Ohm’s Law for Electronics Repair
Ohm’s law is one of the most useful relationships in electronics repair because it links voltage, current and resistance. Once two values are known, the third can be calculated. Add the power equations and the same measurements can also tell you how much heat a resistor, load or circuit section is dissipating.
For quick calculations, use our free Ohm’s Law & Power Calculator.
The three basic Ohm’s law equations
Voltage: V = I × R
Current: I = V ÷ R
Resistance: R = V ÷ I
Voltage is measured in volts, current in amps and resistance in ohms. In repair work, current is often measured in milliamps, so remember that 1000 mA = 1 A.
Worked example: current through a resistor
Suppose 12 V is applied across an 8 Ω resistive load. Current is 12 ÷ 8 = 1.5 A. The power dissipated is 12 × 1.5 = 18 W. A resistor used in that position would therefore need a power rating comfortably above 18 W, not exactly 18 W.
Power equations
Electrical power can be calculated several ways:
- P = V × I
- P = I² × R
- P = V² ÷ R
These forms are useful when you cannot measure every quantity directly. For example, if you know the voltage drop across a resistor and its resistance, V² ÷ R gives the resistor’s power dissipation without separately measuring current.
Using voltage drop for fault-finding
A voltage drop across a known resistor can be turned into a current measurement. If a 1 Ω resistor has 50 mV across it, Ohm’s law gives 0.05 V ÷ 1 Ω = 0.05 A, or 50 mA. This is why low-value current-sense resistors are common in amplifiers and power supplies.
Why resistance measurements can mislead in-circuit
A resistance reading taken with the component still connected may include parallel paths through other components. If a resistor appears lower than expected, the resistor itself may be correct and the meter may simply be seeing another path. Lifting one end of the component can isolate it for a more meaningful measurement when appropriate.
DC calculations versus AC impedance
Basic Ohm’s law is straightforward for DC and purely resistive loads. Loudspeakers, transformers, capacitors and inductors behave differently with AC because their impedance changes with frequency. For speaker cabinet work, use our Speaker Impedance Calculator rather than treating a loudspeaker as a fixed resistor.
Power rating is not the same as resistance
A 1 kΩ 0.25 W resistor and a 1 kΩ 5 W resistor have the same nominal resistance but very different abilities to dissipate heat. Always select both the correct resistance and a suitable power rating for the circuit.
Bench safety
Calculations do not make a circuit safe to probe. Mains equipment and valve amplifiers can contain hazardous or lethal voltages. Measure only where you are competent to do so and use appropriately rated test equipment.
Need the quick version? Open the Ohm’s Law & Power Calculator, or contact Electronic Repairs UK if you need help diagnosing faulty equipment.