Choosing Capacitors

By Jorge J. Ramirez

In any electrical design, when choosing a capacitor its size and shape are as important as its electrical characteristics. The designer should check for correct lead spacing, maximum working voltage, and polarization. 

Radial capacitors have both ends coming out of the same end.

Axial capacitors have the same shape as resistors with a wire coming out of each end.

Electrolytic and tantalum capacitors are polarized and must be fitted the correct way. 

Typical capacitors applications are:

1. Remove power supply spikes ( place 0.01 to 0.1uf across power supply pins of digital ICs).

2. Smooth out rectified AC voltage into steady DC voltage.

3. Block DC signal while passing AC signal.

4. Filter out unwanted portions of a fluctuating signal.

5. Bypass AC signal around a circuit or to ground.

6. Use with resistor to differentiate a fluctuating signal.

Figure 1. RC differentiator circuit

This circuit is used to create trigger pulses. The RC time constant must be at least 1/10 or less  the period of the incoming signal.

7. Use with a resistor to integrate a fluctuating signal.

Figure 2. RC integrator circuit

The RC time constant must be at least 10 times the period of the incoming signal otherwise it will reduce the amplitude of the incoming signal. The circuit becomes a low pass filter blocking high frequencies.

8. Use in timing applications.

Figure 3. RC basic timing circuit

The circuit will charge quickly C and then it will discharge slowly through R at a time given by the time constant RC.

9. Store a charge to keep a transistor turned off or on.