Two-winding transformers are often equiped with taps and switchgear to let them maintain a steady voltage. The problem with this is that when the input voltage to the autotransformer changes, so does the output. The solution was to control the actuation of the relays based on the voltage output so that it would dynamically adjust based on the output, esentially making a control system for the output voltage.

Iterating the Design: Electronics & Software

The hardware and software went through a few iterations. At first, a transformer and rectifier were used to step down the voltage and convert it to DC so the controller could read it but this proved to be sub-optimal because the resistors change in value as they heat up, throwing off the calibration. To solve this, a transducer was solved along with a low tolerance resistor.

The software was revised to include multiple menus by pressing a button. This allows the operator to control settings like tolerance of the desired value vs. the read value and reaction time for the realays. It also includes a calibration sequence that saves values into EEPROM to be accessed during the main routine.


After the hardware was agreed on a formal schematic was made to facilitate putting it together. As can be seen in the schematic, a mechanical encoder module that includes a push button was chosen to minimize the number of components and a capacitors and resistors were used to attenuate the bouncing noise.

A switching mode power supply was used to power the controller and power the pair or relays needed for the actuation of the motor.


A PCB was measured and cut to fit into a box where all the controller hardware would be housed. Holes were made for the wires, encoder and LCD and components were lined up. There was plenty of room left over.

Voltage Stabilizer

As can be seen in the video the autotransformer is now a voltage stabilizer. The voltage feedback loop allows it to turn the voltage up or down depending on the set point. A quick search for an autotransformer similar to this one priced at ~$800. A voltage stabilizer with similar characteristics to this one costs about $4,000.