What is Voltage

Posted by Josh Carton on

What is Voltage?

Voltage can be described as the “pressure” of an electrical circuit. In Automotive we deal mostly with DC or direct current circuits (because the vehicles power source is a DC Battery) and so voltage is the pressure” of this DC Battery in the circuit and it allows energy to flow through the circuit and will power or “flow through” motors and lights (loads) to turn them on.

Voltage, notated as “V” or “VDC” in most manuals, write ups or tools is most noted as a number when talking about a system of the vehicle’s power source such as the battery, or components that make up the vehicles system to indicate the operating level of those systems. For example a 12 Volt battery, or 12 volt system will run 12 volt components, motors and circuits and normally cannot be replaced with components rated for other voltages.

A normal 12 volt simple circuit in a vehicle can be seen below powering a fan:

Simple DC Circuit with Fan, 12VDC battery, 12VDC Fan, What is Voltage, How does a fan work

In the circuit above the battery is the power source which provides the voltage (pressure) of 12 Volts. The image shows the fan operating and that the circuit is “live” (On) and that electrons are flowing (orange arrows) from the battery’s negative terminal (-) through the ground wire into the fan, and out through the power lead (through a fuse for safety’s sake) and back into the battery’s positive terminal (+) as current.

Simple DC Voltage

The Voltage in a DC Circuit is unable to flow until there is a complete connection between the battery’s negative pole (-) and positive pole (+), in effect closing the circuit. The pressure or Voltage is often described as a difference of potential or a potential difference between the poles (potential meaning the potential energy between the starting point and ending point of the circuit). The potential is often describing the amount of work that is available to get done through the circuit by the electrons flowing through it and as measured in Volts (V). So in other words the Volts describes the amount of potential work that can get done between the two points (-/+) in the circuit, in the case above 12 units of Voltage (V) or 12V.

Is bigger better?

Depends on who you’re asking :-) but the bigger the V or battery means the more amount of work that can get done. For example if you have a 24 VDC or 48 VDC battery you can get more work done by having the availability of more electrons to push through a circuit with a single power source. This also means less Amps, but we will get into that later.

Why do we need to measure voltage?

In automotive in general and now more than ever, we are heavily reliant on electrical power to start our circuits, power our computers, run sensors, send messages between systems, operate safety and infotainment systems and as always produce spark at the motor. Voltage in a vehicle is now just a part of every day life in our world and it will only become more integral and important to understand it and it’s relationship to the operation of vehicles. In most cases a component (motor, light, switch, circuit or other set of circuits) in a vehicle with be listed and rated with a voltage, 12V motor fan for example or a 5V computer circuit or 115V power port. As already described the V number represents the amount of work that a circuit can do and in measuring voltage on vehicles circuits (and knowing what the voltage value is supposed to be) can help us determine the operating status of that circuit as well as indications towards its health (we can know if the circuit is operating normally or out of parameter).

How to measure voltage?

With your multimeter such as an N2 Neuron there are two ports on the meter (just like the two poles on a vehicles battery) that are for (V) Voltage and (C) Common or Earth (Often identified as C) for the Ground or (-) measurements. The two ports on the meter connected to the circuit’s positive and negative sides can help us measure the voltage of the circuit.

N2 Neuron with Fan Circuit, N2 Neuron Measuring voltage, how to measure voltage, how to use n2 neuron, what is n2

What if my voltage measurement isn't what is it should be?

Voltage measurements on a vehicles circuit can be different from what you expected for a number of reasons. In most cases we need a wiring diagram to be absolutely sure that we are looking at and testing the correct sides of a circuit and to know what it is connected to, but there are general rules and logic that we can follow when testing for voltage. As seen in the figure below, when testing a vehicles circuit the ground side should always test at a "0" or between 0 and .5 Volts. this indicated a "Good" ground (more on testing this later) and shows that power should be able to flow through this side of the circuit uninhibited. If there is voltage on the ground side of a circuit it may be an indication that there is a bad ground on the circuit and that when powering up that circuit operation may be faulty, seemingly low and create sporadic operating conditions. On the power side we should almost always see 12V (if we are operating on a 12VDC system or unless otherwise indicated for sub systems). If we see voltage lower than 12 Volts on the power side of a circuit it may indicate that there is a problem between that point of the circuit and the positive post of the vehicles battery, there may be a problem with the connections, wires, switches OR there may be a problem with the vehicles battery itself and therefore it is always important to test the battery first before attempting to diagnose any other electrical issues in a vehicle.

how to measure battery voltage using meter on engine fan


It is important to confirm a vehicles battery (and charging system) is operating correctly before checking for any other electrical issues in the vehicle as the "chain of custody" of issues can begin here, at the power source. If the battery is not correctly operating and sending power through the system, all other systems will be affected. Additionally if charging system is causing faulty charing of the battery system it can cause issues in the rest of the system when the charging system is operating or attempting to charge the vehicles battery.

Is Voltage the Only thing I need to know?

Unfortunately voltage is not the only value of a circuit we need to be aware of in making determination about its health and operating status. Other factors we need to be aware of in a circuit that allow us to determine if it is within “spec” or correct operating parameters  (specifications) are Ohms (resistance) notated as Ω and Amps notated as A. Together all 3 of these readings provide us with the ability to fill in ohm’s law (V=IR) calculations understandings about the circuit (More on this later). Briefly, Amps is the current flow of electrons from the negative side of the battery to the positive. Resistance is the opposition or “road block” to the current flow (of electrons) in the circuit and we will definitely be getting into this more in the next set of articles. Together when we know just 2 of any of the values of ohm’s law we can calculate the missing 3rd value (using math) and make determinations about a circuits operation. For example if we only know the voltage of a circuit and the amperage draw on that circuit we can calculate the resistance of the circuit and this will help us identify if there the circuit is in good “spec” OR if the circuit has more resistance on it than is supposed to be. *Note that resistance can only be measured on a circuit without voltage flowing on it as it is quite literally the measure of how much “Blockage” is between two points on a circuit and any voltage flow (Other than that from the testing device) will disrupt the true value. High resistance on a circuit can lead to very hot circuits causing fires in some cases, low voltage to the intended components and thusly operating effectiveness of that component or operation all together, miscommunication between sensors and computers, mis communication between computer systems and eventual failure of the circuit overall depending on the cause of the resistance.

Understanding the aspects of a vehicles electrical system is helpful when trying to solve problems related to systems that come in for repair in the shop.

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