current through the resistor is then the voltage drop divided by the resistance value. That makes the voltage drop proportional to the value of the resistance and the current flowing through it. The disadvantage is obvious – to prevent the voltage drop from affecting the circuit parameters, one needs a very low value resistor when the current involved is high. Additionally, as the current reduces, so does the voltage drop. That involves amplification of the voltage drop, creating additional circuit complexity.

Ideally, current sensors should not use any power when detecting the current in the circuit. However, real current sensors do require a part of the energy from the circuit for providing the information. For sensing AC currents, current sense transformers are typically useful. A single wire from the circuit acts as the primary of the transformer or the primary may be a single turn winding on the transformer.

The AC current sense transformer develops a current in the secondary, proportional to the sensed primary current. The secondary current is allowed to flow through the terminating resistor to produce an output voltage. As the turns ratio of the transformer decides the secondary current, a low turns ratio (pri/sec << 1) minimizes the current through the terminating resistor. A balance of the transformer ratio and low-enough current through the terminating resistor ensures adequate output voltage. You select the appropriate AC current sensor based on the frequency range and current rating of the sensor for the conditions of your application. The highest flux density to prevent saturation of the sensor core will then depend on the worst-case current and frequency conditions in the circuit. The requirement is to generate a voltage output from the sensor that will vary linearly with the current being sensed. If the core saturates, the output becomes non-linear, and the output voltage is no longer strictly a representative of the input current. Sensors come in surface mount or through hole types, with different turns ration and overall dimensions. As noted earlier, you can have a sensor only type, which has a conductor integral to the application serving as the primary. The other is a current transformer type, where the primary is an included winding. Current transformer manufacturers offer online selection tools for selecting the right current sensor for the specific application. Initially, the user selects either an SMT sensor or a leaded type of sensor. The tool then requires the user to input the maximum sensed current expected, the input frequency, the duty cycle of the primary current waveform and the desired output voltage. The output voltage being the desired output voltage for the maximum input current the user expects. Based on the maximum input current, the number of secondary turns and the output voltage necessary, the tool suggests the required terminating resistor value. For this calculation, the tool assumes a single-turn primary. The tool also provides the maximum flux density based on the above parameters and the maximum operating frequency, making sure the value does not exceed 2K Gauss to ensure linearity.