How to Measure an Inductor?

Accurately determining the inductance of a coil is crucial for electronics enthusiasts and professionals alike. Whether you’re verifying component values, troubleshooting circuits, or designing new projects, knowing the precise inductance (measured in Henrys) is essential for optimal performance. Various methods and tools can be employed to measure an inductor, each with its own level of accuracy and complexity.

This guide will walk you through the fundamental techniques for measuring inductance. We’ll explore the use of specialized instruments like LCR meters and impedance analyzers, as well as simpler methods involving multimeters and basic circuit setups. Understanding these approaches will empower you to confidently assess the inductive properties of your components.

What Are Inductors

Inductors are passive electrical components that store energy in a magnetic field when an electric current flows through them. Typically, an inductor consists of a coil of wire wound around a core, which can be air or a ferromagnetic material like iron or ferrite. The coiled structure and the core material work together to enhance the magnetic field generated by the current, thereby increasing the inductor’s ability to store energy magnetically.

A key characteristic of an inductor is its inductance, measured in Henrys (H), which quantifies its opposition to changes in current. According to Lenz’s Law, when the current through an inductor changes, it induces a voltage (electromotive force or EMF) that opposes this change. This property makes inductors crucial in various electronic circuits for tasks such as filtering signals, storing energy in power supplies, and forming resonant circuits when combined with capacitors.

How to Measure Inductors

Accurately measuring the inductance of a component is a fundamental skill in electronics, whether you’re troubleshooting a circuit, verifying a component’s value, or designing a new project. Several methods and tools are available for this purpose, ranging from specialized instruments to more basic techniques.

Understanding these approaches will allow you to determine the inductance of your coils effectively and ensure your circuits perform as intended.

Tools Needed

Before diving into the measurement methods, let’s outline the essential tools you might need:

• LCR Meter: This is the most direct and accurate instrument for measuring inductance (L), capacitance (C), and resistance (R). It typically applies a test signal to the component and measures the impedance at a specific frequency to determine the inductance. LCR meters range from handheld portable units to more sophisticated benchtop models.
• Impedance Analyzer: A more advanced instrument capable of measuring impedance over a wide range of frequencies. It provides detailed information about the inductor’s behavior across different frequencies and is often used for component characterization and analysis.
• Multimeter with Inductance Measurement Capability: Some digital multimeters (DMMs) come equipped with a built-in inductance measurement function. While generally less accurate than dedicated LCR meters, they can provide a quick and convenient way to get an approximate inductance value.
• Oscilloscope and Signal Generator: These tools can be used together to measure inductance indirectly by analyzing the inductor’s behavior in a test circuit, such as an RL or RLC circuit. This method requires more calculations but can be useful when a dedicated inductance meter is unavailable or for analyzing frequency-dependent behavior.
• Known Resistor and Capacitor: For indirect measurement methods using an oscilloscope and signal generator, you’ll need precision resistors and capacitors with known values to create the test circuit.

Using an LCR Meter

An LCR meter is the most straightforward and accurate tool for measuring inductance. To use one, first ensure the meter is calibrated and set to the inductance (L) measurement mode. Connect the inductor to the appropriate terminals on the LCR meter, paying attention to the polarity if the inductor has one (though most simple inductors are non-polarized).

Once the inductor is connected, select the desired test frequency on the LCR meter. The inductance of a component can vary with frequency, so choosing a frequency relevant to your application is important for obtaining a meaningful measurement. After setting the frequency, initiate the measurement. The LCR meter will apply a test signal and display the measured inductance value, often along with other parameters like the Q-factor (quality factor) and series resistance.

Using a Multimeter with Inductance Function

How to measure an inductor with a multimeter?

Some digital multimeters (DMMs) include a function for measuring inductance. To use this feature, select the inductance (L) measurement mode on the multimeter, typically indicated by an ‘L’ symbol or ‘mH’ for millihenries. Connect the inductor to the designated test jacks on the multimeter, ensuring good electrical contact.

Once the inductor is connected, the multimeter will apply a test signal and display the measured inductance value on its screen. Keep in mind that the accuracy and frequency range of inductance measurements on a multimeter are generally lower compared to dedicated LCR meters. However, for many basic applications and quick checks, a multimeter with inductance capability can provide a sufficiently accurate reading.

