# Component: Inductors

Testing: Setup a simple switching circuit with a MOSFET to ground, an inductor, and a freewheel diode. Then generate a narrow pulse at some arbitrary rate of change (you will understand quickly as you begin to look). Put a low ohm resistor in the ground below the FET so that you can look at the current. Add an IA to get out of the noise. When you pulse the fet, the current will ramp up as a triangle. If you work out the applied voltage and the DI/DT, then you can figure out the inductance fairly easily. V=1/2L(DI/DT). Then you widen the pulse width (generate it with a saw and a comparator so that you can vary the duty, unless you own a cool wave gen). At some point, you will see the current flatten out. This is generally the current saturation level of the core material. You will rarely find it roll over at a single point, it just kind of curves over.

It works on power inductors and transformers, too. On transformers, leave all secondaries disconnected.

Richard Prosser says

As the inductor goes into saturation, the inductance decreases - therefore di/dt increases. The flattening of the display is more likely due to the resistance in the circuit limiting the maximum. The droop you note is probably due to the FET resistance increasing as it warms up - thereby7 reducing the current. (I'm assuming the FET Rds is >> coil resistance & the dc supply is OK for the peak current)

Magnetic Amplifiers: Controlling high current AC with DC signal

Inductors act like a large resistor at higher frequencies, but a small resistor at DC. XL is inductive reactance [similar to "impedance"]:

XL = 2*PI*f*L

For example a 10 uH inductor will act like a resistor of 63 ohms @ 1 Mhz and 630 ohms @ 10 Mhz.

See also (thanks to M. Adam Davis)

See:

Questions:

• spamnajeeb_qspam at hotmail.com asks:
I want to create a 1.2nH inductor on PCB. How I will calculate the lenght X width of a trace and its geometry (to create inductor). frequency is 2.4GHz
+

See also:

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