How to Calculate Voltage Drops Across Resistors

By Mark Stansberry

To pass electronic technician certification tests and to be able effectively fix electronic circuits, electronics students need to be able to calculate voltage drops across resistors. If you want to become an electrician, to get licensed, you will have to solve problems that involve calculating the voltage drop across a resistor and across resistive wires. But those are not the only ones who need this knowledge. Scientists, electrical engineers and even mechanical engineers often will have to calculate voltage drops across resistors.

Determine the voltage supply that powers the circuit. All resistors in electronic circuits are attached to a power supply, such as a battery. The higher the voltage level of the power supply that powers the resistors, the higher the current through the resistors and the higher the voltage drop across the resistors.

Determine the topology of the resistive circuit. Resistors are arranged in two basic types of topologies: the series circuit and the parallel circuit. When two resistors are connected in series to a battery: the positive battery terminal is connected to the left end of the first resistor, the right end of the first resistor is connected to the left end of the second resistor, and finally, the right end of the second resistor is connected to the negative terminal of the battery.

This series resistor circuit forms a closed loop such that electric current can flow from the positive terminal of the battery, through the resistors back into the negative terminal of the battery. In a series circuit the current throughout the entire loop is the same. The current that leaves the battery is the same as the current that enters the battery. And the current that flows through each resistor is the same.

If two resistors are connected in parallel to a battery, the batteryâ€™s positive terminal is connected to one end of the first resistor and one end of the second resistor. Similarly, the negative terminal of the battery is connected to the other end of the first resistor and the other end of the second resistor. Current flow in this type of arrangement is split between each resistor. In other words, the current flow is not always the same in resistors that are in parallel.

Calculate the total resistance of the circuit. If the circuit is a series circuit, add the resistance value of the two resistors together. If one resistor was 5 ohms and the other was 15 ohms, the total resistance of the series circuit would be 20 ohms.

To calculate the voltage drop in each of the resistors in the parallel circuit for this example, the total resistance is not needed.

Calculate the current flow through each circuit. The current through the circuit is found by dividing the total voltage applied to the circuit divided by its total resistance.

For the series circuit in this example, the voltage is 10 volts and the total resistance is 20 ohms, so the current through the resistor loop is 0.5 amperes, since 10 divided by 20 is 0.5.

For this example, the current through each of the resistors in the parallel circuit is not needed to calculate the voltage drop.

Calculate the voltage drops. The voltage drop in each of the series resistors is equal to the current though the loop multiplied by the resistor value. Since there is 0.5 amps through each resistor, the voltage drop across the first 5-ohm resistor is 2.5 volts, and the voltage drop across the second 15-ohm resistor is 7.5 volts. And thatâ€™s because 0.5 times 5 is 2.5 and 0.5 times 15 is 7.5. Notice that these two voltage drops add up to the battery supply voltage, 10 voltsâ€”as they should in a series circuit.

The voltage drop across resistors in parallel is always the same. For the parallel circuit, the voltage at one end of each of the resistors is 10 volts and the voltage at the other end of the resistor is at 0 volts. So the voltage drop across the each of the resistors is 10 volts, since 10 minus 0 is 10. Note that the current that flows from the battery of the 10-volt supply in the parallel circuit is equal to the battery voltage divided by the total resistance. Since the total resistance is 2/3 ohms and the battery voltage is 10 volts, the current is 15 amperes.

Tip

If the circuit is a parallel circuit, the total resistance would not be added. Instead, to obtain the total resistance, calculate the reciprocal of the first resistor, then add that to the reciprocal of the second resistor. Then take the reciprocal of that result. For example, if one of the resistors had a value of 1 ohm and the other had a value of 2 ohms, the reciprocal of the first resistor would be 1, since 1 divided by 1 is 1. The reciprocal of the second resistor would be 0.5 since 1 divided by 2, is 0.5. The sum of the reciprocals would be 1.5 or 3/2, since 1 plus 0.5 is 1.5. The total resistance of the parallel circuit would then be 2/3 since the reciprocal of 3/2 is 2/3, or 0.66 in decimal form.