Indirect Measurement with Oscilloscope and Signal Generator

Inductance can be measured indirectly using an oscilloscope and a signal generator by analyzing the inductor’s behavior in a known circuit. One common method involves creating an RL circuit by connecting the inductor in series with a known resistor. Apply a square wave signal from the signal generator to the RL circuit and observe the voltage waveform across the resistor on the oscilloscope.

The time constant (τ=L/R) of the RL circuit can be determined from the exponential rise or fall of the voltage waveform across the resistor (the time it takes to reach approximately 63.2% of its final value). Once the time constant and the resistance (R) are known, the inductance (L) can be calculated using the formula L=τ×R. Ensure the frequency of the square wave is low enough to allow the circuit to reach steady state during each pulse.

Another indirect method involves creating a resonant RLC circuit with a known resistor and capacitor in addition to the inductor. By finding the resonant frequency of the circuit (the frequency at which the impedance is minimum or maximum, depending on the configuration), and knowing the values of R and C, the inductance L can be calculated using the formula for resonant frequency: fr​=2πLC​1​, which can be rearranged to L=(2πfr​)2C1​. You can find the resonant frequency by sweeping the frequency of the signal generator and observing the point of maximum (or minimum) impedance on the oscilloscope.

How to Measure DC Resistance of Inductor

Accurately measuring the DC resistance (DCR) of an inductor is important for understanding its power losses and overall performance in a circuit. Unlike measuring inductance, which involves AC signals and frequency dependence, DCR is a purely resistive measurement at 0 Hz (DC). It represents the resistance of the wire used to wind the inductor’s coil. Here’s how to measure it effectively.  

Tools Needed

The primary tool needed for measuring the DC resistance of an inductor is a multimeter capable of measuring low resistance values accurately.

• Digital Multimeter (DMM): A DMM with a low ohms range (typically capable of measuring fractions of an ohm) is essential. For very low DCR values (in the milliohm range), a more sensitive multimeter or a dedicated low-resistance ohmmeter might be required.  
• Kelvin Clips (Four-Wire Measurement – Optional but Recommended for Low Values): For highly accurate measurements of very low DCR, using Kelvin clips or a four-wire measurement technique can minimize the impact of lead and contact resistance.  
• Test Leads: Standard multimeter test leads are sufficient for most inductors with DCR in the ohm range.

Using a Standard Multimeter

The most common way to measure the DC resistance of an inductor is using a standard digital multimeter:

1. Select the Resistance Mode: Turn on your multimeter and select the resistance (Ω) measurement mode. Choose the lowest resistance range available on your meter to obtain the best possible resolution, especially if you suspect the DCR is low. Some multimeters have an auto-ranging function that will automatically select the appropriate range.  
2. Connect the Test Leads: Connect the multimeter’s test leads to the terminals of the inductor. Ensure good electrical contact to get an accurate reading. If the inductor is part of a circuit, it’s generally best to remove it from the circuit to avoid the influence of other components that might provide parallel resistance paths, leading to an incorrect measurement.
3. Read the Resistance Value: Once the leads are connected and the reading stabilizes, the multimeter will display the DC resistance of the inductor. Note down this value. Be aware that the resistance reading might fluctuate slightly, especially for very low values, due to contact resistance in the probes and connections.  
4. Consider Lead Resistance: For inductors with very low DCR (close to or below 1 ohm), the resistance of your multimeter’s test leads themselves can become a significant portion of the measurement. Some advanced multimeters have a “relative” or “zero” mode that can be used to subtract the lead resistance. To use this, short the test leads together and press the “relative” button to zero the reading before measuring the inductor.  

Using an LCR Meter for DCR Measurement

While LCR meters are primarily used for measuring inductance, capacitance, and AC resistance/impedance, many also have a function to measure DC resistance:  

1. Select DC Resistance Mode: Turn on your LCR meter and select the DC resistance (often labeled as DCR or Rdc) measurement mode.
2. Connect the Inductor: Connect the inductor to the appropriate terminals on the LCR meter. Again, ensure good electrical contact.
3. Read the DCR Value: Once connected, the LCR meter will apply a DC signal and measure the resistance. The displayed value will be the DC resistance of the inductor. LCR meters often offer better accuracy and resolution for low resistance measurements compared to standard multimeters.  

