Voltage and Current
The easiest way to think about voltage and current is
the
analogy of
electricity to water, and the analogy of a conductor to a pipe. In
simple terms, voltage is the electrical "pressure" that pushes charge
carriers through a conductor, while current refers to the volume of
charge carriers moving through the conductor. The standard measurement
for voltage is the volt,
abbreviated V. The standard measurement of
current is the ampere,
abbreviated to amp or A.
To complicate matters, there are multiple terms for
voltage,
although they all mean the same thing. Alternate references to voltage
include electromotive force or
EMF, potential, and less commonly these
days, tension.
A DC voltage source
has a positive pole and a negative pole. You can't
really say the the current flows from one to the other, since you could
either say that electrons flow
from the negative pole to the positive pole, or that charge carriers
called holes flow from the
positive pole to the negative pole. Knowing this isn't really
important, but it's surprising how many people think electricity flows
in some arbitrary direction. Despite their common meaning, there's
nothing inherently positive or
negative about the positive or negative charges. It's just a convenient
label, similar to the concept of using up and down to refer to
different types of quarks. Positive and negative voltages may be
referenced to each other, or to ground,
which is the electrical potential of the actual ground, and is always
considered to be 0V. Many rectifiers that convert AC to DC only provide
either a
positive or negative pole, and allow the ground to act as the other
pole.
An AC voltage is one that is constantly varying. Common
household
electricity in the US alternates between positive and negative 60 times
per second (60 cycles/sec or 60 Hz), with the voltage referenced to
ground. An audio signal from an amplifier is another example of AC.
Measuring AC voltage is somewhat more complicated than measuring DC,
since just giving the "average" voltage would result in 0 for an AC
voltage with a constant frequency, and would change with frequency with
a variable voltage like an audio signal. Fixed frequency AC voltages
like mains power are usually given in a measurement called root mean
square (RMS). A standard US wall outlet operates at 120V RMS. Another
method of measuring AC voltage is to measure the peak to peak voltage,
that is, the difference between the maximum positive potential and the
maximum negative potential. It is commonly abreviated pkpk. This is
the measurement most commonly used in measuring signals, for example a
standard composite video signal has a voltage of about 1V pkpk. It's
important not to substitute one for the other, since the differences
can be dramatic. For example, the 120V RMS wall outlet mentioned
earlier would be over 300V pkpk.
Resistance
Whether AC or DC, the current traveling through a line
is
affected by
its resistance, which is measured in ohms, abbreviated as Ω. Higher
resistance offers more impedance to current without affecting voltage.
If you know the voltage and resistance, you can calculate the amount of
current that can flow. Let's try it now, using 12 volts and 1000Ω of
resistance, using the formula known as Ohm's law: current = voltage / resistance.
Calculating 12V/1000Ω gives us 0.012 amps or 12mA, meaning that 12mA
will flow through a 1000Ω resistor connected to a 12V source.
If you know the voltage and current used by the system,
you
can
calculate its power consumption, which is usually measured in watts and
abbreviated W. The formula to do this is voltage × amperage. For
example, a lamp drawing 1 amp of current at 1 volt is using 1 watt of
electricity. Another term for this is voltampere, which is abbreviated
as VA. Since all of these equations are reciprocal, you can rearrange
them to find out an unknown third value when two or more variables are
known. For example, if we know that a lamp is using 60W at 120V, we can
calculate the current flowing through it as 60W/120V = 0.5A. Now that
we know the lamp's current, we can even calculate its resistance by
dividing current and voltage 120V/0.5A= 240Ω. Same deal if we knew the
current and resistance, and wanted to find the voltage, 0.5A ×
240Ω = 120V. Once you're familiar with these formulas, you'll be
surprised how often they come in handy.
Balanced and Single Ended Signals
There are two ways of determining the amplitude of a
signal;
balanced, and single ended. In the type of electronics engineering
you'll be doing, the two major things that can be balanced or single
ended are transmission lines and amplifiers.
In a single ended signal, there is a single waveform,
and its
voltage is referenced to ground. Single ended transmission lines
usually have an outer shield that serves as the ground, as in coaxial
cable. Single ended amplifiers have a one conductor input with all
internal and external voltages referenced to a common ground.
A balanced or differential signal has two identical
waveforms,
which are 180° out of phase with each other, which means that
the peak of one wave is happening at the same time as the trough of the
other one. A balanced circuit may have an external ground, or the
voltage can be derived from the difference in the two waveforms. The
advantage to a balanced circuit is a reduction in noise, especially for
transmission lines. When the differential signal is received by
something like an amplifier, one waveform is inverted, bringing them
into phase with each other, and the two waveforms can be summed. Since
the noise riding on one of the waveforms was inverted and the other one
wasn't, the noise is
180°
out of phase itself, and adding the two waveforms together cancels out
the noise without damaging the signal. Common examples of balanced
circuits are phone lines, ethernet networks, XLR cables, 300Ω twin lead
antenna cable, and most amplifiersonachip.
