HOW DOES A POWER SUPPLY WORK

This article describes in (hopefully) straightforward terms the operation of a basic power supply unit of the type used with telecommunications equipment (switchboards, house exchange systems, plansets etc).

The term power supply is more commonly abbreviated to PSU, this will be used from hereon in.

Telecommunications equipment is designed to operate on voltages lower than the domestic Mains voltage. In order to reduce this voltage a PSU is used.

To provide a useable low voltage the PSU needs to do a number of things:-

• Reduce the Mains AC (Alternating current) voltage to a lower level.

• Convert this lower voltage from AC to DC (Direct current)

• Regulate the DC output to compensate for varying load (current demand)

• Provide protection against excessive input/output voltages.

Reduction of AC Mains
This is achieved by using a device known as a Transformer an electromagnetic device consisting of an ferrous iron core which has a large number of turns of wire wound around it, known as the Primary Winding

The ends of these turns of wire being connected to the input voltage (in this case Mains AC).

A second number of turns of wire are wound around the Primary Winding, this set being known as the Secondary Winding.

The difference between the number of turns provides us with a way of reducing (in our case) a high AC voltage to a lower one.

Conversion of AC to DC
To convert our now low AC voltage to DC we use a Rectifier Diode connected to the Secondary Winding.

This is a silicon diode, which has operation analogous to a bicycle tyre valve (as the valve only allows air to flow into the tyre, the diode only allows current to flow in one direction)

As our low AC voltage will be working at a frequency of 50Hz (Mains AC frequency) it is desirable to reduce the inherent hum on this to a lower level.

This is achieved by a technique known as Smoothing ('Ironing' out the bumps in the AC).

A simple way to reduce the hum is to use Full Wave Rectification.

Today this is usually done by four diodes in a bridge configuration known as a Bridge Rectifier. (This can be four individual diodes or a dedicated self contained package)

Regulation of Output Voltage
The Electrolytic Capacitor is a device capable of storing energy the amount of energy and the time it remains stored depending on the value.

In a simple PSU the easiest way to provide regulation to compensate for varying load conditions is to use a pair of relatively high value Electrolytic Capacitors.

Their values in this case being in the region of 470uF to 2000uF depending on the application and the amount of current required from the output of the unit.

One of these capacitors is connected across the DC output of the rectifier diode(s) or bridge, this capacitor also providing an extra degree of smoothing the output waveform.

The second capacitor is connected via a low value, medium to high wattage resistor, which assists in limiting the current demand.

Protection against excessive voltages
In a simple PSU the easiest way to do this is by providing fuses at the input to the transformer, generally in the live side of the mains supply, also at the DC outputs.

In the event of an excessive input voltage, or excessive current being drawn from the output, one of these fuses should normally blow protecting the PSU and the equipment connected to it.

The transformer may also be fitted with an internal or external thermal fuse, which will open if the transformer becomes hot due to the aforementioned conditions.

Other PSU's
There are lots of other types of PSU, some of which are much more complex in their design and operation and are beyond the scope of this overview.

Many thanks to Andrew Tebbutt for this article

Last revised: July 23, 2025