What is the Difference Between AC and DC Resistance?

What is the Difference Between AC and DC Resistance? . Electrical resistance is a fundamental concept in physics and electrical engineering, representing the opposition to the flow of current. Resistance occurs due to collisions between electrons and atoms in a conductor. Understanding resistance is crucial for designing efficient electrical systems.

Difference Between AC and DC Resistances – How to Calculate it?

Resistance The property of a substance or material which oppose the flow of electricity through it is called resistance OR,

Resistance is the ability of a circuit or element (which is called resistor) to oppose the flow of current through it.

Examples of Resistors with the ability of high resistance are Wood, Air, Mica, Glass, Rubber, Tungsten etc.

Unit of Resistance is “Ohm” and it is denoted by Ω and it is represented by “R”.

Difference-between-AC-and-DC-Resistance-and-How-to-calculate-it

What is DC Resistance?

We know that there is no concept of Inductive and capactive reactances in DC Circuits. i.e. capacitive and inductive reactances in DC circuits zero because there is no frequency in DC circuits, i.e. magnitude of DC current is constant. Therefore, only the original resistance of wire comes into play.

Good to know:

That’s why the resistance offered by a wire is lower for DC than AC as will as AC lines need more insulation than DC.

Measuring DC Resistance

Electrical Resistance Formulas

In DC Circuits, we calculate the resistance by Ohm’s Law.

R = V/I.

Good to know:

When solving electric circuits for finding resistance and you are not sure which one should you take into account whether AC or DC resistances, then, if the current passed is AC, then take AC resistance else if the current passed is DC, take DC resistance.

Applications of DC Resistance in Circuits

DC resistance plays a critical role in:

Power supplies: Ensuring minimal voltage drops.
Battery systems: Preventing overheating.
Resistive components: Such as heaters and incandescent bulbs.

What is AC Resistance?

In Simple words, Resistance in AC circuits is called Impedance. Or

The Overall resistance (Resistance, Inductive reactance and Capacitive reactance) in AC circuits is called Impedance (Z).

Explanation:

When AC Current pass through a wire (resistor, inductor, capacitor), then current produces a magnetic field across that wire which opposes the flow of AC Current in it along with the resistance of that wire. This oppose cause is called Inductance or Inductance is the property of Coil (or wire) due to which opposes any increase or decrease of current or flux through it. Also, we know that inductance is only exist in AC because the magnitude of current continuously changing

Inductive Reactance X_L, is the property of coil or wire in an AC circuit which opposes the change in the current. The unit of Inductive reactance is same as Resistance, capacitive reactance i.e. Ohm (Ω) but the representative symbol of capacitive reactance is X_L.

Likewise,

Capacitive Reactance in a capacitive circuit is the opposition to current flow in AC circuits only. The unit of capacitive reactance is same as Resistance, Inductive reactance i.e. Ohm (Ω) but the representative symbol of capacitive reactance is X_C.

Measuring AC Resistance

Electrical Resistance & Impedance Formulas in AC Circuits

In AC Circuits (Capacitive or inductive Load), Resistance = Impedance i.e., R = Z

Z = √ (R² + X_L²)… In case of Inductive Load

Z = √ (R² + X_C²)…In case of Capacitive Load

Z = √ (R² + (X_L– X_C)²…In case of both inductive and capacitive Loads.

Good to know:

Where;

X_L= Inductive reactance

X_L= 2πfL…Where L = Inductance in Henry

And;

X_C = Capacitive reactance

X_C = 1/2πfC… Where C = Capacitance in Farads.

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Applications of AC Resistance in Circuits

AC resistance is essential for:

Power transmission lines: Managing impedance losses.
Transformers: Optimizing core and winding resistances.
RF circuits: Minimizing losses in high-frequency systems.

Basic Principles of Electrical Resistance

Both AC and DC resistance depend on:

Material Conductivity: Good conductors like copper have lower resistance.
Conductor Geometry: Longer and thinner wires exhibit higher resistance.
Temperature: Resistance typically increases with temperature for most materials.

Relationship between DC and AC Resistance

From the characteristics described above, we can summarize the relationship between DC and AC resistance as follows:

At very low frequencies, AC resistance ≈ DC resistance in reverse bias
As frequency increases, AC resistance decreases while DC resistance remains unchanged
DC resistance represents a lower limit for AC resistance but AC resistance can never fall below DC resistance
At very high frequencies, AC resistance approaches dynamic resistance which is higher than DC resistance
DC resistance is determined by the bulk resistance of the diode material
AC resistance depends on frequency and junction capacitance along with bulk resistance

Therefore, we can conclude that AC resistance is always greater than or equal to DC resistance for a diode. But the exact relationship varies depending on the frequency. The AC resistance equals the DC value only under static conditions at low frequencies. As frequency increases, the AC resistance starts decreasing due to capacitive effects while the DC resistance remains constant.