Four-Wire (Kelvin) Measurement (Advanced)

For the most accurate measurement of very low DCR, especially in the milliohm range, the four-wire Kelvin measurement technique is recommended. This method eliminates the error caused by the resistance of the test leads and contact resistances:  

1. Use Kelvin Clips: Connect the four Kelvin clips to the inductor’s leads. Each clip has two contacts: one for sourcing current and one for measuring voltage.  
2. Connect to the Meter: Connect the current-carrying leads to a stable current source (some specialized low-resistance meters have this built-in). Connect the voltage-sensing leads to a sensitive voltmeter or the voltage-measuring inputs of a four-wire capable ohmmeter or LCR meter.
3. Apply a Known Current: Apply a known, stable DC current through the inductor using the current source leads.
4. Measure the Voltage Drop: Simultaneously measure the voltage drop across the inductor using the voltage-sensing leads.
5. Calculate Resistance: Use Ohm’s law (R=V/I) to calculate the DC resistance of the inductor, where V is the measured voltage drop and I is the known current.  

By using these methods, you can accurately determine the DC resistance of an inductor, which is a crucial parameter for analyzing its performance and losses in various electronic applications. Remember that the DCR is primarily determined by the length and gauge of the wire used in the coil and can vary slightly with temperature.

How to Measure Inductance of an Inductor

Accurately determining the inductance of a component is a fundamental skill in electronics, whether you’re troubleshooting a circuit, verifying a component’s value, or designing a new project. Several methods and tools are available for this purpose, ranging from specialized instruments to more indirect techniques. Understanding these approaches will allow you to measure the inductance of your coils effectively and ensure your circuits perform as intended.

Tools Needed

Before diving into the measurement methods, let’s outline the essential tools you might need:

• LCR Meter: This is the most direct and generally the most accurate instrument for measuring inductance (L), capacitance (C), and resistance (R). It typically applies a test signal to the component and measures the impedance at a specific frequency to determine the inductance value. LCR meters range from handheld portable units to more sophisticated benchtop models offering higher accuracy and more features.
• Impedance Analyzer: A more advanced instrument capable of measuring impedance over a wide range of frequencies. It provides detailed information about the inductor’s behavior across different frequencies, including inductance, impedance magnitude, phase angle, and quality factor (Q). Impedance analyzers are often used for thorough component characterization and analysis in research and development settings.
• Multimeter with Inductance Measurement Capability: Some digital multimeters (DMMs) come equipped with a built-in inductance measurement function. While generally less accurate and with a limited frequency range compared to dedicated LCR meters, they can provide a quick and convenient way to get an approximate inductance value for many common inductors.
• Oscilloscope and Signal Generator: These tools can be used together to measure inductance indirectly by analyzing the inductor’s behavior in a test circuit, such as an RL or RLC circuit. This method requires understanding circuit theory and performing calculations but can be useful when a dedicated inductance meter is unavailable or for analyzing frequency-dependent behavior in specific circuit conditions.
• Known Resistor and Capacitor: For indirect measurement methods using an oscilloscope and signal generator, you’ll need precision resistors and capacitors with known values to create the test circuit. The accuracy of your inductance measurement will depend on the tolerance of these reference components.

Using an LCR Meter

An LCR meter is the most straightforward and commonly used tool for measuring inductance directly. Here’s how to use one effectively:

1. Select Inductance Mode: Turn on the LCR meter and select the inductance (L) measurement mode. This is usually indicated by an ‘L’ symbol on the meter’s display or function selector. Ensure the meter is properly calibrated according to the manufacturer’s instructions for accurate readings.
2. Connect the Inductor: Connect the inductor to the appropriate terminals on the LCR meter. Many LCR meters have dedicated terminals or test leads for component measurement. Ensure a secure and clean connection to minimize contact resistance, which can affect the accuracy of low impedance measurements, especially at higher frequencies. Note if the inductor has any specific connection requirements or polarity markings, although most basic inductors are non-polarized.
3. Set the Test Frequency: Select the desired test frequency on the LCR meter. The inductance of a component can vary with the frequency of the applied test signal, especially if the inductor has significant parasitic capacitance or core losses. Choosing a test frequency that is relevant to the inductor’s intended application will yield the most meaningful measurement. Common test frequencies include 100 Hz, 1 kHz, 10 kHz, and 100 kHz.
4. Initiate Measurement and Read Value: Once the inductor is connected and the test frequency is set, initiate the measurement on the LCR meter. The meter will apply a test signal and display the measured inductance value on its screen, typically in Henrys (H), millihenrys (mH), or microhenrys (µH). Many LCR meters also display secondary parameters such as the Q-factor (quality factor), series resistance (Rs), or parallel resistance (Rp), which can provide additional insights into the inductor’s performance.

