Voltage Regulator

A voltage regulator can be used to clean-up voltage, regulate the speed of motor and to give great flexibility for the voltage coming out of your transformer. Look at TEMCo's guide for selecting a voltage regulator, and then call us to buy one today.

What is a voltage regulator?

A voltage regulator is another term for a variable transformer or variac, which will have a range of output voltages. Most often variacs will be autotransformers, but there are isolated variacs as well.

To make the secondary connection, the variable autotransformer uses a sliding brush, which makes a for easy control of the output voltage. Low voltage applications use autotransformers.

The autotransformer coil will be made of insulated magnet wire. The top of the coil will have insulation removed so the bare wire is exposed.

Isolated variacs will have a brush on the secondary coil, not on the primary.

How does a transformer work?

Transformers are designed to transfer energy from one circuit to another, through the coils of the transformer.

A transformer contains two coils: a primary and a secondary. The current in the primary coil creates a varying magnetic field in the transformer core. The current creates voltage when it is transferred to the secondary coil.

Transformers, in most cases, change the voltage running from a power source to a piece of equipment, though they can also be used to clean up inconsistent voltage.

Regular transformers contain taps, which are predetermined connection points along a transformer coil, which allow for a set amount of voltage.

Different Types of Transformers


Unlike regular transformers, autotransformers have one coil instead of two. The single coil will act as both the primary and secondary. Autotransformers will also have at least three taps.

Because they have fewer windings, and a smaller core, autotransformers weight less, are smaller and less expensive than standard transformers.

Isolation transformers:

An isolation transformer has two coils, one inside the other, which serves to buffer the voltage coming from the source and the voltage going to the device. Isolation provides safety from electric shock hazards and blocks interference caused by ground loops.

Isolation transformers can withstand higher voltages, because the wires are insulated. They are used to clean up messy voltage.

They can also allow for increased speed control, as rotations per minute (RPM) are determined by voltage.

Why would you choose a voltage regulator?

A voltage regulator allows for much higher flexibility in terms of the amount of induced voltage being transmitted between the coils.

A variac has what is, essentially, a tap that can go anywhere along the coil. This allows for the coil to produce any voltage in the range provided by the transformer.

Variacs can also be used for testing out old equipment that has not been used for a long period of time, such as radios and amplifier tubes. Using a variac to slowly raise the voltage in these cases, instead of running full voltage immediately, is recommended, as too much voltage at once may be too much for an old piece of equipment.

A large variety of applications can use a voltage regulator, such as light dimmers, resistive loads, heating elements, lab gear, switchgear. They can also be used to test the voltage capabilities of appliances.

Voltage Regulator Specifications

Constant Current Rating

Variable transformers have a constant current rating. In most cases, the current and voltage have an inverse relationship: as voltage goes down, current goes up, and vice versa. But because current is constant in the case of variacs, when the voltage changes, instead of the current changing, the power consumption will change by the same percentage as the voltage.

You should know the power consumption, or wattage, of your transformer. If your variac exceeds the voltage of your load, please be advised that your amp draw will also begin to increase. Exceeding the maximum load-rating of the transformer may lead to problems.

Phase - single or three phase

Voltage regulator transformers will be come with single phase or three phase power.

The difference between the two phases is the environment in which the transformer will be used. Three phase power is found in industrial areas that run heavy equipment, while single phase power is typically used in residential areas.

A three phase variac is actually three single phase variacs tied together in either a Y connection, or a delta connection, which looks like a triangle.

Input and output voltage

Input voltage comes from the power source; output voltage goes into the connected device.


Transformers have fixed frequencies, so they will not be changed along with the voltage.

A variac typically has a frequency of 50, 60, 50/60 or 400 Hz.

A 50 Hz transformer will be able to operate on 60 Hz power, but a 60 Hz transformer cannot be used on 50 Hz power.

Constant impedance rating

When voltage is applied, the current will experience impedance, which is the opposition to the current provided by the circuit.

The ratio of output impedance to input impedance is the same as the ratio of output voltage to input voltage squared. If the impedance ratio is changed,then the voltage ratio will also change, but only by a small amount.

The constant impedance rating is the current that can be carried when the current is proportional to the voltage output.

Ambient Temperature

A variac has an ambient temperature rating, which indicates that the surrounding area must not top a particular temperature. If the ambient temperature is exceeded, the current has to be reduced.


There are two types of voltage regulators: bench and open frame.

Bench models have a metal shield that covers the transformer from outside contamination. They can have an on/off switch, a fuse, an AC outlets, a line cord and a voltage meter.

Open frame models do not have the metal shield. They are meant to be mounted or incorporated into another piece of equipment, but otherwise work the same as bench models.

Some variacs will come with a voltmeter and/or an ammeter. A voltmeter measures the potential in electrical difference between two points in a circuit. An ammeter measures the current in a circuit.

Bench Model Voltage Regulator

External Devices

A voltage regulator can come with additional features, like an on/off switch and an input plug, which portable a voltage regulator would most likely have.

Voltage Regulator
Variac Models and Pricing

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