How to Design an AutoTransformer?


We have already received some good knowledge regarding transformers in a couple of previous articles here in Bright Hub. We have studied how an AC mains voltage applied to the primary winding of a transformer is magnetically induced to its secondary winding.

In this article we will learn how to design an autotransformer through simple manipulations of ordinary transformers and get them converted into auto transformers. These devices can be effectively used to step-up or step-down our AC mains voltage manually whenever required.

What are Auto Transformers

Basically, auto transformers consist of a long single winding over an iron core with a single input and multiple outputs. These outputs may be arranged in steps, starting from voltages less than the input AC mains voltage and ending with voltages higher than the input voltages. In more sophisticated auto transformers the output voltage can be varied continuously, right from zero to much higher voltages than the input supply voltage.

Understanding Auto Transformers through this simple Experiment

Autotransformer Schematic, Image

An ordinary 0-24 volts/ 5 amps transformer, can easily be converted into an auto transformer. As it should be able to handle power up to 100 watts, a 5 ampere current rating becomes imperative. As in the case of any other transformer, it will consist of a primary input and a secondary step down winding. Interesting things start to happen when one end of the primary winding is connected to one of the two ends of the secondary winding as shown in the diagram.

The procedure of conversion is as follows:

  • Take the help of the diagram and connect the lower end of the primary winding to the upper end of the secondary winding to form an auto transformer.
  • Apply AC mains voltage to this connection and to the free end of the primary winding.
  • Measure the voltage across the free ends of the two windings.
  • The voltage at this point will be less than the input AC mains voltage by 24 volts.
  • On the contrary, in the next auto transformer configuration, when both the lower ends of the transformer are joined and the above procedure followed, voltage will be higher than the input mains by 24 volts.
  • Finally by adding a voltmeter and a DPDT toggle switch, the input AC mains voltage can be monitored and changed as required.
  • Since the whole experiment is at DANGEROUS MAINS POTENTIAL extreme care should be taken while going through the procedure.

Principle of Operation

Again referring to the diagram, due to the inherent property of transformers, the instantaneous polarity (since the AC is changing its polarity 50 to 60 times per second) of the current flowing through the primary winding will just be the opposite to that of the secondary winding at that instant.

Therefore when the opposite ends of the two windings are connected, the secondary voltage gets deducted from the applied input voltage. When similar ends are joined, the magnitude of the secondary voltage gets added to the input mains voltage, because in this case the instantaneous currents in both the windings are in the same direction or in favor of each other.

If you are still confused regarding how to design an autotransformer, you simply need to stop thinking and start commenting (needs moderation and will take some time to appear).