This begins a series of circuit analysis emails that focuses on the ground side of the circuit with a total of 9 articles. This is Part 1 with 8 more articles to follow until we have the subject of electron current through the ground circuit fully covered.
Positive Electron Current Flow
There is a lot of misunderstanding about ground circuits in automobiles, trucks and heavy equipment. Let’s clear up the misunderstanding caused by teaching positive current flow in circuits. Positive current flow states that current flows from the positive (+) terminal to the negative (-) terminal to explain how circuits work. Since the positive post of the battery is the one that causes sparks when it contacts ground, technicians assume that is where the “juice” is. They note that if you ground the negative post of the battery it doesn’t cause sparks to fly because there is no “juice” there. Just the opposite of what is true due to misunderstanding.
It seems logical to assume that when the circuit is connected to a battery, the current flows from the positive terminal, through the circuit and goes to ground. Once the current goes to ground it “disappears” and you don’t have to worry about it anymore.
When the ground side of the circuit develops a bad connection due to corrosion, a voltage appears on the ground side of the circuit. I have heard technicians ask: “How does voltage feeds back into ground?” It is clear that positive current flow doesn’t explain how circuits really work. And it certainly doesn’t answer the question of how “voltage feeds back into ground” which it doesn’t although it appears that it does due to misunderstanding.
The solution is to understand the ground circuit and how electron flow occurs in a circuit requires electron flow (-) to (+). Electrons are negative charges. A large number of electrons accumulate in the negative terminal of the battery producing a negative (-) charge. The battery negative terminal is labeled -BATT (say minus BATT) or B- (say B minus). The minus sign indicates a negative charge because all the electrons have been accumulated there. The positive side of the battery, labeled +BATT (say plus BATT) or B+ (say B plus), is marked with a ”+” sign since it is positive, the opposite of the negative side. This indicates there are much fewer electrons at the positive terminal than at the negative terminal. The difference between the two battery terminals is referred to as battery voltage.
Battery voltage can be measured with a voltmeter to indicate the difference in electrons on the negative compared to how many electrons are on the positive. If a vehicle battery is fully charged, which means the negative terminal is full of electrons and the positive terminal has as few as possible, the voltmeter will indicate 12.66 Volts measured between the battery posts.
I can tell you a lot of confusion arises when technicians think electron flow and voltage on the ground circuit are the same thing. They are not. Electron flow is measured with an ammeter or an inductive current clamp. Circuit voltages are measured with a voltmeter. That alone tells you electron current and voltage are not the same thing.
Figure 1 Simple vehicle electrical circuit – dual battery ground
The schematic diagram above is a simple vehicle electrical circuit including the ground circuit shown in detail. Several circuit components are connected together to form a complete vehicle electrical circuit containing a typical vehicle voltage source (battery and generator) and an ignition switch to control the voltage applied to vehicle circuits. Notice that Lamp Circuit #2 is connected to Hot-At-All-Times B+ which is a direct connection to B+ supply. Lamp circuit #2 will operate any time switch S3 is closed regardless if the Ignition Switch is ON (closed) or OFF (open). Lamp circuit #1 can only operate when the ignition switch is CLOSED and switch S2 is CLOSED.
Our focus is on the ground side of the circuit. Notice all the components connected to the “ground side,” which is nothing more than the negative side of the circuit.
The case of the generator is the negative side of the generator -GEN (say minus GEN) or B- (say B minus) and is bolted to the engine block in most vehicles. The negative post of the battery, -BATT (say B minus), is connected to the engine block by two cables. One cable is the engine ground cable connected to the engine block.
A second cable is called the accessory ground or sheet metal ground and is connected by a cable from -BATT to sheet metal (1). A ground strap connects sheet metal (1) to sheet metal (2) which completes the ground circuit. In the next article we will begin to discuss how electron current flows through the circuit.
You may be surprised how this series of articles will reveal some things about circuits that you probably never heard or knew before.