Control Engineering

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What’s the best way to sense electric current for control applications?
September 1, 2008

The best way to sense electric current depends on the size of the current and the impedance of the circuit it’s passing through. My three favorite methods are shunt resistors, op-amp current-to-voltage convertors (CVCs), and Hall effect current sensors. All are ideal in their application “sweet spots” and can be problematical otherwise.

A shunt resistor is the simplest method for measuring current. It is simply a small-value resistor connected in series with the load. Typically, they are installed in the low-voltage side of the load where it connects to ground or neutral so that measuring equipment and personnel are not exposed to high voltage. In most applications, the resistor value is well under an Ohm. 

Shunt resistors use a 4-wire connection, where the load current flows through the shunt resistor by the most direct path, with additional connections made across the resistor for signal leads. Source: Empro Mfg. Co., Inc.

The problem is that the added resistance affects the current in the circuit and burns power. In the high-current circuits where shunt resistance measurements are most effective, these problems are the worst. Electric motor currents, for example, are very high and their load impedances are very low, making additional series resistance a big problem. Lowering the shunt resistance to ameliorate the problems, lowers the measurement voltage, introducing different problems. 

Sensing current with a shunt resistor affects the current being measured.

My predilection for op-amp circuits makes the current-to-voltage converter my favorite means of measuring current. The neat thing about this circuit is that it leaves the current to be measured virtually undisturbed. The transfer function sensitivity equals the feedback resistor’s value. 

Current-to-voltage converters based on op-amps sense moderate currents without disturbing the circuit being monitored.

Op-amps are designed with high input impedances and enormous open-loop gains. All the “action” happens by virtue of feedback loops. The feedback loop feeds back to the input a current equal in magnitude and opposite in sense to the measured current. This nulls out any current flowing between the negative and positive inputs, making the voltage drop across the shunt resistor virtually zero. The only current flowing there is a small bias current, which is effectively nulled by making the resistor’s value equal to that of the feedback resistor.

The op-amp’s enormous open-loop gain makes the voltage difference between the + and – inputs virtually zero. Coupled with the effectively zero voltage drop across the shunt resistor, that virtually connects the neutral side of the load directly to ground. The limiting factor for this circuit is the op-amp’s ability to source the feedback current. The real current bypasses the op-amp input and sinks to ground through its output and power supply.

Hall sensors have no high-current restriction. The Hall-effect current sensor is a semiconductor device inserted into a bead of soft ferromagnetic or paramagnetic material. Current flowing through a wire threading the bead’s hole induces a magnetic field in the bead. The Hall sensor then puts out a voltage proportional to the field strength. The down side of Hall sensors is that they require support electronics. The Hall controller comprises a highly stable excitation current source along with an instrumentation amplifier to boost the signal strength to a usable level. The amplifier also helps isolate the sensor from external noise. 

Hall sensors can pick up a dc current’s magnetic field without disturbing the measured circuit.

There are, of course, other methods of measuring current, but I’ve found these to be the most effective ways to sense current for data acquisition and control applications.

Posted by Charlie Masi on September 1, 2008 | Comments (2)