Resistors are components
that have a predetermined resistance. Resistance
determines how much current will flow through a
component. Resistors are used to control
voltages and currents. A very high resistance
allows very little current to flow. Air has very
high resistance. Current almost never flows
through air. (Sparks and lightning are brief
displays of current flow through air. The light
is created as the current burns parts of the
air.) A low resistance allows a large amount of
current to flow. Metals have very low
resistance. That is why wires are made of metal.
They allow current to flow from one point to
another point without any resistance. Wires are
usually covered with rubber or plastic. This
keeps the wires from coming in contact with
other wires and creating short circuits. High
voltage power lines are covered with thick
layers of plastic to make them safe, but they
become very dangerous when the line breaks and
the wire is exposed and is no longer separated
from other things by insulation.
Resistance is given in units of
ohms. (Ohms are named after Mho Ohms who played
with electricity as a young boy in Germany.)
Common resistor values are from 100 ohms to
100,000 ohms. Each resistor is marked with
colored stripes to indicate it�s resistance.
Variable
Resistors
Variable resistors are
also common components. They have a dial or a
knob that allows you to change the resistance.
This is very useful for many situations. Volume
controls are variable resistors. When you change
the volume you are changing the resistance which
changes the current. Making the resistance
higher will let less current flow so the volume
goes down. Making the resistance lower will let
more current flow so the volume goes up. The
value of a variable resistor is given as it�s
highest resistance value. For example, a 500 ohm
variable resistor can have a resistance of
anywhere between 0 ohms and 500 ohms. A variable
resistor may also be called a potentiometer (pot
for short).
Diodes
Diodes are components
that allow current to flow in only one
direction. They have a positive side (leg) and a
negative side. When the voltage on the positive
leg is higher than on the negative leg then
current flows through the diode (the resistance
is very low). When the voltage is lower on the
positive leg than on the negative leg then the
current does not flow (the resistance is very
high). The negative leg of a diode is the one
with the line closest to it. It is called the
cathode. The postive end is called the anode.
Usually when current is
flowing through a diode, the voltage on the
positive leg is 0.65 volts higher than on the
negative leg.
LED
Light Emitting Diodes
are great for projects because they provide
visual entertainment. LEDs use a special
material which emits light when current flows
through it. Unlike light bulbs, LEDs never burn
out unless their current limit is passed. A
current of 0.02 Amps (20 mA) to 0.04 Amps (40 mA)
is a good range for LEDs. They have a positive
leg and a negative leg just like regular diodes.
To find the positive side of an LED, look for a
line in the metal inside the LED. It may be
difficult to see the line. This line is closest
to the positive side of the LED. Another way of
finding the positive side is to find a flat spot
on the edge of the LED. This flat spot is on the
negative side.
When current is flowing
through an LED the voltage on the positive leg
is about 1.4 volts higher than the voltage on
the negative side. Remember that there is no
resistance to limit the current so a resistor
must be used in series with the LED to avoid
destroying it.
Switches
Switches are devices
that create a short circuit or an open circuit
depending on the position of the switch. For a
light switch, ON means short circuit (current
flows through the switch, lights light up and
people dance.) When the switch is OFF, that
means there is an open circuit (no current
flows, lights go out and people settle down.
This effect on people is used by some teachers
to gain control of loud classes.)
When the switch is ON
it looks and acts like a wire. When the switch
is OFF there is no connection.
