By: Lj Ristic, Ted Stokes and Maziar Amirani, Crocus Technology
This Design Note from Crocus Technology explains how MLU magnetic sensors can be used in industrial applications to measure currents from several amperes to several hundred amperes.
It is well known that a simple technique for measuring the current flowing in a conductor is based on Faraday’s Law of induction. Typically it involves placing a coil around the conductor, thereby the current flowing in a conductor produces an output equivalent to the rate of change of current. Integrating this output produces a voltage proportional to the current, which can then be monitored using an instrument such as an oscilloscope. This approach is non-invasive and it does not require direct electrical connection. Since the coil is isolated from the current in the conductor being measured, the method is also safe for measuring high currents. However it also has a drawback – the coil can only generate a response when an alternating current is measured.
Luckily, this limitation can be overcome by using MLU magnetic sensor from Crocus Technology that offers capability of measuring both DC and AC currents in the wide dynamic range from several amperes to thousand amperes in many of industrial applications.
Crocus has designed a closed loop solution based on the MLU magnetic sensor to accommodate measuring high currents in a wide dynamic range. The solution includes two CTSR218 current sensors that are connected in differential mode. The sensors are physically mounted on the PCB under a vertically adjustable bus bar that is used to pass high currents. The busbar can adjust distance from sensors; this way the solution span from several amperes to a thousand amperes. When AC or DC current passes through the busbar, the sensors detect the magnetic field generated by current and provide an output voltage that is directly proportional to the current. Figure 1 shows the physical setup for this solution.
The CTSR218 sensors are connected in a half bridge configuration (differential mode) and they are part of a closed-loop solution shown in Figure 2. The output voltage of the half bridge is used in the circuit to provide feedback currents to the input of sensors that set biasing point. The closed-loop circuit is designed to provide a feedback path for the bias currents for both CTSR218 sensors so that both sensors are kept at the same bias point. This is accomplished by first sensing the output voltage of the half bridge circuit and then zeroing the sensors’ output by changing the input current of the sensors. The feedback in the field line current is used to essentially cancel the effects of the external magnetic field and keep the sensors in the set bias point.
By way of circuit analysis, two CTSR218 sensors comprise a voltage divider (Sensor1 and Sensor2) biased by the sensor supply (VDD). In an initial state of no current on the busbar and no external magnetic field, the sensor supply voltage is split by the differential sensor half bridge circuit. This value changes as external current (external magnetic field) flows in the bus bar. Due to the opposite placement-orientation of the sensors with respect to the busbar, changes of external magnetic field will have opposite effect on each of the two sensors – one will increase in resistance while the other will decrease. Therefore, to zero the output of each sensor under the effect of external magnetic field, the feedback currents will need to act opposite, one to decrease and the other to increase. For this reason, output of the half bridge is first buffered via an Op-Amp and then compared to a VDD/2 and then amplified. The output of the amplifier is then fed back into the sensor input in order to keep the sensors in their linear region. The output signal of the comparing circuit is the signal out of the closed-loop circuit.
High Currents Sensing Using Close-Loop Solution Based on MLU Magnetic Sensor
The evaluation board for closed-loop solution for high current measurements is shown in Figure 3. High dynamic range from several amperes to thousand amperes is achieved by adjusting the distance of the busbar from the two sensors. The higher the current to be measured, the further away the busbar is placed from the sensors.
The output signal of the closed-loop solution is shown in Figure 4. In this example, the measured current spans from 1A to 200A. One can see the excellent linearity of this solution with the non-linear error below 0.5%. To measure higher currents one just needs to place the busbar further away from sensors.
The Crocus sensor CTSR218 is an excellent choice for high current measurements. Table 1 shows major parameters for the CTSR218 part suggested for high current industrial applications.
Table 1 : Main parameters of the CTSR218 high-sensitivity MLU magnetic sensors
|Output Resistance, RO||18kΩ|
|Input Resistance, RIN||30Ω|
|Voltage Supply (Typical)||5V|
|Input Bias Current||10mA|