Polarised capacitors (large values, 1µF +)

Examples:       Circuit symbol:   

Electrolytic Capacitors

Electrolytic capacitors are polarised and they must be connected the correct way round, at least one of their leads will be marked + or -. They are not damaged by heat when soldering.

There are two designs of electrolytic capacitors; axial where the leads are attached to each end (220µF in picture) and radial where both leads are at the same end (10µF in picture). Radial capacitors tend to be a little smaller and they stand upright on the circuit board.

It is easy to find the value of electrolytic capacitors because they are clearly printed with their capacitance and voltage rating. The voltage rating can be quite low (6V for example) and it should always be checked when selecting an electrolytic capacitor. If the project parts list does not specify a voltage, choose a capacitor with a rating which is greater than the project's power supply voltage. 25V is a sensible minimum for most battery circuits.

Smoothing

Smoothing is performed by a large value electrolytic capacitor connected across the DC supply to act as a reservoir, supplying current to the output when the varying DC voltage from the rectifier is falling. The diagram shows the unsmoothed varying DC (dotted line) and the smoothed DC (solid line). The capacitor charges quickly near the peak of the varying DC, and then discharges as it supplies current to the output. 

 

Note that smoothing significantly increases the average DC voltage to almost the peak value (1.4 × RMSvalue). For example 6V RMS AC is rectified to full wave DC of about 4.6V RMS (1.4V is lost in the bridge rectifier), with smoothing this increases to almost the peak value giving 1.4 × 4.6 = 6.4V smooth DC.

Smoothing is not perfect due to the capacitor voltage falling a little as it discharges, giving a small ripple voltage. For many circuits a ripple which is 10% of the supply voltage is satisfactory and the equation below gives the required value for the smoothing capacitor. A larger capacitor will give less ripple. The capacitor value must be doubled when smoothing half-wave DC.

There is more information about smoothing on the powersupplyschematics.blogspot.com/ website.

Smoothing capacitor for 10% ripple, C =	5 × Io
	Vs × f

C  = smoothing capacitance in farads (F)
Io  = output current from the supply in amps (A)
Vs = supply voltage in volts (V), this is the peak value of the unsmoothed DC
f    = frequency of the AC supply in hertz (Hz), 50Hz in the UK 

Transformer + Rectifier + Smoothing + Regulator

 

The regulated DC output is very smooth with no ripple. It is suitable for all electronic circuits.

Further information: Transformer | Rectifier | Smoothing | Regulator

Transformer + Rectifier + Smoothing

 

The smooth DC output has a small ripple. It is suitable for most electronic circuits.

Further information: Transformer | Rectifier | Smoothing