Make Electronics

Switching Basics and More 61

Experiment 8: A Relay Oscillator

Adding Capacitance

Add a 1,000 μF electrolytic capacitor in parallel with the coil of the relay as
shown in the diagram in Figure 2-65 and the schematic in Figure 2-66. Check
Figure 2-14 if you’re not sure what a capacitor looks like. The 1,000 μF value will
be printed on the side of it, and I’ll explain what this means a little later.

Make sure the capacitor’s short wire is connected to the negative side of the
circuit; otherwise, it won’t work. In addition to the short wire, you should find
a minus sign on the body of the capacitor, which is there to remind you which
side is negative. Electrolytic capacitors are fussy about this.

When you press the button now, the relay should click slowly instead of buzz-
ing. What’s happening here?

A capacitor is like a tiny rechargeable battery. It’s so small that it charges in a
fraction of a second, before the relay has time to open its lower pair of con-
tacts. Then, when the contacts are open, the capacitor acts like a battery, pro-
viding power to the relay. It keeps the coil of the relay energized for about one
second. After the capacitor exhausts its power reserve, the relay relaxes and
the process repeats.

FundAmentAls

Farad   basics

The Farad is an international unit to measure capacitance. Modern circuits

usually require small capacitors. Consequently it is common to find capacitors

measured in microfarads (one-millionth of a farad) and even picofarads (one-

trillionth of a farad). Nanofarads are also used, more often in Europe than in the

United States. See the following conversion table.

0.001 nanofarad 1 picofarad 1 pF

0.01 nanofarad 10 picofarads 10 pF

0.1 nanofarad 100 picofarads 100 pF

1 nanofarad 1,000 picofarads 1,000 pF

0.001 microfarad 1 nanofarad 1 nF

0.01 microfarad 10 nanofarads 10 nF

0.1 microfarad 100 nanofarads 100 nF

1 microfarad 1,000 nanofarads 1,000 nF

0.000001 Farad 1 microfarad 1 mF

0.00001 Farad 10 microfarads 10 mF

0.0001 Farad 100 microfarads 100 mF

0.001 Farad 1,000 microfarads 1,000 mF

(You may encounter capacitances greater than 1,000 microfarads, but they are

uncommon.)

12v DC from

AC adapter

Figure 2-65. Adding a capacitor makes the

relay oscillate more slowly.

680

12v

DC

Figure 2-66. The capacitor appears at the

bottom of this schematic diagram.