Adjustable Power Supply

An adjustable power supply refers to any power supply unit where the output voltage may be adjusted. Some units use AC current as input, other start with DC power.

This article will focus on DC output.

Most adjustable power supply units include a regulator to keep the voltage steady despite changes in the load.

Here's an example: an electric motor pulls a chain over a pulley to hoist a loaded bucket. There is little load while the chain is slack; when the bucket is lifted, the load on the motor increases.

This article will ignore non-regulated power supply units.

There are two kinds of regulators: linear and switching.

Linear Regulators

Linear regulators adjust an internal resistance to keep the output voltage steady.
 
If the unit draws from a DC source, then either the maximum output voltage will be 2 volts less than the input, or a special low-dropout regulator is required.

Linear regulators are simpler and less expensive than switching regulators, but less power-efficient under most circumstances.

Switching Regulators

Also known as a switched-mode power supply, or SMPS, this regulator turns the current flow on and off at a very high frequency to provide a steady output. The longer the current flows, the higher the power output.

Switching regulators are smaller and lighter than linear regulators of the same power capacity.

An SMPS may produce high-frequency interference, although this can be avoided by adding more complexity to its internal circuitry. As well, the DC output may be "noisy" - with higher or lower voltage than desired - unless more circuitry is added to smooth the output.

An SMPS may also fail, or even damage itself, if the load is dropped from the output. Again, there are ways to prevent these problems.

A switching regulator is able to produce a higher DC output voltage than its input DC voltage; this is beyond the capabilities of a linear regulator.

More Concerning Regulators

Both linear and SMPS regulators can be adjustable.

Hybrid regulators start with an efficient SMPS that produces somewhat noisy DC output. A linear regulator takes in the noisy DC, and outputs a smoother DC at a lower voltage. In this case, the linear regulator would be adjustable.

Uses for an Adjustable Power Supply

Why would one purchase an adjustable DC power supply? It may be more cost-effective than having a collection of single-voltage power supplies.

Electronics hobbyists who build different projects may require different DC voltages.

Manufacturers - especially in their product design and engineering departments - as well as science laboratories use high-quality units.

Industrial testing laboratories also use these power supplies, to determine either the operational boundaries or the ideal voltage values for equipment they are testing.

Hobbyists may build their own adjustable power supply. There are many schematics or plans available.

Commercially-available units may include LED readouts of the output voltage and current.

While most units are adjusted by hand, some are built to be remote-controlled by computer.

Summary of Features

The two primary features are the range of output voltages and currents.

Other features include: how smooth the output is; self protection from over-voltage on the input side, or short circuits on the output side; heat dissipation; efficiency; and expected lifetime.

From a hobbyist's basement to an engineering workbench, an adjustable power supply is a versatile workhorse.