Inductance is the ability of a coil to prevent electrical current from flowing through it. An inductor can block one current and pass another. For example, in televisions and radios, inductors are used to receive and tune to various channels. Typically, inductance is measured in millihenries or microhenries. As a rule, a frequency generator and an oscilloscope or RLC meter (immittance meter) are used to measure it. Inductance can also be calculated from the slope of the voltage-current relationship by measuring the current flowing through the coil.
Method 1 of 3: Measuring inductance with a resistor
Step 1. Find a 100 ohm resistor with an accuracy of 1%
Resistors have colored stripes that indicate their resistance. The 100 ohm resistor should have brown, black, and another brown stripe. The last strip at the far end will also be brown, which corresponds to a 1% tolerance. If you have a set of resistances, choose a resistor with these strips.
New resistors are labeled, but they can be easily confused after you take them out of the box. Always use a known resistance resistor to measure inductance to obtain correct results
Step 2. Connect the inductor to the resistor in series
A series connection means that the current will flow in series through the individual parts of the circuit. To get started, place the coil and resistor side by side so that they touch each other with one contact. Close the circuit: to do this, connect the power wires to the free contacts of the resistor and the inductor.
- Wires can be purchased from an electrical store or ordered online. They are usually red and black for easy identification. Connect the red wire to the free end of the resistor and the black wire to the opposite free terminal of the coil.
- If you don't have a breadboard, consider purchasing one. Holes in the breadboard make it easy to connect wires and components.
Step 3. Connect the function generator and oscilloscope to the circuit
Connect the output pins of the function generator to the oscilloscope. Then turn on both devices and check that they work. Then connect the red output of the function generator to the red wire of the circuit. Connect the black output of the oscilloscope to the black wire of the circuit.
- A function generator is a test device that feeds electrical waves into a circuit. It allows you to monitor the signal flow through the circuit so you can accurately measure inductance.
- An oscilloscope is used to detect and display a voltage signal passing through a circuit. With it, you will be able to see the signal that is supplied by the function generator.
Step 4. Run current through the circuit using a function generator
The function generator simulates the currents that would flow through a coil and resistor during operation. Use the control knob on the generator to drive current through the circuit. Try setting the function generator to 100 or 50 ohms. Make sure the generator is tuned to sine waves so that continuous large curved waves can be seen on the display.
Check the generator settings and change the wave type if necessary. In addition to sine waves, function generators can create square, triangle, and other waves that are not suitable for measuring inductance
Step 5. Observe the input voltage and voltage across the resistor on the display
On the oscilloscope screen, you will see a pair of sine waves. One of them can be controlled with a function generator. The second, lower sinusoid, corresponds to the junction of the coil and resistor. Adjust the function generator frequency so that the voltage seen on the display at the junction is half the initial input voltage.
- For example, set the frequency on the generator so that the voltage between the peaks of both waves on the oscilloscope display is 1 volt. Then change it until the voltage is 0.5 volts.
- The voltage at the junction of the coil and resistor corresponds to the difference between the sinusoids on the oscilloscope display. Make sure that it is half the original voltage of the signal generator.
Step 6. Find the frequency of the function generator current
This frequency will be displayed on the oscilloscope. Look at the number at the bottom of the screen, which is in kilohertz (kHz). Write down this number - you will need it to calculate the inductance.
If you need to convert hertz (Hz) to kilohertz, remember that 1 kHz = 1000 Hz. For example, 1 Hz = 0.001 kHz
Step 7. Calculate the inductance using the formula
Use the following formula: L = R * sqrt (3) / (2 * pi * f) where L is the inductance. Thus, you will need the resistance (R) and frequency (f) values that you defined earlier. Another way is to enter the measured values into an inductance calculator, for example
- First, multiply the resistor's resistance by the square root of 3. For example, 100 ohms x 1.73 = 173.
- Then multiply 2, pi and frequency f. For example, if the frequency was 20 kHz: 2 * 3, 14 * 20 = 125, 6.
- Finally, divide the first number by the second. In our case, 173/125.6 = 1.38 millhenry (mH).
- To convert millihenry to microhenry (μH), multiply the resulting value by a thousand: 1.38 x 1000 = 1378 μH.
Method 2 of 3: Determining inductance with an RLC meter
Step 1. Turn on the RLC meter and wait for it to start
A standard RLC meter, or immittance meter, is very similar to a conventional multimeter that measures voltage and current. Most immittance meters have a display that will show 0 after pressing the power button. If the display does not show 0, press the reset button to reset the meter to zero.
- There are also larger electronic devices that further simplify the testing process. Such devices often have a socket for connecting an inductor, which allows for a more accurate result.
