Resistors in AC Circuits

In the previous tutorials we have looked at resistors, their connections and used Ohm’s Law to calculate the voltage, current and power associated with them. In all cases both the voltage and current has been assumed to be of a constant polarity, flow and direction, in other words Direct Current or DC.

But there is another type of supply known as alternating current or AC whose voltage switches polarity from positive to negative and back again over time and also whose current with respect to the voltage oscillates back and forth. The oscillating shape of an AC supply follows that of the mathematical form of a “sine wave” which is commonly called a Sinusoidal Waveform. Therefore, a sinusoidal voltage can be defined as V(t) = Vmax sin ωt.

When using pure resistors in AC circuits that have negligible values of inductance or capacitance, the same principals of Ohm’s Law, circuit rules for voltage, current and power (and even Kirchoff’s Laws) apply as they do for DC resistive circuits the only difference this time is in the use of the instantaneous “peak-to-peak” or “rms” quantities.

When working with AC alternating voltages and currents it is usual to use only “rms” values to avoid confusion. Also the schematic symbol used for defining an AC voltage source is that of a “wavy” line as opposed to a battery symbol for DC and this is shown below.

Symbol Representation of DC and AC Supplies

ac and dc symbols


Resistors are “passive” devices, that is they do not produce or consume any electrical energy, but convert electrical energy into heat. In DC circuits the linear ratio of voltage to current in a resistor is called its resistance. However, in AC circuits this ratio of voltage to current depends upon the frequency and phase difference or phase angle ( φ ) of the supply. So when using resistors in AC circuits the term Impedance, symbol Z is the generally used and we can say that DC resistance = AC impedance, R = Z.

It is important to note, that when used in AC circuits, a resistor will always have the same resistive value no matter what the supply frequency from DC to very high frequencies, unlike capacitor and inductors.

For resistors in AC circuits the direction of the current flowing through them has no effect on the behaviour of the resistor so will rise and fall as the voltage rises and falls. The current and voltage reach maximum, fall through zero and reach minimum at exactly the same time. i.e, they rise and fall simultaneously and are said to be “in-phase” as shown below.

V-I Phase Relationship and Vector Diagram

volatge current phase relationship


We can see that at any point along the horizontal axis that the instantaneous voltage and current are in-phase because the current and the voltage reach their maximum values at the same time, that is their phase angle θ is 0o.  Then these instantaneous values of voltage and current can be compared to give the ohmic value of the resistance simply by using ohms law. Consider below the circuit consisting of an AC source and a resistor.

resistance in ac circuit


The instantaneous voltage across the resistor, VR is equal to the supply voltage, Vt and is given as:

ac resistor voltage


The instantaneous current flowing in the resistor will therefore be:

instantaneous resistor current


As the voltage across a resistor is given as VR = I.R, the instantaneous voltage across the resistor above can also be given as:

instantaneous resistor voltage


In purely resistive series AC circuits, all the voltage drops across the resistors can be added together to find the total circuit voltage as all the voltages are in-phase with each other. Likewise, in a purely resistive parallel AC circuit, all the individual branch currents can be added together to find the total circuit current because all the branch currents are in-phase with each other.

Since for resistors in AC circuits the phase angle φ between the voltage and the current is zero, then the power factor of the circuit is given as cos 0o = 1.0. The power in the circuit at any instant in time can be found by multiplying the voltage and current at that instant.

Then the power (P), consumed by the circuit is given as P = Vrms Ι cos Φ in watt’s. But since cos Φ = 1 in a purely resistive circuit, the power consumed is simply given as, P = Vrms Ι the same as for Ohm’s Law.

This then gives us the “Power” waveform and which is shown below as a series of positive pulses because when the voltage and current are both in their positive half of the cycle the resultant power is positive. When the voltage and current are both negative, the product of the two negative values gives a positive power pulse.

Power Waveform in a Pure Resistance

voltage current and power relationship


Then the power dissipated in a purely resistive load fed from an AC rms supply is the same as that for a resistor connected to a DC supply and is given as:

resistor power formula
  • Where:
  • P  is the average power in Watts
  • Vrms  is the rms supply voltage in Volts
  •  Irms   is the rms supply current in Amps
  • R  is the resistance of the resistor in Ohm’s (Ω) – should really be Z to indicate impedance

The heating effect produced by an alternating current with a maximum value of Imax is not the same as that of a DC current of the same value. To compare the AC heating effect to an equivalent DC the rms values must be used. Any resistive heating element such as Electric Fires, Toasters, Kettles, Irons, Water Heaters etc can be classed as a resistive AC circuit and we use resistors in AC circuits to heat our homes and water.

