Variable Power Supply

A "variable power supply" is a power supply where the output voltage may be changed or adjusted.

This article will discuss only regulated DC variable power supply units. "Regulated" means that the output voltage is maintained despite changes in the load being driven.

Linear regulators and switched-mode power supply (SMPS) regulators have different characteristics as regulators. Used in series - SMPS and then linear - the complete unit is called a hybrid" regulator.

Applications for a Variable Power Supply

Low voltage DC - under 16 volts - is used in many electronic circuits. Your personal computer, for example, has 3.3, 5 and 12 volt circuits. Hobbyists build many electronics projects using low-voltage DC power. There are also commercial and industrial applications.

But your computer's power supply should be very consistent. The 5-volt output should always be 5 volts; your computer would not appreciate a fluctuation in the voltage.

A hobbyist may have many projects, however, and each with different power requirements. It is more cost-effective to have one device capable of supplying power during testing, than to have a lot of power supplies.

The hobbyist may also want to test a new circuit by supplying very low voltage to start, increasing the power gradually, and watching for smoke or sparks. This troubleshooting can save a project from a meltdown.

In testing laboratories or in industrial engineering, it is important to test prototypes at different power levels. This will reveal what voltage is ideal for the application, or the range of values that allow the equipment to operate at all.

Basic Parameters

Computer programmers say that input and output are important. This is true for a variable power supply unit, too. Do you plan to use your building's AC power? Or are you taking DC power from a battery or solar cell?

What range of outputs do you need? Along with the variable DC voltage, you also need to know the required wattage or amperage. (Watt's law states that Power = Voltage X Current, and we use the units Watts = Volts X Amperes).

Linear versus SMPS

This section compares regulators with the same input and output parameters.

Efficiency is important. An SMPS regulator will normally be more energy-efficient, smaller, and lighter than a linear regulator.

A linear regulator will probably cost less, and produce output that is less "noisy" (with fewer small spikes in the DC voltage).

If the initial power source is DC, then a linear regulator will produce a lower maximum output voltage - usually about 2 volts lower. An SMPS can produce a higher output voltage.

Simpler versions of an SMPS regulator may create high-frequency interference.

Additional Considerations

Noise reduction and reducing high-frequency interference may be important factors in choosing one commercial unit over another.

How does the unit dissipate heat? Low power units use free air convection. Medium or high-power units may require cooling fans. You don't want to burn yourself on a hot case.

Check whether the unit is advertised as having protection against voltage overloads (on the input side), and overloaded demand or short circuits on the output side.

Finally, some commercial units - such as the Sorensen XBT series of variable power supply equipment - may be controlled from a computer through a USB interface.