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Voltage Drop: Definition & Calculation

Instructor: Kip Ingram

Kip holds a PhD in Engineering from The University of Texas at Austin and was an occasional substitute lecturer in engineering classes at that institution.

Voltage drop specifies the amount of electric power produced or consumed when electric current flows through the voltage drop. Voltage drops can be measured in a circuit with an appropriate instrument, but also can be calculated in advance using appropriate equations.

What Is Voltage Drop?

Most of us understand that water at high elevation can be used to do work. For example, the water above a dam is at higher elevation than the water below the dam, and that water can be used to turn electrical generators as it moves downward through the elevation change.

The water above the dam has greater potential energy than the water below the dam, and it releases that energy for other purposes as it descends. The amount of energy delivered by each bit of water is precisely proportional to the elevation change that bit of water experiences.

'Voltage drop' refers to a similar process that occurs in electrical circuits. In circuits, flowing water is replaced by flowing electrical charge, also known as current, and elevation change is replaced by voltage drop.

Each point in a circuit can be assigned a voltage that's proportional to its 'electrical elevation,' so to speak. Voltage drop is simply the arithmetical difference between a higher voltage and a lower one. The amount of power (energy per second) delivered to a component in a circuit is equal to the voltage drop across that component's terminals multiplied by the current flow through the component:

P = V*I

Here V is the voltage drop in volts, I is the current flow in amperes, and P is the power in watts. Obviously, if either V or I is zero, no power or energy is delivered to that component, so it can't fulfill any useful purpose. So voltage drop (along with current flow) is a vital feature of all electric circuits and is planned and controlled very carefully by the engineers that design those circuits.

How Is Voltage Drop Calculated?

Voltmeters

One way to determine the voltage drop across a circuit component is to build the circuit and measure the drop, using a tool called a voltmeter. Voltmeters are designed to disturb the operation of the circuit they're connected to as little as possible. They achieve this by minimizing the current that flows through the voltmeter to the smallest possible value (i.e., they draw as little power as possible from the circuit).

If this were the only way to determine voltage drops, circuit design would be a very trial-and-error process. Fortunately, engineers can write equations based on the components that form the circuit and the manner in which they're connected.

KVL & KCL

The solution of these equations provides knowledge of all voltage drops and all current flows in the circuit. Engineers can then adjust the various component values to obtain a final circuit that serves its purpose in some optimal way (lowest noise, fastest speed, lowest total power consumption, etc.)

Equations expressing the connectivity of a circuit are based on either:

  • Kirchoff's Voltage Law (KVL) - states that the sum of voltage drop around any closed path in a circuit is zero. KVL equations are expressions of energy conservation.
  • Kirchoff's Current Law (KCL) - states that the total current flow into or out of any junction of wires in the circuit is zero. KCL equations are expressions of charge conservation.

KVL / KCL equations can be written for a circuit without regard for the nature of the actual components in the circuit - all that matters is the pattern of their interconnections (also known as the topology of the circuit). But KVL and KCL alone aren't enough and by themselves yield a system of equations that contains more unknown values than there are equations. Such an underspecified system doesn't have a unique solution.

Constitutive Equations

To resolve this problem, engineers also include the constitutive equation for each circuit component. Constitutive equations express the physics of the components themselves (without regard for how they are interconnected), and vary by component type.

For example, the constitutive equation for a resistor (V = I*R, known as Ohm's Law) is entirely different from one for an inductor or a capacitor. Inclusion of both the KVL / KCL equations and all of the constitutive equations always results in a system of equations that has a unique solution.

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