Ohm's Law for AC Circuits

Article : Andy Collinson

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The familiar Ohm's Law triangle used for DC circuits can only be used at AC if the load is purely resistive. Most AC circuits however, contain series or parallel combinations of resistance, capacitance and inductance. This leads to the voltage and currents being out of phase and the load becomes complex. In purely capacitive circuits the current waveform leads the voltage waveform, whereas in inductive circuits the voltage will lead the current. Circuits containing both inductors and capacitors, the voltage and current waveform will not be in phase except at resonance. The general term for AC resistance is impedance and given the symbol Z. The impedance triangle is shown below:

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The familiar Ohm's Law triangle used for DC circuits can only be used at AC if the load is purely resistive. Most AC circuits however, contain series or parallel combinations of resistance, capacitance and inductance. This leads to the voltage and currents being out of phase and the load becomes complex. In purely capacitive circuits the current waveform leads the voltage waveform, whereas in inductive circuits the voltage will lead the current. Circuits containing both inductors and capacitors, the voltage and current waveform will not be in phase except at resonance. The general term for AC resistance is impedance and given the symbol Z. The impedance triangle is shown below:

The triangle is a used exactly the same as Ohm's Law at DC except that impedance now replaces resistance. It should be noted that when measuring ac voltages or currents, your meter will only indicate correct values over a limited frequency range. This is usually valid from DC up to 400Hz but can be found by checking the specifications for your meter.

For AC circuits where the voltage and current are in phase ONLY, the following pie chart may be used.