B+ and B-
The high voltage power supply for the vacuum tube’s Plate is referred to as the “B” voltage supply with B+ and B- polarity. The positive side “B+” is connected to the vacuum tube’s Plate circuit. When the switch is closed as shown in Figure VT03 the high positive voltage attracts electrons from the electron cloud.
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Electrons leave the “B” voltage supply from the B- (negative) terminal, channel through ground, enter the grounded Cathode circuit and are drawn up through the Cathode. Electrons enter the electron cloud and the electrons continue to stream through the vacuum tube. Electron current passes through the load which begins to operate. The B+ voltage on the plate of the early vacuum tubes was often more than 100 V to create sufficient electron current to operate the load. One way to control the electron current through a vacuum tube was two vary the B+ voltage supply. The higher the B+ voltage the higher the electron current through the vacuum tube.
Notice that electrons in vacuum tube circuits are flowing from the negative terminal (B-), through the vacuum tube and the load and return to the positive terminal (B+). Electrons always travel (-) to (+) just as they do when a vehicle battery is providing electron current to vehicle circuits.
The term “B+” has hung around long after vacuum tubes have gone the way of history being replaced by transistors. In fact, some generators, alternators (now called generators again), have the term “B+” embossed on the case near the voltage “output” terminal. From the beginning of vacuum tube electronics, the term B+ continues to define the highest positive voltage in a circuit that allows solid-state circuits and transistors to operate in today’s vehicles. The B+ voltage in typical automotive and truck gasoline and diesel engines is in the range of 13.8 V to a high of 15.1 V. However, in some hybrid vehicles B+ can be over 300 V DC. It is still “B+.”
Voltage: The generator output terminal is called the “output terminal” because that is where the B+ voltage originates or first appears in the electrical system.
Electron Current: As far as electron current is concerned, electrons leave B- (the generator case) and flow through the circuit and back into the B+ terminal. Remember to keep voltage and electron current separate from each other. They are not the same.
Tip #1: If you ever find yourself confused about an electrical principal or what is happening in an electrical circuit, ask yourself this question. Am I thinking of voltage and electron current as the same thing? You probably are. First identify the voltage source and its (+) and (-) terminals. Then trace the electron current from the (-) terminal through the circuit and see it return to the (+) terminal. It will clear up confusion as you see the voltage at the voltage source and the electron current flowing through the circuit.
Tip #2: Voltage is measured in units of volts with a voltmeter. Electron current is measured in units of amperes (amps) with an ammeter or a current clamp. Voltage and electron current are two different parameters to be measured in different ways and with different meters.
When? - Why? - Who? - How?
Since the earliest days of electronics using vacuum tubes in the 1900s, it was universally understood that electron current is the movement of (-) charged electrons flowing through a circuit.
The unanswered question is this! What happened in the automotive and truck service industry when explaining automotive electrical circuits? When did people in the industry begin to refer to electrical current moving from the (+) positive terminal and through the circuit to (-) ground. I have no idea who started this incorrect idea sometimes referred to as “positive” current flow. As you can see, from the earliest days of electronics and vacuum tubes, electron current was understood as (-) to (+).
In conclusion, it is important that a technician understands how electrical circuits actually operate, how electrical circuits fail and how to troubleshoot electrical circuits to quickly identify a problem.
One on the keys to this understanding is how electron current flows through a circuit. Two of the most effective ways of troubleshooting an electrical circuit is measuring voltage and measuring electron current. Then compare readings with known good values to identify circuit problems.
Always trace the electron current from the voltage source (-) through the circuit and back to the voltage source (+). This simple but effective technique identifies which ground is used, indicates all the circuit components that comprise the circuit, whether the control switch is on the voltage side or the ground side of the load and the fuse location that protects the circuit.
We learn a lot about a circuit as we come to understand how electrons flow through the ground circuit especially in a vehicle where the ground circuit has more than one way to be wired up. Different ground circuit configurations can change the way we measure electron current in vehicle ground circuits and troubleshoot a variety of possible failures. In future parts of this series we will see different ways to wire up the ground circuit as we trace electrons through the ground circuit with different ground wiring configurations.