Electronic Design

Build Automatic Tap Changer

The automatic tap changer comes in handy when automatically adjusting for large line variations seen from poorly regulated line power. It's also useful when automatically adjusting for small differences in line power ranges that occur from country to country (100/115 V ac or 200/230 V ac, Japan/U.S.). The circuit discussed here is intended for new equipment possessing electromechanical components (ac motors, high CFM fans), as well as linear or unregulated power supplies. It can also be packaged separately and used as a line conditioner on to existing equipment to improve performance and life.

An automatic tap changer can be particularly effective when it comes to ac fans. Some ac fan manufacturers report that operating a fan more than 5% above its nominal ac rating can decrease fan life as much as 50%. Running an ac fan too far below its nominal rating can significantly reduce its CFM output, causing unnecessary heating in the components that it's cooling. With this circuit, however, line variations to the load are cut in half, optimizing life and performance (see the figure). The device reduces the heat dissipation on the series pass element of linear supplies. And in some less demanding applications, with the regulation the tap changer provides, a series regulator can be eliminated.

Simplicity defines the circuit: Its fundamental components are two electronic ac switches and a control circuit that decides when a tap change is necessary (see the figure, again). Each ac switch consists of two VMOS transistors connected in series, with their sources forming a common node. The control circuit is powered from the ac voltage difference that's developed between the two tap positions. This ac voltage is rectified by the intrinsic diodes present in each VMOS transistor pair that form each switch. That's one reason why one switch uses a p-channel pair and the other employs an n-channel pair. The rectified voltage is filtered by C1. Because C1's dc voltage level is proportional to the input line voltage, it's used to determine the input line voltage threshold at which to change taps.

The LM358 in the circuit has two op amps in one package. One op amp acts as a comparator, the other as an inverter. R1 and R2 along with CR1 determine the line voltage threshold to change taps. C3 and R3 are used for the hysteresis that's necessary to keep the circuit from continuously changing taps when the input line is at the threshold voltage. The device's auto-transformer was built by properly phasing an isolation transformer. An auto-transformer with a 10% tap is quite small when compared to the power it's supplying (a 10-VA transformer can be used for a 100-VA load).

Although the circuit shown has two discrete tap ranges, it can be extended to include more taps. Another variation could have a smoothing inductor and high-frequency (PWM style) operation to maintain a well-regulated output voltage.

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