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Ground circuits — Part 2

A look at battery electron current
Thursday, March 30, 2017 - 07:00
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This is Part 2 in a series of online-exclusive electrical circuit articles that focus on the ground side of the circuit. Read the first article, Ground Circuits — Part 1, by clicking here.

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The first electron current that occurs in a vehicle’s primary electrical circuit, when the engine is not running, battery cables are connected to a charged battery and circuits are turned ON. (For clarity in this article, we are intentionally ignoring normal key off battery drain of a few milliamps, which by the way, would follow the same path as described in this article.)

Any circuit connected to “hot at all times” voltage has battery voltage available to it “all-the-time” as shown by Fuse F2 at the top of the diagram. The Lamp Circuit #2 (on the right) can operate if switch S3 is CLOSED because battery voltage is available through Fuse F2 regardless of the condition of the ignition switch. Lamp Circuit #2 does not depend on the ignition switch position. However, Lamp Circuit #1 is connected to Switched Ignition voltage and cannot operate until the ignition switch is CLOSED and switch S2 is CLOSED.

Figure 1: Battery electron current path











We are using two simple lamp circuits to illustrate the path of battery electron flow through a vehicle’s electrical system. It doesn’t matter if the lamps are replaced by more complex circuitry. The path of electron current through the ground circuit is the same. We are using simple electrical circuits to illustrate a very important concept of electron current through the ground circuit.

Later on, in this series of articles, we will add more complex circuits grounded to the sheet metal and see what happens. The path of electron flow would not be any different with complex circuits so why not use two simple circuits to explain principles of ground electron flow? I’ve often found it easier to explain new or complex electrical or electronic circuit principles using simple circuits. Direction of electron flow doesn’t change with how simple or how complex a circuit might be. It flows through the circuit in the same way (-) to (+) as long as the path is complete (the circuit’s control switch is CLOSED and voltage is available).

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To place a vehicle in operation the ignition switch is CLOSED as shown in Figure 1. Normally, schematic diagrams always show switches in the rest or OFF position. In Figure 1 the switches are drawn CLOSED. Battery voltage is available to provide electron current to any circuit that is wired to switched ignition B+. Did you notice that battery voltage and electron current are two separate parameters in the circuit? This article deals with the parameter electron current.

Electron current flows from the negative battery terminal through the ground circuit and returns to the positive battery terminal. Specifically, electrons leave the battery negative terminal and flow down the accessory ground cable to G101, ground/sheet metal (1). Electrons then “channel” (flow) through a direct path in the sheet metal to enter all circuits grounded. Notice in Figure 1 we have accessory ground/sheet metal (1) connected to sheet metal (2) by a ground strap. This supplies electrons from sheet metal (1) to sheet metal (2). Any circuit grounded to either sheet metal will have access to electron current as each circuit is turned ON.

Get used to the idea that electrons enter a circuit at the ground connection. Electrons enter Lamp circuit #1 at G200 and travel up through the circuit when S2 is CLOSED turning the lamp ON. Electrons flow through the lamp causing it to operate, continue through the fuse, through the closed ignition switch to the battery positive terminal which completes the circuit for Lamp Circuit #1.

Electrons leave sheet metal (1) and flow through the ground strap to supply electrons to sheet metal (2). If switch S3 is CLOSED a path for current exists through Lamp Circuit #2. Electrons enter G300 and flow up the ground wire, through lamp circuit #2 turning the lamp ON. Electrons continue flowing through the closed contacts of switch S3, fuse F4, fuse F2 and return to the battery positive terminal. Trace the path of battery electron current leaving the battery negative terminal and traveling through the ground circuit and up each grounded circuit then returning to the battery positive terminal.

If switch S2 is switched OFF or “OPEN,” electron current through Lamp Circuit #1 will stop but Lamp Circuit #2 will continue to operate. The opposite is true. If S3 is switched “OFF” or “OPEN” electron current through lamp circuit #2 will stop but lamp circuit #1 will continue to operate.

Figure 1 again for easy reference.

Notice the engine block battery cable connection point is labeled G100. We did not mention any electron current flowing down the engine ground to the engine block. If any circuit were grounded to the engine block there would be some battery electron current flowing down the engine ground cable into the engine block.

The only circuit we show connected to the engine block in this diagram is the generator. If this generator has an initializing circuit that is used to start the generator producing electricity as the engine begins running, there would be a small electron current flowing through the engine ground cable before engine RUN to initialize the voltage regulator. If the generator does not use an initializing circuit (no internal voltage regulator) there would be no current through the engine ground at this time (Crank sequence not yet initiated.)

From this simple circuit explanation, we can see why the ground circuit is so important to good circuit operation. Electron flow enters any circuit that is grounded (connected to sheet metal) and turned ON.

If ground G200 or ground G300 were corroded, resistance is created that will reduce the electron flow through that circuit. This understanding will form the basis for explaining how to measure electron flow through the ground circuit and how to troubleshoot ground circuits.

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