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Use a Resistor to Measure Inductance

Inductance is usually measured in units called millihenrys or microhenrys. It is commonly measured by using an LCM multimeter. It can also be calculated using a function generator and an oscilloscope. A function generator is a piece of electronic test equipment that sends electrical waves through the circuit. It allows you to control the signal moving through the coil so you can accurately calculate the inductance. An oscilloscope is used to display the signal voltage running through the circuit.

With the resistor method, you adjust the frequency of a sine wave until the AC voltage accross the resistor and inductor are equal. Then using a formula to calculate the inductance.

Choose a 100-ohm resistor with 1% resistance. Resistors have colored bands that can help you tell them apart. A 100-ohm resistor will have a brown, black, and brown band. The final band at the far end will also be brown to represent 1% resistance.

Connect the inductor coil in series with the resistor. In series means the current passes through the coil one after the other. Start setting up a circuit by placing the coil and resistor next to each other. Make sure they have one terminal touching. To finish the circuit, you will also need to touch wires to the exposed ends of the resistor and inductor.

Wire a function generator and an oscilloscope into the circuit. Take the output leads from the function generator and plug them into the oscilloscope. Then, turn on both devices to make sure they are working. Once they are both on, take the function generator's red output lead and connect it to the red power wire in your circuit. Connect the oscilloscope's black input lead to the black wire in your circuit.

Run a sine wave current through the circuit with the function generator.

The function generator simulates currents the inductor and resistor would receive if they were actually being used. Use the control knob on the device to start the current. Make sure the generator is set to sine waves so you see big, curving waves flowing steadily across the screen.

Monitor the input voltage and resistor voltage on the screen.

Look to the oscilloscope screen for a pair of sine waves. One will be controllable through the function generator. The other, smaller wave comes from where the inductor and resistor meet. Adjust the function generator's frequency so the junction voltage listed on the screen is half of the original input voltage.

For example, set the generator frequency so the voltage between the peaks of both waves is listed as 1 V, which you will see on the oscilloscope. Then, change it until the voltage is 0.5 V.

The junction voltage is the difference between the sine waves on the oscilloscope. You need it to be half of the signal generator's original voltage.

Find the frequency of the functional generator current.

This will be displayed on the oscilloscope. Check the numbers on the bottom of the readout to find one in kilohertz, or kHz. Note this number, since you will need to use it in a calculation to find the inductance.

If you need to convert hertz (Hz) to kilohertz, remember that 1 kHz = 1,000 kHz. For instance, 1 Hz / 1,000 kHz = 0.001 kHz.

Calculate the inductance using a mathematical formula.

Use the formula L = R * sqrt(3) / (2 * pi * f). L is the inductance, so you need the resistance (R) and the frequency (f) you figured out earlier. Another option is to type your measurements into an inductance calculator, such as at Daycounter.

Start by multiplying the resistance of the resistor by the square root of 3. For instance, 100 ohms x 1.73 = 173.

Next, multiply 2, pi, and the frequency. For example, if the resistance was 20 kHz: 2 * 3.14 * 20 = 125.6.

Finish by dividing the first number by the second number. In this case, 173 / 125.6 = 1.38 millihenries (mH).

To convert millihenries into microhenries (uH), multiply by 1,000: 1.38 x 1,000 = 1378 uH.


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