An electrically isolated variable transformer is actually an obligatory piece of equipment in every electronics lab. However, many people are put off by the high price of such a device. In a hobby lab, a professional variable transformer is usually grossly over-dimensioned, so it’s possible to save a lot of money with a little bit of handiwork. All that you need is a transformer kit for the desired power level and a few electronics (Figure 1). This approach can be used to construct a binary-controlled transformer for ac voltages in the 1 to a 255-V range with 1-V steps. The working principle is based on a binary-valued series of eight isolated transformer secondaries, each having a voltage that is twice that of the previous one (1, 2, 4, 8, 16, 32, 64 and 128).
These can be combined to achieve the desired ac output voltage. In order to allow the output voltage to be continuously adjusted (to the extent that this can be said to be the case with 1-V steps), a rotary pulse generator connected to an up/down counter is used. The pulses from the generator are offset with respect to each other, which makes it possible to determine the direction of rotation. After passing through a network that suppresses contact bounces, the pulses are applied to the cascaded counters to cause them to count up or down.
The counter outputs energize a set of relays via the ULN2803 power driver, and the relay contacts tap off the corresponding voltages. The output voltage should be checked by connecting an ac voltmeter. Please observe the usual safety precautions for equipment carrying mains voltages when building this digital transformer.