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999001 - Basic DC Electronics Basic DC Electronics
(Voltage/Current/Resistance)
Document# 999001
Publication, Duplication, or Retransmission Of This Document Not Expressly Authorized In Writing By The Install Doctor Is Prohibited. Protected By U.S. Copyright Laws. © 1997,1998,1999,2000. What are: Voltage, Current, and Resistance?
Voltage
(letter symbol: V; unit of measure: volts), Current (letter symbol: I; unit of measure: amps), and Resistance (letter symbol: R;
unit of measure: ohms ) are the 3 most fundamental principals in electronics. To understand how these 3 principals relate together, lets
look at an everyday garden item, a water hose. Everything needed to understand these 3 basics of electronics can be seen in a simple
garden hose. (This concept is so strong that medical researchers use voltage, current, and resistance to describe blood movement
through blood vessels; very similar to the garden hose concept.
Voltage:
The technical definition of voltage is the electric energy potential between 2 points. Potential means that the possibility
exists between the 2 points. If the 2 points are never connected, the potential is zero. When the points are connected, then voltage in
the circuit actually exists and there is no longer a potential. The garden hose definition of voltage would be the water pressure in a
garden hose. You can control the amount of water flowing from the garden hose by turning a control valve to allow more or less water
to flow from the hose. Voltage can be either DC or direct current, or AC or alternating current. Batteries, the primary power
supply in you automobile, are DC or direct current. The voltage in your house is AC usually noted as 120V AC. AC voltage
flips between (+) positive and (-) negative every other wave. AC current is more stable and can be transmitted very long distances
over wires; you might recognize these as the power wires running on towers and power poles throughout the world. But, for automo-
tive purposes, we will concentrate on DC voltage or battery voltages, usually noted as 12V DC. In automobiles, the battery in your
vehicle contains acids which, through the miracles of chemistry and physics, eat away at metal plates inside the battery. This erosion of
the plates creates free electrons, which when grouped inside the battery, create an electric potential between the (+) positive and (-)
negative battery posts. A typical automotive battery will produce 10 to 16 Volts DC.
Resistance:
To understand current, we must first look at resistance. The technical definition of resistance is simply the opposition to
the flow of electrons in an electric circuit. The garden hose definition is two fold: the first resistance would be the diameter and length
of the garden hose. A large diameter garden hose will offer low resistance and allow the water to flow easily. A second resistance of a
garden hose would be a sprayer or sprinkler attached to the end of the water hose. The sprayer or sprinkler would restrict the flow of
water out of the end of the garden hose. Remember, voltage is the potential difference between two points. Restricting the flow of
water would restrict the flow of water between the two ends of a garden hose. Every item or component in an electric circuit will have
resistance, even the wires that connect the components of the circuit has a small resistance. The great physicist Albert Einstein
developed the Law of Conservation. What The Law of Conservation stated was that NO energy can ever be mysteriously lost. ALL
energy MUST be accounted for.
When an electric circuit is connected and power is applied, the voltage through the circuit can be measured using a multimeter or other
voltage measurement devices. Resistance plays a key role in how voltage is distributed throughout the circuit. To visualize this, we
can set up a basic circuit where we can measure the voltage at different points in the circuit. When measurements are taken, the voltage
at the end of the circuit is less than the voltage at the beginning of the circuit. So in an automotive application, when the 12V DC
battery power is turned on, the circuit draws what power it needs to operate. Lets say all 12 Volts were needed by the circuit. It you
were to apply the multimeter to measure the voltage at the very end of the circuit you would notice that the multimeter would NOT
read 12 Volts. What happened to all the voltage? According to Einsteins Law of Conservation, the circuit MUST have absorbed the
lost voltage. And, he is correct. The resistance of every item in the circuit actually absorbs or dissipates voltage in the circuit. In the
world of electronics, resistance acts an a voltage drop in a circuit. A circuit cannot exist without a resistance load in the circuit. A
load is the main object of the circuit. A light bulb would be the load of the lighting circuit. In a car stereo application, a speaker
(speakers are usually 2, 4, or 8 ohms in resistance) is an example of a load when connected to an amplifier.
Current:
Now that you have a basic understanding of both voltage and resistance, current will become a little clearer to understand.
The technical definition of current is the flow of electrons when an electric circuit is activated. The garden hose definition is that the
volume flow of water itself is the current. When the water is turned off, water still resides in the garden hose, but there is no move-
ment. Nothing is forcing it to move. When the water is turned on, new water is forced into the hose. When the sprinkler or sprayer is
turned on at the other end, the water begins to flow. As the sprayer is adjusted (a change in resistance), current changes, or the volume
flow of water through the garden hose changes. This is current. And current changes as resistance and voltage change. As stated
earlier, the diameter and length of the garden hose are some of the factors that determine the resistance to the flow of water through the
hose. Current is directly proportional to BOTH voltage and resistance. If resistance is high ( the garden hose is small and thin or the
sprayer is set to only let a small amount of water to leave the hose) then volume flow of water through the hose would be low - or low
current. If the garden hose has a large diameter, the water will have less resistance. Since the water will move through the hose with
less resistance, the volume flow of water through and out of the hose will be high - or high current. The same applies to voltage. If
less water pressure is allowed to flow into the hose, then the volume flow of water through the hose would also be low - or low current.
If the water is turned up all the way, the garden hose would be full of water and the water pressure would be higher allowing a high
volume of water to flow though the hose - or high current. Again, the circuit must be active for there to be current. In the garden hose
definition, the sprayer or sprinkler must be turned on to allow water to flow. Voltage, Current, and Resistance Applied To Mobile Electronics
Car Stereos and Speakers:
The electronics inside your stereo are designed to operate on 12 Volts DC. Strangely, your speakers only
operate when AC voltage is applied to them. How does this work? The amplifier inside your stereo is powered by the same 12 Volts
DC that the radio is powered by, but an amplifier takes an audio signal and converts it to alternating AC voltages. Speakers move in
and out when alternating voltage is supplied to a speaker through speaker wires. A speaker, on the outside looks relatively simple, but
on the inside a speaker is a complex electromechanical machine. A speaker makes sound by moving in and out, moving and vibrating
air which is the sound we hear. The funnel shaped part of the speaker we see is called the cone. Attached to the rear of the cone is a
tube of cardboard which extends to the rear of the speaker. Surrounding the tube of cardboard is a magnet. Wrapped around the tube
of cardboard is a very thin wire. Each end of this wire is connected to the (+) positive and (-) negative terminal of a speaker. When
AC voltage from your radios amplifier is supplied to the speaker via speaker wires connected to these terminals, the speaker begins to
move. As the AC voltage increases and decreases due to the music, the wire wrapped around cardboard tube induces an electromag-
netic field which alternatesbetween (+) positive and (-) as the music changes. The magnet will either attract or repel the electromag-
netic field, repelling or pulling the cardboard tube and wire wrapped around it in and out, moving the speaker in and out, creating
sound.
Types Of Circuits Common In Mobile Electronics
Parallel Circuits:
Parallel circuits are very common in mobile electronic applications. Circuits are hard to explain and the best way
to understand a circuit is to see a picture of a circuit: (the most common form of this type of