How to Calculate a Current Through a Resistor

Written by jane tyne
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Calculating the current through a resistor is one of the first elements learnt in basic circuit analysis. Ohm's Law states that the current is equal to the voltage divided by the resistance. So the current through any resistor can be found by dividing the voltage across that resistor by its resistance. It's not always possible to measure current through a circuit without disturbing it, so you measure the voltage drop across the resistor and calculate the current. Here's how to find the current through a resistor in a simple series DC circuit.

Skill level:
Moderate

Things you need

• Multimeter
• Pencil and paper

Instructions

1. 1

Read the resistance, according to the value or colour code (see Resources) printed on the resistor. You may also measure the resistance with the ohmmeter function of your multimeter if the resistor can be disconnected from the circuit. Write down the resistance value. As an example, assume the value of the resistor, R1, is 250 ohms.

2. 2

Measure and record the voltage drop across the resistor, using the voltmeter function of your multimeter.

Assume a voltage drop of 50 volts across the resistor R1, as an example.

3. 3

Divide the measured voltage drop across the resistor by the resistance in ohms of the resistor to find the current through the resistor. In this example, find the current through R1 by dividing 50V by 250 ohms: 0.2 amps.

Tips and warnings

• Measure the voltage while the circuit is connected and on.
• Measure the resistance, if needed, while the circuit is off, and the resistor is disconnected.
• The values used in this example are only mathematical examples and do not reflect any real-world circuit values.
• Do not use the ohmmeter function while there is power to the circuit.
• Respect the polarity when making voltage measurements and connect positive to positive, negative to negative.

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