Electronics Made Easy - a Complete Introduction to Electronics - Martin Denny [2]
The impedance (Z) of an inductor is the vector sum of its DC resistance and its AC reactance.
Z = R + xl Ω
where xl = 2πFL Ω
SWITCHES & RELAYS
Switches are manufactured in all shapes and sizes. Limiting factors in size are contact area and contact separation, ie current and voltage rating. Switch contacts are made from a variety of conductors dependant on the estimated life of the component, and the contact resistance. In some cases mercury is used either to wet the contacts (anti-bounce), or in liquid form to provide low resistance high current switching (arc suppression), for example tilt switches. Restrictions in some countries prevent the use of mercury for health reasons.
The most common forms of switch use toggle (up/down), push button (in/out) or rotary action. Micro switches can be best defined as a push button switch operated by a lever arm. These switches are used mainly as limits to mechanical operations, for example voltage "cut outs" on equipment doors, movement restriction on machinery and even as a reversing switch on some windscreen wiper motors.
Reed/Relay switches are also quite common and can be operated either by a relay coil, or a fixed magnet. They are available as single, double, and change-over options. Construction of the relay is simplified as contacts are deflected by the magnetic field rather than an arm operated by the electro-magnet as in more conventional relays. The contacts are sealed usually in a glass envelope, which allows mercury wetted contacts to be used. The construction of reed relay switches enables them to be used in hazardous environments, and at high frequency. They are also useful in applications where high packing density is required due to their relatively small size. Disadvantages are relatively low current switching and due to their size low voltage rating.
Common Terms:
Figure number 1 shows an example of a latching relay circuit. When RL1 is energised via PB1 the energised contacts are made. RL1b contact supplies the load whilst RL1a provides an alternative supply to the relay coil. To reset the relay PB2 is depressed removing the relay coil supply. Diode D1 provides protection against back EMF generated by the inductance of the coil.
Figure number 2 shows an example of a DC motor reverse control circuit. With the relay de-energised the positive connection is made at the top motor terminal and negative to the bottom terminal. When SW1 is closed RL1 is energised thus reversing the power connections. Diode D1 provides protection against back EMF generated by the inductance of the coil.
BATTERIES
Three components are required to produce a voltage cell, two metallic electrodes of dissimilar material, and a suitable electrolyte. Batteries can be separated into two types, non-rechargeable and rechargeable.
Internal Resistance increases with both cycling and chronological age. Rising internal resistance causes the voltage at the terminals to drop under load, which reduces the maximum current that can be drawn. Eventually the internal resistance reaches a point at which the battery can no longer operate the equipment for an adequate period.
CRYSTALS
Certain natural crystalline materials exhibit what is known as the piezo-electric effect. If a slice is cut in a particular direction and a metallic film applied to two opposite faces so that the arrangement can function as a capacitor, the application of a potential difference (V) to the electrodes produces a mechanical strain in the crystal; if the potential is reversed, so is the strain.
The strain will tend to set up vibrations in the structure of the crystal which will then vibrate at its natural frequency (resonance). The natural frequency is dictated by the thickness of the slice of crystal.
Quartz crystal has good temperature stability and can