Control switches

An electrical switch is any device used to interrupt the flow of electrons in a circuit. Switches
are essentially binary devices: they are either completely on ("closed") or completely or ("open").
There are many different types of switches, and we will explore some of these types in this chapter.
Though it may seem strange to cover this elementary electrical topic at such a late stage in this
book series, I do so because the chapters that follow explore an older realm of digital technology based
on mechanical switch contacts rather than solid-state gate circuits, and a thorough understanding of
switch types is necessary for the undertaking. Learning the function of switch-based circuits at the
same time that you learn about solid-state logic gates makes both topics easier to grasp, and sets
the stage for an enhanced learning experience in Boolean algebra, the mathematics behind digital
logic circuits.

The simplest type of switch is one where two electrical conductors are brought in contact with
each other by the motion of an actuating mechanism. Other switches are more complex, containing
electronic circuits able to turn on or off depending on some physical stimulus (such as light or
magnetic field) sensed. In any case, the final output of any switch will be (at least) a pair of
wire-connection terminals that will either be connected together by the switch's internal contact
mechanism ("closed"), or not connected together ("open").

Any switch designed to be operated by a person is generally called a hand switch, and they are
manufactured in several varieties:

Toggle switch
Toggle switches are actuated by a lever angled in one of two or more positions. The common
light switch used in household wiring is an example of a toggle switch. Most toggle switches will
come to rest in any of their lever positions, while others have an internal spring mechanism returning
the lever to a certain normal position, allowing for what is called "momentary" operation.

Push button switch
Push button switches are two-position devices actuated with a button that is pressed and released.
Most push button switches have an internal spring mechanism returning the button to its "out," or
"unpressed," position, for momentary operation. Some push button switches will latch alternately on
or off with every push of the button. Other push button switches will stay in their "in," or "pressed,"
position until the button is pulled back out. This last type of push button switches usually have a
mushroom-shaped button for easy push-pull action.

The transformers

A transformer is a device made of two or more inductors, one of which is powered by AC,
inducing an AC voltage across the second inductor. If the second inductor is connected to a
load, power will be electromagnetically coupled from the first inductor's power source to that
load.

The powered inductor in a transformer is called the primary winding. The unpowered inductor
in a transformer is called the secondary winding.

Total primary current in an unloaded transformer is called the exciting current, and is comprised of magnetizing current plus any additional current necessary to overcome core losses. It
is never perfectly sinusoidal in a real transformer, but may be made more so if the transformer
is designed and operated so that magnetic flux density is kept to a minimum.

Core flux induces a voltage in any coil wrapped around the core. The induces voltage(s) are
ideally in phase with the primary winding source voltage and share the same wave-shape.


Any current drawn through the secondary winding by a load will be "reflected" to the primary
winding and drawn from the voltage source, as if the source were directly powering a similar
load.

Batteries

"battery" is a set of voltaic cells designed to provide
greater voltage and/or current than is possible with one cell alone.

The symbol for a cell is very simple, consisting of one long line and one short line, parallel to
each other

the voltage produced by any particular kind of cell is determined strictly
by the chemistry of that cell type. The size of the cell is irrelevant to its voltage. To obtain greater
voltage than the output of a single cell, multiple cells must be connected in series. The total voltage
of a battery is the sum of all cell voltages.

The cells in an automotive battery are contained within the same hard rubber housing, connected
together with thick, lead bars instead of wires. The electrodes and electrolyte solutions for each cell
are contained in separate, partitioned sections of the battery case. In large batteries, the electrodes
commonly take the shape of thin metal grids or plates, and are often referred to as plates instead of
electrodes.

For the sake of convenience, battery symbols are usually limited to four lines, alternating
long/short, although the real battery it represents may have many more cells than that. On occasion,
however, you might come across a symbol for a battery with unusually high voltage, intentionally
drawn with extra lines.

The ideal battery, in a short circuit with 0 ­ resistance, would be able to supply an infinite
amount of current. The real battery, on the other hand, can only supply 50 amps (10 volts / 0.2 ­)
to a short circuit of 0 ­ resistance, due to its internal resistance. The chemical reaction inside the
cell may still be providing exactly 10 volts, but voltage is dropped across that internal resistance
as electrons °ow through the battery, which reduces the amount of voltage available at the battery
terminals to the load.