Autoformer / Auto Transformer

An auto transformer or autoformer is a special type of transformer that employs a single non-isolated winding to step up or step down the available line voltage to match the requirements of the load.

It forgoes the electrical isolation provided by a multi-winding transformer in exchange for smaller size, lower cost, improved efficiency, and better voltage regulation. This can be accomplished because only a portion of the power is being “transformed”.

A typical application might be to adapt a piece of equipment manufactured in one country to operate in another where the line voltage is different. For this type of application, it is acceptable to use a non-isolated transformer because the output is supplying a piece of equipment that would otherwise be connected directly to the line.

The degree to which an autoformer can reduce the size and cost is determined by the difference in the input and output voltages. This ratio can be calculated by dividing the difference between the input and output voltages by the higher of the two voltages.

Consider a piece of equipment designed to operate at 120 V that is to be installed where the available line voltage is 240 V. If the machine draws 10 A, we need a transformer capable of suppling 1200 VA (the product of the output voltage multiplied by the output current).

If we elect to use an autoformer in place of an isolation transformer, the actual physical size of the transformer can be reduced by ½! (240-120)/240 = ½! If we take that same machine, ship it to Japan and need to operate it from a 100 V source, the size of an autoformer becomes even smaller (120-100)/120 = 1/6 the size of an equivalently rated isolation transformer!

In summary, an autoformer can be a smart choice if the difference between the input and output voltages is small, and isolation between the input and output is not required. Foster offers a standard 1500 VA autoformer that can be used to adapt equipment to virtually any available line voltage. If your needs are different, please contact the factory.

Standard Autoformer

  • UL / C-UL Recognized. File: E19598
  • TUV Certified to EN60950-1. Certificate: R72132324.
  • UL Recognized Class 180 (H) Insulation System.
  • Rated Output: 115 V @ 1500 VA
  • Input Voltages Taps: 100 V, 115 V, 127 V, 200 V, 220 V, 230 V, and 240 V, 50/60 Hz.
  • Can be wired in parallel for increased capacity.
  • Optional resettable thermal overload to protect against overheating

The Multi-Tapped Autoformer is available for sale online.

Ideal for use in virtually all types of equipment including office automation, medical and dental, commercial appliance, vending and other OEM equipment. This transformer is specifically designed to allow equipment originally configured to operate from an 120 V source to be operated virtually anywhere in the world.

Standard termination is through a common AMP Universal Mate-N-Lok connector allowing a single connection point and pre-termination of mating connectors to minimize wiring errors. Optional thermal overload protection guards against overheating and multiple units can be wired in parallel for additional capacity.

SELV Isolating Transformers
SELV Isolating Transformers
SELV Isolating Transformers
Wire Color Pin Connections Voltage
White 6 & 8 COM
Black 7 & 9 100V
Blue 10 & 12 115V
Gray 5 127V
Yellow 4 200V
Orange 3 220V
Red 2 230V
Brown 1 240V


  1. Autoformers can be used in virtually any piece of electrical equipment.

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