Resistors in AC Circuits Example No1

A 1000W heating element is connected to a 250v AC supply voltage. Calculate the impedance (AC resistance) of the element when it is hot and the amount of current taken from the supply.

ac resistance

Resistors in AC Circuits Example No2

Calculate the power being consumed by a 100Ω resistive element connected across a 240v supply.

As there is only one component connected to the supply, the resistor, then VR = VS

resistors in ac circuits example


Then to summarise, in a pure ohmic AC Resistance, the current and voltage are both said to be “in-phase” as there is no phase difference between them. The current flowing through the resistor is directly proportional to the voltage across it with this linear relationship in an AC circuit being called Impedance. As with DC circuits, Ohm’s Law can be used when working with resistors in AC circuits to calculate the resistors voltages, currents and power.


Join the conversation!

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  • S

    I heard that resistors also have a frequency does resistance change when we apply very high frequencies?

    • Wayne Storr

      The resistive value of most types of ceramic, carbon, metal film and pure resistors are not affected or changed by frequency and as such their is no frequency component associated with their calculations. Wirewound, power resistors and the other types of non-linear resistor that use wire in the form of a coil, are affected by frequency due to the inductive reactance of the coil. Resistors are affected more by variations in temperature than frequency.

  • A

    Why amplitude of V is greater than I when ac is connected with resistance ……

    • Wayne Storr

      The amplitude of the voltage across a resistance is determined by the supply, while the amplitude of the current flowing through it is determined by its resistance as given be Ohms Law.

  • G

    I have an AC to AC transformer giving 12volts Ac. How can I reduce the output to 5VoltsAC which I need. I can no longer find any such transformer that does the job.
    Many Thanks

  • A

    My switch board’s resistor got damaged so I used 1kohm resistor of 0.5watts and this resistor also got burnt so what kind of resistor shall I use now with their values please tell me

  • J
    JohnDale Daugherty

    I need a 100 ohm resistor on my motorcycle,it is of course, a 12v system.The local electronics store sell a100ohm resistor.If it is for AC can I still use it? If not where can I get whatI need? To night is my first study of resistors, so plz,don’t expect too much out of me? Thanks in advance

  • D

    thanks. your explanation make electronics very manageable

  • R

    Resistors do consume electrical energy. That is why they are used to drop voltage in a circuit. They consume this voltage and dissipate it as heat. My question, will a 100 ohm resistor drop the voltage in a AC circuit, the same amount as the drop in voltage in a DC circuit?

  • M
    Muhammad Ibrar Khan

    useful info. thanks

  • J
    John Cushard

    Thank you, had been an ambition of mine to one day teach basic electronics. It was where my heart always lay, but in life things sometimes go in unexpected directions. After many jobs and attempts to finish my degree… well I fell a few credits short. somehow I wound up playing electrician for over 30 years and raised a family. I made the occasional use of my electronics training, and built computers and other gadgets to keep my mind sharp. One day I’ll retire I told myself then I can pass what I know on. I waited too long and the fog of age stole away at my knowledge…advances dimmed the desire of students, the throw away society no longer seem to care why electronics worked, they simply bought a new device…. So to see your site was refreshing, perhaps there are the few, you have done well from what I have explored so far. Best wishes, John

    • T

      Well, that’s what exactly happened to me too. Graduated as Electronic Engineer who love working with Digital Circuits. Before graduated, went into programming and later took Master degree in software engineering. It’s been 13 years since I ever touch any circuit design and now all my knowledge has gone to waste. Whenever I think about it, I want to cry.

    • David Miles

      Hey John,
      There’s still plenty of people who enjoy learning the “how it works” part. I’m a hobbyist and in order to design/build circuits to suit certain needs, I have to first know how it works.
      There isn’t a product-for-purchase for every circuit one may need. The landscape has definitely changed and very few build computers from capacitors/transistors/resistors/etc. But there are new needs and challenges facing us and therefore the need for new circuitry. And there’s also the enjoyment of building something for fun. A painter will use the same palette of paint, yet will come up with a plethora of different pictures they can paint by changing how the combine the colors.
      You still have much to contribute I’m sure. Look for someone to mentor. While your mind may not be as sharp, it doesn’t mean you don’t know things that a student/hobbyist keeps getting stumped on. That’s the beauty of it in my opinion.

  • g
    gouse bala

    i love this

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