Power Supplies

Smoothing circuits

By adding a choke L, and a capacitor C_S as in Fig 6.6, the fluctuations in V_dc, 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 C_S 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.