0.1 Discrete components  (Page 4/6)

Electrolytic and tantalum capacitors are made in this shape. They also tend to have large capacitive values. I generally think of them in the 10-µF to 500-µF range, although they are available in ranges both below and above this. These capacitors are often used as storage elements, or perhaps in a power supply.

Another shape is a flat shape that looks a bit like a coin with two wires sticking out of its edge. These capacitors are usually used for signals in a circuit. Their capacitive values are in the pF range to units of µFs. And of course, they can go beyond this range.

Finally, there are surface-mount capacitors. Unlike the previous two shapes, these do not have leads on them. Rather than having leads that go through holes in the printed circuit board, these are mounted on the surface of the printed circuit board.

Uses

For the purposes of this discussion, I’ll limit the list of uses for a capacitor to:

• A storage element.
• A filter element.
• A timer element.

Virtually every power supply has some sort of a storage element on its output to make sure that there is enough energy stored to handle variations in current demand while keeping a relatively constant voltage. Typically, a large electrolytic capacitor is used to give the necessary buffering.

In many DC-to-DC converters, a toroid-based transformer allows a higher frequency through the transformer (many simple power supplies use 50 or 60 Hz from the power grid), giving higher efficiencies. But, it also has frequency components on the output of the DC-to-DC converter that are difficult for a large electrolytic capacitor to filter out; this is a result of the significant inductance component found in the capacitor. In these cases, a ceramic capacitor is placed in parallel with the larger electrolytic. Given its construction, the ceramic capacitor has very little inductance and therefore can filter out the higher frequencies that the larger electrolytic capacitor does not see.

To extend this idea, ceramic capacitors work well with resistors (and inductors) to create filter elements. Adding active components such as transistors or operational amplifiers can perform significant filtering of a signal.

Finally, you can use capacitors along with resistors to create timing elements and clock generators. Once again, this is best accomplished with the inclusion of active components.

Picking a part

Table 3 is a chart to help you pick the best part for your design. I have included the Digi-Key part number for each to make it easy for you to order the right part.

Inductors

Inductors are probably the least used of the passive components – or at least it seems that way. The reasoning for their somewhat rare use is the difficulty in integrating an inductor onto a silicon substrate. With the advent of digital filters, you can easily do most of the signal filtering in the digital domain. But that is far too narrow of a view of how inductors are used.