- Inductance cannot be measured with conventional multimeters, as they do not have a similar function. Fortunately, there are fairly inexpensive handheld RLC meters available on the internet.
Step 2. Set the RLC meter to "L", that is, the inductance measurement function
With the RLC meter, you can measure various quantities that are indicated around the rotary switch. "L" stands for the inductance you need. If you have a portable RLC meter, turn the switch so that it points to “L”. If you are using an electronic device, set the display to "L" using the buttons.
RLC meters have a variety of options - make sure you pick the one you want. "C" is for capacitance and "R" for resistance
Step 3. Set the instrument to 100 kHz and 1 volt
Typically, RLC meters have several settings. Typically the lowest inductance value is around 200 μH. Optimum values for most benchtop instruments are 100 kHz and 1 volt.
Incorrect settings will negatively affect measurement accuracy. Most RLC meters are designed for low currents and higher currents should be avoided than the inductor can handle
Step 4. Connect the wires to the RLC meter
Like a multimeter, an RLC meter has a black and a red lead. The red wire goes to the positive, and the black one goes to the negative socket of the device. Touch the output terminals of the device under test to flow current through it.
Some RLC meters have jacks to which you can connect devices under test, such as capacitors or inductors. Insert the terminals of the device into the connectors to test it
Step 5. Look at the inductance value shown on the display
RLC meters can measure inductance almost instantly. Once connected, you will immediately see the inductance value on the display. The meter will display the inductance in microhenry (μH). Then you can turn off the RLC meter and disconnect it from the coil.
Method 3 of 3: Calculating inductance from the slope of the voltage-current relationship
Step 1. Connect the inductor to the impulse voltage source
The easiest way to get a pulse current is to purchase a pulse generator. It works in the same way as a normal function generator and connects to the circuit in the same way. Connect the generator output lead to the red power lead, which should be connected to the current sense resistor.
- The impulse voltage can also be obtained by yourself. It can damage nearby electronic devices, so be careful.
- Pulse generators allow for better current control than specially assembled circuits, so it is best to use a generator if you can.
Step 2. Adjust the current with a current sense resistor and an oscilloscope
A current sense resistor must be used in the circuit. Connect it behind the inductor and make sure there is contact before connecting to the opposite end of the red power wire. Then connect the oscilloscope - connect its black output to the black power wire that is connected to the coil.
- Check the readings after you connect everything. If everything is normal, you will see the generated current pulses on the oscilloscope display.
- Current sense resistors are a special kind of resistors that consume the least amount of power. Also called shunt resistors, they are essential for accurate voltage measurement.
Step 3. Set the pulse width to 50% or less
Look at the pulse image on the oscilloscope screen. The upper points correspond to the active phase of the signal. They should deviate from zero by about the same as the low points. The pulse period corresponds to the length of one full wave on the oscilloscope display.
For example, the active phase of the pulse can last one second, then there is no signal for another second. This will produce symmetrical waves on the oscilloscope display, since the pulse will only be active half the time
Step 4. Note the maximum current value and the time interval between voltage pulses
Find these values from the pulse image on the oscilloscope display. The maximum current value corresponds to the peak of the wave you see on the screen, it is measured in amperes. The time span between the two peaks will be shown in microseconds. With these two values, you can calculate the inductance.
One second has a million (1,000,000) microseconds. If you need to convert time to seconds, divide the number of microseconds by 1,000,000
Step 5. Multiply the voltage and pulse duration
Use the formula L = V * Ton / Ipk to find the inductance, where V is the impulse voltage, Ton is the time interval between impulses, Ipk is the impulse current that you measured earlier. All quantities included in this formula should be displayed on an oscilloscope.
- For example, if a 50 volt pulse is applied every 5 microseconds, we have 50 x 5 = 250 volt-microseconds.
- You can also enter the measured values into an inductance calculator, for example
Step 6. Divide the product by the peak current to find the inductance
Determine the maximum (peak) current value from the oscilloscope. Plug it into the formula and you get the desired inductance!
- For example, 250 volt-microseconds / 5 amps = 50 microhenry (μH).
- Although the math is straightforward, this method requires more complex measurements than others. However, once everything works for you, you can easily calculate the inductance!
- When several coils are connected in series, their total inductance is equal to the sum of the inductances of each coil.
- If you connect the coils in parallel, their total inductance is much lower. To find the total inductance in this case, divide 1 by the inductance of each coil, add these values, and then divide 1 by the result.
- Inductors can be in the form of simple coils, core rings, or thin film. The more turns or more area a coil has, the higher its inductance.
- Long coils usually have less inductance than shorter ones due to their shape.