# Power Supplies

## Smoothing circuits

By adding a choke L, and a capacitor CS as in Fig 6.6, the fluctuations in Vdc, ie the ripple, can be greatly reduced or 'smoothed'. In fact the output of the rectifier circuit as shown in Fig 6.5 consists of a direct voltage with an alternating voltage, ie the ripple, superimposed upon it. Thus L, functions as a smoothing choke by opposing the alternating voltage and CS provides a low-impedance path to earth for this voltage.

Fig 6.6. Smoothing circuit

A low-value resistor can be used in place of the choke but is not so effective. In the high-current supplies often required by transistor circuits, a smoothing choke is unacceptably large and expensive. In this situation a single high-value capacitor (eg 68,000μF) would be used for smoothing.

Fig 6.7. Practical power supply circuit

A power supply may contain other components as in Fig 6.7. The primary circuit contains the mains switch and a fuse. A metallic screen, placed between the primary and secondary windings, is connected to the earth terminal. This screen helps to reduce mains-borne interference, and to protect the secondary winding from the voltage on the primary, should a short-circuit occur between them. This screen is generally made of thin copper (0.1mm thick) and of course is not continuous, otherwise it would act as a short-circuited winding. C1 may have a value of 1,000μF and C2 4,700μF. R1 will help with smoothing but must have a comparatively low value to avoid excessive voltage drop across it. The light-emitting diode indicates the ON condition. The resistor R2 will be 470Ω 1/4W. Note the symbol used in Fig 6.7 for a bridge rectifier.

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