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:-
Reduction of AC
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.
AC to DC
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
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)
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.
against excessive voltages
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.
Last revised: December 20, 2010