Welcome to the exciting
world of electronics. Before we can build
anything we need to look at a couple of things.
Anytime you have an electrical circuit, you have
voltage and current. We build circuits to
control voltage and current.
Current is what flows
through a wire. Think of it as water flowing in
a river. The current flows from one point to
another point just like water in a river.
Current flows from points of high voltage to
points of low voltage. Current can be shown in
circuit diagrams by using arrows as in Figure 1.
The arrow shows which way the current is
flowing. An I is usually included beside the
arrow to indicate current.
The unit of measurement
for current is the Ampere, or Amp for short, and
abbreviated as A. (The name Ampere comes from
Mr. Ampere who played with electricity as a
small boy in Vermont.) Common currents are 0.001
Amps (0.001A) to 0.5 Amps (0.5A). Since currents
are usually small, they are usually given in the
form of milliAmps (abbreviated mA.) The milli
means divided by 1000, so 0.001 Amps equals 1
milliAmp (1 mA) since 1 / 1000 = 0.001. Also,
0.5 Amps equals 500 milliAmps (500mA) since 500
/ 1000 = 0.5.
Voltage indicates the
power level of a point. Voltage is measured in
volts. If we continue the river comparison, a
point at the top of a hill would be at a high
voltage level and a point at the bottom of a
hill would be at a low voltage level. Then, just
as water flows from a high point to a low point,
current flows from a point of high voltage to a
point of low voltage. If one point is at 5 volts
and another point is at 0 volts then when a wire
is connected between them, current will flow
from the point at 5 volts to the point at 0
A measurement of
voltage is much like a measurement of height. It
gives you the difference in voltage between
those two points. If point A is at 10 volts and
point B is at 2 volts then the voltage measured
between A and B is 8 volts (10 -2). This is
similar to measuring height. We measure the
height of hills the same way. We say the sea
level is at zero feet and then compare other
points to that level. On top of Mary’s Peak you
are 4000 ft high (compared to sea level). In the
same way we call the lowest voltage in a circuit
zero volts and give it the name ground. Then all
other points in the circuit are compared to that
ground point. Rivers always flow towards sea
level and currents always flow towards ground.
A battery is similar to
a dam. On one side is a lot of stored up energy.
When a path is formed from that side to the
other side then current flows. If there is no
path then current does not flow and the energy
just stays there waiting for a path to form to
the other side. The path can be a big path with
lots of current flowing or a small path with
just a little bit of current flowing. With a
dam, a little bit of water flow could go on for
a long time, but flow through a big path that
lets all the water go at once would only last a
short while. A battery is the same. If there is
big path from the high voltage side to the low
voltage side then the battery will not last
There are two special
cases that we give names. One is when the
current is zero (open circuit) and the other is
when the voltage is zero (short circuit).
An open circuit is when
two points are not connected by anything. No
current flows and nothing happens. If a wire in
your vacuum cleaner breaks it can cause an open
circuit and no current can flow so it does not
do anything. There may be a voltage between
those two points but the current can not flow
with out a connection.
A short circuit (or
short) is when two points with different voltage
levels are connected with no resistance (see
resistors) between two points. This can cause a
large amount of current to flow. If a short
circuit happens in your house, it will usually
cause a circuit breaker to break or a fuse to
blow. If there is no device to limit the
current, the wires may melt and cause a fire.
This situation is something like a dam breaking.
There is a large amount of energy suddenly free
to flow from a high point to a low point with
nothing to limit the current.
A series connection is
when two components are joined together by a
common leg and nothing else is connected to that
point as shown in Figure 2.
A parallel connection
is when two components are joined together by
both legs as shown in Figure 3.