How Does Resistance Differ in AC and DC Circuits?

The primary difference lies in the nature of the current:

DC Circuits: Resistance is static and linear.
AC Circuits: Resistance includes reactance, a component caused by inductance and capacitance. This combination is called impedance.

Which One is More – AC or DC Resistance ?

As we know that frequency in DC supply is zero, so there is no skin effect (a behavior of alternating current to flow through the surface i.e. outer layer of a conductor instead of the core of the wire). in DC circuits. Due to the skin effect, AC resistance are more in AC circuits than DC supply in DC circuits.

Skin Effect Formula

δ = √(2ρ/ωµ)

Where;

δ = Skin effect depth
ρ = Specific resistance
ω = 2πf = Angular frequency
µ = Permeability of the conductor

In short, frequency is directly proportional to the skin effect i.e. if frequency increase, skin effect also increase where, there is no frequency and skin effect in DC.

As a rule of thumb;

AC Resistance = 1.6 x DC Resistance

Read More : How to Read Capacitor Value?

What causes the difference between DC and AC resistances?

The key factors that cause the DC and AC resistances to be different are:

1. Junction Capacitance

Every PN junction has an inherent junction capacitance which depends on the area of the junction, doping levels and voltage applied.
In reverse bias, the depletion region at the junction acts as the dielectric of a parallel plate capacitor causing capacitance.
Under AC, this capacitance provides a reactive path for the current to flow by charging and discharging.
Hence, AC resistance decreases with increase in frequency due to capacitive reactance.

2. Minority Carrier Injection

Under DC, only majority carriers contribute to conduction which depends on the bulk resistance.
But in AC, minority carriers also get injected into the junction when it is forward biased during one half cycle.
These extra carriers increase the conductivity and lower the dynamic resistance under AC conditions.

3. Temperature Effects

DC resistance has a positive temperature coefficient – increases with temperature due to higher lattice vibrations.
But AC resistance and capacitance are negatively affected by temperature rise.
So heating causes DC resistance to increase but AC resistance decreases due to reduced capacitive reactance.

4. Non-linear I-V Characteristics

The diode does not follow Ohm’s law. Instead it has an exponential I-V relationship in forward bias.
So AC resistance becomes dependent on the operating point unlike the DC case.
Significant non-linearity causes DC and AC resistances to diverge.

5. Transit Time Effects

At high frequencies, the diode’s transit time for carriers starts affecting the AC resistance.
Transit time acts as a small inductance, thereby increasing impedance.
Thus transit time effects also contribute to the difference between DC and AC resistances.

Temperature Effects on AC and DC Resistance

Temperature changes can:

Increase resistance for conductors like copper.
Decrease resistance for materials such as carbon.

The resistance-temperature relationship is governed by the temperature coefficient of resistivity.

FAQs About AC and DC Resistance

Q1. Why does AC resistance decrease with increase in frequency?

Ans: AC resistance decreases with increase in frequency because of the junction capacitance of the diode. At higher frequencies, the capacitive reactance XC becomes low, providing an alternate lower resistance path for AC current to flow through capacitance by charging and discharging effect.

Q2. Is AC resistance affected by temperature?

Ans: Yes, AC resistance is affected by temperature. Increase in temperature causes the junction capacitance to decrease due to increase in intrinsic carrier concentration. This causes the AC resistance to increase with temperature.

Q3. Is AC resistance higher or lower than DC resistance?

Ans: AC resistance is always higher than or equal to DC resistance. It equals DC resistance only at very low frequencies when capacitive effects are negligible. At higher frequencies, AC resistance becomes lower than DC resistance due to capacitive reactance but never falls below the DC resistance value.

Q4. Does a diode follow Ohm’s law?

Ans: No, a diode does not follow Ohm’s law. It has an exponential I-V relationship in forward bias due to its PN junction properties. This non-linear V-I curve causes the AC resistance to become dependent on the operating point.

Q5. Why does transit time affect AC resistance?

Ans: At high frequencies, the diode’s transit time for charge carriers starts affecting the AC resistance. Transit time acts as a small inductance that increases impedance and hence increases AC resistance at very high frequencies.

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What is the Difference Between AC and DC Resistance?