Balancing Coil

A pair of coils is arranged in 90-degree angled vectors off the pivot point of a magnetically responsive display needle. The coil on the left (low) side of the display is angled so that if the display needle were to exactly align with it, this would represent the lowest gauge display reading. Similarly, the coil on the right (high) side of the display is angled so that if the display needle were to precisely align with it, this would represent the highest gauge display reading. When both coils are supplied with the same voltage and fully grounded, the high-side coil is designed to produce the stronger magnetic field, biasing the display needle toward it. Both coils are supplied with a nonvariable voltage value such as V-Bat or V-Ref. The low-side coil is direct grounded. This means that its electromagnetic field intensity is constant. While the high-side coil is provided with the same voltage as the low side, it is grounded by means of a variable resistor.

To explain the operation of a balancing coil gauge, the example we use is that of an NTC thermistor connected to the ground circuit of the high-side coil. When the NTC thermistor is cold, resistance in the high-side coil is high, choking down on current flow. The result is low electromagnetic field intensity in the high-side coil biasing the display needle to the low side of the display gauge. As the NTC thermistor is heated, its internal resistance goes down, allowing more current to flow through the high-side coil. More current flowing through the high-side coil intensifies its magnetic field, pulling the display needle toward it. At the highest expected temperature, the NTC thermistor produces almost no resistance on the ground side of the high-side coil. Because the high-side coil is designed to produce the stronger magnetic field, when the NTC internal resistance is lowest, the display needle is pulled fully over to the high side of the gauge.

Although we have used the example of an NTC thermistor here, balancing coil gauges can be used for analog display of a range of status signals. The only requirement is that a means of varying the resistance on one of the two coils is provided.

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