Power Supplies

Rectifying circuits

Silicon diodes are used as the rectifying elements. Fig 6.1 shows the half-wave rectifying circuit in which current flows in the transformer secondary circuit (ie load, rectifier and secondary winding) only during the positive half-cycle. If the diode is reversed, then it is the negative half-cycle which is rectified.

FIg 6.1. Half-wave rectifier and waveforms

Fig 6.2 shows the full-wave rectifier circuit. The diodes conduct on alternate half-cycles, and so this is a combination of two half-wave circuits. The load current waveform varies considerably in amplitude but, as it does not change polarity, it is a direct current.

Fig 6.2. FulI-wave rectifier and waveforms

This variation in the amplitude of the output voltage (or current) of a rectifier circuit is known as the 'ripple'.

The frequency of the ripple in the full-wave arrangement is 100Hz, while in the half-wave circuit it is 50Hz.

Fig 6.3. Bridge rectifier circuit

Fig 6.3 shows the bridge rectifier circuit. The output is full-wave, ie 100Hz ripple. At any one instant two of the diodes are in series carrying current. The transformer secondary winding does not have a centre tap and is required to supply a voltage of Vac only (compare the full-wave arrangement of Fig 6.2 where the transformer supplies 2 x Vac).

The above animation demonstrates the process of transforming and rectifying AC mains, to raw DC.

The red arrows represent the direction of current flow during the positive part of the AC cycle, and the yellow during the negative part in the animation.

The AC mains is across the primary of the transformer (left), which is then 'transformed' to a higher or lower voltage on the secondary on the right. The actual AC voltage output at the secondary, depends on the turns ratio between the primary and secondary windings. Twice as many turns on the primary as the secondary, will result in the input voltage being divided by 2. 240v input (primary) will therefore produce 120v output at the secondary.

The waveform on the left, is one which appears on both the primary, and the secondary of the transformer.

The bridge rectifier then changes the AC output of the secondary, into a raw DC voltage. Raw DC, is simply an AC voltage which has had all the negative excursions of voltage removed, or changed by rectification into positive ones. This can be seen in the waveform on the right, which is that which would be found across the load (R_L). If the frequency of the supply is assumed to be 50Hz, this would produce raw DC at a frequency of 100Hz on the right.

Raw DC is generally of little use and will usually be converted into smooth DC voltage by the addition of smoothing circuits (see the following pages). To provide an accurate DC voltage, independent of load, a DC regulator circuit may also be employed.