Inductor DC Resistance (DCR)
Although an ideal inductor has no resistance, every real inductor contains copper wire that has a small electrical resistance. This resistance is known as DC Resistance (DCR). It causes voltage drop, power loss and heat whenever current flows through the inductor. Understanding DCR is essential when selecting inductors for power supplies, DC-DC converters, audio equipment and other high-current electronic circuits.
What Is DC Resistance?
DC resistance is the resistance of the inductor's winding when measured using direct current (DC). It is caused by the electrical resistance of the copper wire and is measured in ohms (Ω).
Unlike inductive reactance, which depends on frequency, DC resistance is present even at 0 Hz (pure DC).
Why DCR Matters
Every ampere of current flowing through the winding produces a voltage drop and generates heat. Lower DCR improves efficiency, reduces temperature rise and allows the inductor to carry higher current safely.
| Low DCR | High DCR |
|---|---|
| Higher efficiency. | Lower efficiency. |
| Lower voltage drop. | Higher voltage drop. |
| Less heat generation. | More heat generation. |
| Better high-current performance. | Reduced current capability. |
Voltage Drop Across the Winding
When DC current flows through the winding, the voltage lost is determined by Ohm's law.
Voltage Drop = Current × DCR V = I × R
Example:
Current = 5 A DCR = 0.04 Ω Voltage Drop = 5 × 0.04 = 0.2 V
Power Dissipation
The winding also converts electrical energy into heat.
Power Loss = I² × R
Example:
Current = 5 A DCR = 0.04 Ω Power Loss = 5² × 0.04 Power Loss = 1 W
Higher current increases power loss rapidly because it depends on the square of the current.
Factors Affecting DCR
| Factor | Effect |
|---|---|
| Wire Diameter | Thicker wire lowers DCR. |
| Wire Length | Longer wire increases DCR. |
| Copper Purity | Higher purity reduces resistance. |
| Temperature | Resistance increases as temperature rises. |
| Number of Turns | More turns generally increase DCR. |
Typical DCR Values
| Inductor Type | Typical DCR |
|---|---|
| RF Inductor | Less than 1 Ω |
| SMD Power Inductor | 5–200 mΩ |
| Buck Converter Inductor | 10–100 mΩ |
| Audio Crossover Inductor | 0.1–1 Ω |
| Industrial Power Choke | Depends on size and current rating. |
Measuring DC Resistance
- Digital multimeter (low-resistance range).
- Four-wire (Kelvin) resistance meter for high accuracy.
- LCR meter with DCR measurement.
- Precision ohmmeter.
For very low resistance values, a four-wire measurement provides much greater accuracy because it eliminates the resistance of the test leads.
Applications Where DCR Is Critical
| Application | Importance |
|---|---|
| DC-DC Converters | High efficiency and low heat. |
| CPU Voltage Regulators | High-current operation. |
| Battery Chargers | Reduced power loss. |
| Solar Power Systems | Improved energy efficiency. |
| Electric Vehicles | High-current power conversion. |
Selecting an Inductor Based on DCR
- Choose the lowest practical DCR for high-current circuits.
- Ensure the current rating exceeds the maximum operating current.
- Consider both DCR and saturation current together.
- Balance low resistance against size and cost.
- Verify DCR in the manufacturer's datasheet.
Common Misconceptions
| Myth | Reality |
|---|---|
| An ideal inductor has measurable DCR. | An ideal inductor has zero resistance. DCR exists only in real inductors. |
| Higher inductance always means higher DCR. | Core size, wire gauge and construction also affect DCR. |
| Inductance alone determines efficiency. | DCR is a major contributor to copper losses. |
Interesting Facts
- Modern high-current power inductors often have DCR values below 10 milliohms.
- Large wire diameters reduce DCR but increase component size.
- Copper resistance increases as temperature rises.
- DCR is one of the most important specifications for switch-mode power supplies.
- Manufacturers often specify DCR as a maximum value rather than a typical value.
Key Points
- DCR is the resistance of an inductor's winding.
- It causes voltage drop, heat and power loss.
- Lower DCR improves efficiency in high-current applications.
- Wire diameter, wire length and temperature all affect DCR.
- Always consider DCR together with inductance and saturation current when selecting an inductor.