Using a Multimeter with Inductance Function

Some digital multimeters (DMMs) offer a built-in inductance measurement capability, providing a convenient but generally less precise alternative to LCR meters:

1. Select Inductance Mode: Turn on your multimeter and select the inductance (L) measurement mode. This is usually indicated by an ‘L’ symbol or a unit of inductance (like mH or µH) on the function selector. Be sure to consult your multimeter’s manual for the specific steps to select this mode, as it can vary between models.
2. Connect the Inductor: Connect the inductor to the designated test jacks on the multimeter. These jacks are often labeled with an ‘L’ or shared with other measurement functions. Ensure good electrical contact between the inductor’s leads and the test leads or jacks of the multimeter.
3. Read the Inductance Value: Once the inductor is connected, the multimeter will apply a test signal (often at a fixed, internal frequency) and display the measured inductance value on its screen. The reading will typically be in millihenrys (mH) or microhenrys (µH). Be aware that the accuracy and the fixed test frequency of inductance measurements on a multimeter can limit its applicability for certain types of inductors or applications requiring precise measurements at specific frequencies.

Indirect Measurement with Oscilloscope and Signal Generator

When a dedicated inductance meter is not available, or when you need to analyze an inductor’s behavior in a dynamic circuit, you can use an oscilloscope and a signal generator for indirect measurement:

1. Create an RL Circuit: Connect the inductor (L) in series with a known, non-inductive resistor (R). The value of the resistor should be chosen such that the time constant of the RL circuit (τ=L/R) is within a measurable range for your oscilloscope and signal generator’s frequency capabilities.
2. Apply a Square Wave Signal: Use the signal generator to apply a square wave signal to the RL circuit. The frequency of the square wave should be low enough to allow the current in the inductor to reach its steady-state value during the “on” portion of the pulse and to decay significantly during the “off” portion.
3. Observe Voltage Across the Resistor: Use the oscilloscope to measure the voltage waveform across the resistor. The shape of this waveform will reflect the current flowing through the inductor. When a square wave is applied, the current in the inductor (and thus the voltage across the resistor) will rise exponentially with a time constant τ=L/R during the pulse and decay exponentially with the same time constant when the pulse goes low.
4. Determine the Time Constant: Measure the time it takes for the voltage across the resistor to reach approximately 63.2% (which is 1−1/e) of its final (steady-state) value during the rising edge of the square wave (or to fall to 36.8% (which is 1/e) of its initial value during the falling edge). This time is equal to the time constant τ of the RL circuit.
5. Calculate Inductance: Once you have determined the time constant τ and you know the value of the resistor R, you can calculate the inductance L using the formula: L=τ×R. Ensure that the resistor you use is indeed non-inductive to avoid affecting the circuit’s time constant.

Another indirect method involves creating a resonant RLC circuit and finding its resonant frequency, but this requires knowing the capacitance value accurately as well and is more involved. The RL transient response method is often more practical for estimating inductance with basic lab equipment.

Conclusion

In conclusion, accurately measuring inductance is a fundamental skill for anyone working with electronics. Whether you’re a seasoned engineer or a hobbyist, understanding how to use tools like LCR meters and impedance analyzers ensures you select and troubleshoot components effectively. Even basic methods with a multimeter can provide valuable insights in certain situations.

Mastering these measurement techniques empowers you to verify component values, diagnose circuit issues, and ultimately build more reliable and functional electronic projects. Precise inductance readings are crucial for achieving desired circuit behavior, especially in applications like filtering, energy storage, and resonant circuits where component values directly impact performance.

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