Measure Inductance
"Inductance" refers to either mutual inductance, when an electrical circuit develops voltage as the result of a change in current in another circuit, and self-inductance, the creation of voltage in a circuit resulting from a change in its own current. In either form, inductance is a ratio of voltage to current and is measured in a unit called the henry, defined as 1 volt-second per ampere. As the henry is a large unit, inductance is commonly measured in either millihenries (mH), thousandths of a henry, or microhenries (uH), millionths of a henry. Following are several methods for measuring inductance for an inductor coil.
Contents
Steps
Measuring Inductance on a Voltage Current Slope
- Connect the inductor coil to a pulsed voltage source. Keep the cycle of the pulse below 50 percent.
- Set up current monitors. You'll need to wire a current sense resistor into the circuit, or else use a current probe. Either should be connected to an oscilloscope.
- Read the peak current and the amount of time between voltage pulses. The peak current will be measured in amperes, while the time between pulses will be in microseconds.
- Multiply the voltage delivered in each pulse by the length of each pulse. For example, if a voltage of 50 volts is delivered every 5 microseconds, this figure would be 50 times 5, or 250 volt-microseconds.
- Divide the product of the voltage and pulse length by the peak current. Continuing the above example, if the peak current was 5 amperes, the inductance would be 250 volt-microseconds divided by 5 amperes, or 50 microhenries.
- Although the math is simple, the setup for this method of measuring inductance is more complex than for other methods of measuring inductance.
Measuring Inductance Using a Resistor
- Connect the inductor coil in series with a resistor whose resistance value is known. The resistor should be precise to within 1 percent or less. Wiring in series forces the current to pass through the resistor as well as the inductor being tested; the resistor and inductor should have 1 terminal touching.
- Run a current through the circuit. This is done with a function generator, which simulates currents the inductor and resistor would receive in actual use.
- Monitor both the input voltage and the voltage where the inductor and resistor meet. Adjust the frequency until the junction voltage where the inductor and resistor meet is half of the input voltage.
- Find the frequency of the current. This is measured in kilohertz.
- Calculate the inductance. Unlike calculating inductance from voltage and current, setting up the test is easier, but the math is more complicated. It breaks down as follows:
- Multiply the resistance of the resistor by the square root of 3. If the resistor has a resistance of 100 ohms, multiplying by 1.73 (the square root of 3 to 2 decimal places), this value is 173.
- Divide the result by the product of 4 times pi times the frequency. For a frequency of 20 kilohertz, 4 times 3.14 (pi to 2 decimal places) times 20 is 125.6; 173 divided by 125.6 equals, to 2 decimal places, 1.38 millihenries.
- mH = (R x 1.73) / (6.28 x (Hz / 1000))</p>
- Example: given R = 100 and Hz = 20,000</p>
- mH = (100 X 1.73) / (6.28 x (20,000 / 1000)</p>
- mH = 173 / (6.28 x 20)</p>
- mH = 173 / 125.6</p>
- mH = 1.38</p>
Measuring Inductance Using a Capacitor and Resistor
- Wire the inductor coil in parallel with a capacitor whose capacitance is known. Wiring an inductor coil in parallel with a capacitor creates a tank circuit. Use a capacitor with a tolerance of 10 percent or less.
- Wire the tank circuit in series with a resistor.
- Run a current through the circuit. Again, this is done with a function generator.
- Place probes from an oscilloscope across the tank circuit.
- Sweep the function generator's frequency from its lowest to highest range.
- As you sweep, look for the tank's resonant frequency, where the oscilloscope registers the largest waveform.
- Because an LC circuit's resonant frequency f in Hertz is f = 1/ (2 pi sqrt(L*C)), if you know the frequency f you can compute the inductance L = 1/((2 pi f)^2 * C). For example, if the resonant frequency is 5000Hz, and the capacitance is 1 uF (1.0e-6 farad), the inductance is 0.001 henry, or 1000 uH.
Tips
- When a group of inductors are wired in series, their total inductance is the sum of the inductance of each inductor. When a group of inductors are wired in parallel, however, the total inductance is the reciprocal of the sum of the reciprocals of the values of each inductor in the group.
- Inductors can be constructed as bar coils, ring-shaped cores, or out of thin film. The more turns an inductor coil has for its length, or the greater its cross-sectional area, the greater its inductance. Longer inductor coils have a lower inductance than do shorter coils.
Warnings
- Inductance can be measured directly with an inductance meter, but such meters are not common. Most of the inductance meters that are made measure only at low current.
Things You'll Need
- Function generator
- Oscilloscope with probes
- Resistor or capacitor
