Monitoring the current taken by a mains-powered appliance can be a challenge, particularly if the application demands an inexpensive solution that must provide galvanic isolation for user safety. Common solutions employ either a current-sense resistor or current-sense transformer to convert the line current to an ac voltage that’s then converted into a proportional dc voltage. The dc voltage may then be processed using various techniques to provide a direct indication of the ac current magnitude or to implement a monitoring function that can determine whether the current is above or below a certain threshold.
Current-sense resistors, however, can be problematic. When measuring large currents, the resistance value needs to be very low to avoid excessive power dissipation. This, in turn, often requires considerable gain to boost the sense voltage to a useful level. A simple example will illustrate this point.
Let’s say we need to measure a maximum current of 15 A. A 10-mO resistor connected in a Kelvin-type configuration would generate a sense voltage of 150 mV at 15 A. An amplifier with a gain of, say, 20 would be sufficient to boost this voltage to a useful level.
So far, so good. However, the 10-mO resistor would dissipate 2.25 W at 15 A. We could select a 3-W unit, readily available in surface-mount (SMT) or conventional packages, but the size and cost of a suitably rated unit may prove unacceptable. Furthermore, the heat generated by the resistor might well be a problem, particularly in small enclosures where it may be difficult to apply adequate cooling.
We could minimize the power dissipation by reducing the resistance by a factor of 10. The resulting 1-mO resistor would dissipate just 0.23 W at 15 A—much more manageable. However, we would now require a gain of around 200 to boost the 15-mV maximum sense voltage to a useful level. Even if we could arrive at a resistance value that was acceptable in terms of power dissipation, cost, and the associated amount of amplifier gain, we would still face a significant problem: the sense-resistor technique provides no inherent galvanic isolation whatsoever.
Current-sense transformers, on the other hand, provide the galvanic isolation necessary for operator safety. Generally, these components tend to be available in one of two types. The first consists of a primary winding (the current-sense winding) and a secondary winding, both wound on the same core—much like a conventional power transformer. The second features a secondary winding wound on a toroidal core, resulting in a completely sealed unit. The conductor carrying the current to be measured is passed through the center of this sealed unit, and it therefore functions as a single-turn primary.
In both cases, the turns ratio is usually large, often in the range of 1:50 to 1:1500, so that even relatively small primary currents can generate a large secondary voltage. This obviates the need for high gain amplification.
Current transformers are available to cover a wide range of primary currents, anything from a few amperes up to many hundreds of amperes. However, despite their evident advantages, particularly the inherent galvanic isolation, they’re often bulky and expensive, and certain types suffer from nonlinearities over a wide current range.
It should be clear by now that an “ideal” mains current-sensing solution would be small and inexpensive, and it would feature intrinsic isolation. Furthermore, it should introduce negligible voltage drop into the primary conductor and produce a linear response over the full current range, as well as have power dissipation at close to zero. In addition, it should be possible to fabricate the solution on a printedcircuit board (PCB) using conventional techniques, without the need for any bulky components.
HALL EFFECT
This leads us to the Hall Effect. Working at Johns Hopkins University, Baltimore, in 1879, Dr. Edwin Hall discovered that when a current-carrying conductor was placed in a magnetic field, a voltage proportional to the field was generated. This principle, known as the Hall Effect, is now widely used for sensing both static and alternating magnetic fields.
Furthermore, since a current-carrying conductor generates a magnetic field, a Hall sensor placed in the field can be used to generate a voltage that’s directly proportional to the external current. Combining the Hall sensor with the conductor in a single package results in a current sensor that can be used to measure dc or ac currents.
Allegro Microsystems’ ACS712, an example of this type of sensor, integrates a Hall Effect sensor and low-resistance current conductor in an SO8 package (Fig. 1). Operating on a nominal 5-V dc supply rail and able to sense ac or dc current, it provides 2.1-kV isolation between the sensor circuitry and the current conductor. The current flowing through the conductor generates a magnetic field that’s sensed by the integrated Hall IC and converted into a proportional voltage.
There are three variants of the ACS712, providing sensitivities from 66 mV/A to 185 mV/A with corresponding current ranges of ±30 A to ±5 A. The internal conductor resistance is typically just 1.2 mO, so power dissipation is little more than a watt at maximum current (30 A). Allegro produces a range of larger devices, such as the ACS754, that can handle currents up to 200 A.
Clearly, devices like the ACS712 offer an attractive solution to measuring ac mains current. But priced around $1.60 for large quantities, the ACS712 could prove too expensive for low-cost applications. Furthermore, although not excessive, the internal power dissipation may be troublesome at the top end of the sensed current range.
GOING LOOPY
Fortunately, there’s an alternative approach available, which again exploits the advantages provided by a Hall Effect sensor. In its basic form, the sensor is mounted on one side of a double-sided PCB and positioned to lie in the center of a loop of track on the other side of the board (Fig. 2).
The principle of this technique is simple: mains current flowing around the loop creates an alternating magnetic field that’s concentrated directly on the sensor. The looped track behaves like the U-shaped conductor shown in Figure 1. Since the low-voltage (LV) tracking to the sensor is located on top of the PCB and the hazardous mains tracking is on the bottom, the insulating PCB material itself provides the galvanic isolation required for safety.
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