CODING INTERNATIONAL CO LTD

Current sensing is used to perform two essential circuit functions. First, it is used to measure “how much” current is flowing in a circuit, which may be used to make decisions about turning off peripheral loads to conserve power or to return operation to normal limits. A second function is to determine when it is a “too much” or a fault condition. If current exceeds safe limits, a software or hardware interlock condition is met and provides a signal to turn off the application, perhaps a motor in a stalled condition or short circuit. It is essential to choose the appropriate technology with the necessary robustness to properly withstand the extreme conditions that can exist during a fault.

A signal to indicate the “how much” condition and the “too much” condition is available in a variety of measurement methods:

Resistive (Direct)
a. Current Sense Resistors
b. Inductor DC resistance
Magnetic (Indirect)
a. Current Transformer
b. Rogowski Coil
c. Hall Effect Device
Transistor (Direct)
a. RDS(ON)
b. Ratio-metric
Each method has advantages for current measurement, but also comes with tradeoffs that can be critical to the end reliability of the application. They can also be classified into two main categories of measurement methods; direct or indirect. The direct method means that it is connected directly in the circuit being measured and that the measurement components are exposed to the line voltage, whereas the indirect method provides isolation that may be necessary for design safety.

Current Sense Resistor

The resistor is a direct method of current measurement that has the benefit of simplicity and linearity. The current sense resistor is placed in line with the current being measured and the resultant current flow causes a small amount of power to be converted into heat. This power conversion is what provides the voltage signal. Other than the favorable characteristics of simplicity and linearity, the current sense resistor is a cost-effective solution with stable Temperature Coefficient of Resistance (TCR) of < 100 ppm/°C or 0.01% /°C and does not suffer the potential of avalanche multiplication or thermal runaway. Additionally, the existence of low resistance (< 1 mΩ is available) metal alloy current sense products offer superior surge performance for reliable protection during short circuit and overcurrent events.

Inductor DC resistance

The DC resistance of an inductor can also be used to provide a resistive current measurement. This method is considered “lossless” because of the low resistance value of the copper, typically < 1 mΩ and because it is providing a secondary use of an existing component. In higher current applications; a 30 amp current would provide a 30 mV signal for a 1 mΩ resistance value. This method has two drawbacks; first copper has a high TCR (temperature coefficient of resistivity) of approximately 3900 ppm, which causes the resistance value to increase by 39% for a 100°C rise above room temperature. Because of this high TCR, the temperature must be monitored and compensated to provide an acceptable current measurement. The second drawback is the variance in the resistance of the copper due to dimensional changes that occur due to the conductor being wider or thinner from one lot to the next.

Current Transformer

A current transformer’s three key advantages are that it provides isolation from the line voltage, provides lossless current measurement, and the signal voltage can be large providing a measure of noise immunity. This indirect current measurement method requires a changing current, such as an AC, transient current, or switched DC; to provide a changing magnetic field that is magnetically coupled into the secondary windings (Fig. 1). The secondary measurement voltage can be scaled according to the turns ratio between the primary and secondary windings. This measurement method is considered “lossless” because the circuit current passes through the copper windings with very little resistive losses. However, a small amount of power is lost due to transformer losses from the burden resistor, core losses, and primary and secondary DC resistance.

Rogowski Coil

The Rogowski coil is similar to a current transformer in that a voltage is induced into a secondary coil that is proportional to the current flow through an isolated conductor. The exception is that the Rogowski coil, (Fig. 2), is an air core design as opposed to the current transformer that relies upon a high permeability core, such as a laminated steel, to magnetically couple to a secondary winding. The air core design has a lower inductance providing a faster signal response and very linear signal voltage. Because of its design, it is often used as a temporary current measurement method on existing wiring such as a handheld meter. This could be considered a lower cost alternate to the current transformer.

Hall Effect

When a current carrying conductor is placed in a magnetic field, as shown in Fig. 3, a difference in potential occurs perpendicular to the magnetic field and the direction of current flow. This potential is proportional to the magnitude of the current flow. When there is no magnetic field and current flow exists, then there is no difference in potential. However, when a magnetic field and current flow exists the charges interact with the magnetic field, causing the current distribution to change, which creates the Hall voltage.

The advantage of Hall effect devices is that they are capable of measuring large currents with low power dissipation. However, there are numerous drawbacks that can limit their use, including non-linear temperature drift requiring compensation, limited bandwidth, low range current detection requires a large offset voltage that can lead to error, susceptibility to external magnetic fields, and high cost.