AS9120B, ISO 9001:2015, and FAA AC 0056B ACCREDITED

What Is a Variable Auto Transformer?

Many electrical assemblies benefit from the assistance of a transformer, with variable auto transformers being a particularly effective option for applications that require a high level of control. Simply put, a transformer is a component which transfers electrical energy from one circuit to another, or to multiple circuits. Variable auto transformers accomplish this with the added capacity to adjust the amount of voltage and current output sent to a device, regardless of the input values. In addition, the “auto” prefix refers to the design of the device which uses a single coil to act as both the primary and secondary (high voltage and low voltage) winding. This characteristic comes with several benefits which make it especially useful in aircraft machinery.

How Do Variable Auto Transformers Work?

Variable auto transformers accept utility voltage and provide continuous adjustable output voltage to a connected device. They are able to achieve this because of the way in which they are constructed. A typical variable transformer consists of a copper winding on a doughnut-shaped core which is built from conductive materials. For the control element, a brush, usually made of carbon, rotates around the winding to take off voltage at any value from 0 V to the maximum output voltage. Just a turn of the dial on the device can operate the sliding brush to adjust the output voltage. Each of these mechanisms allow for current to pass through the machine to be converted to the necessary output voltage and current.

Common Uses of Variable Auto Transformers

The most common use for variable auto transformers is to connect systems operating at different voltage classes. To accomplish this, auto transformers come in two major types: step up and step down. Step up auto transformers are designed in such a way that the output voltage exceeds the input voltage and vice versa for its current. Step down auto transformers will do the opposite, releasing an output voltage that is less than the input voltage and an output current which is greater than the input current. Due to their high control and current carrying capacity, variable transformers are also often used to deliver standard electricity or test voltages from lines where voltages fluctuate above and below the normal standard. Due to these abilities, variable transformers are often used in welding equipment, testing instruments, rectifiers, servo stabilizers, battery chargers, and more.

Features of Variable Auto Transformers

Variable transformers have the benefit of being able to enhance an output voltage in excess of twice the input voltage. They can also obtain any desired voltage within the maximum range by adjusting the carbon brush. Apart from their standard transforming capabilities, variable auto transformers, in particular, are advantageous because they are often half the size of a two winding transformer. As a result of this smaller size, they use less materials which both lowers their overall cost and reduces ohmic and core loss.


In conclusion, variable auto transformers are vital instruments for controlling the output of a transferred current. Their weight reduction and capability to adjust voltage also make variable auto transformers a useful device on aircraft. For high quality aircraft equipment and parts like auto transformers, Aerospace and Defense Parts has all that you require. We are an AS9120B, ISO 9001:2015, and FAA AC 00-56B accredited enterprise serving those within the aviation and defense industries. Get started today by browsing the inventory of available parts on our website. For quotes on any of the parts we offer, simply send us a completed Request For Quote (RFQ) form, and a dedicated team member will get back to you with a quote fit to your specific needs in 15 minutes or less!


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