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)

Measureing inductance

Magnetic Amplifiers: Controlling high current AC with DC signal

Stepper Motor Inductance / Torque

Ferrite Bead

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.

book Introduction to Microwave Engineering"

See also (thanks to M. Adam Davis)

• Lenco Electronics McHenry, Illinois Phone: (815) 344 - 2900 Email: lenco-e@ix.netcom.com
• http://hem.passagen.se/sm0vpo/info/pcb-coil.htm This person uses 4 pcbs to create a 20uH coil:
• http://www.eagleware.com/wwwboard/messages/968.html

  Two recent articles I have written cover this subject. The September 1999 issue of RF Design magazine had an article entitled Designing a Low-Noise VCO on FR4. This article used a printed inductor and techniques for reducing effects associated with the poor dielectric of FR4. The October 1999 issue of Applied Microwave & Wireless had an article entitled High-Q Resonators on FR4. This article explained how to obtain printed inductors with Q higher than the inherent Q of FR4. Both articles may be downloaded at this site at the What's New page. Also the Examples manual of Version 7 includes a spiral inductor example in =EMPOWER=,the electromagnetic simulator.

• http://www.i-t.com/engsw/intusoft/magdesgn.htm A demo of a magnetic, inductor and choke design software (with free schematic capture): Looks like you can specify the inductor construction (planar), wire, core and everything. This may be your best bet.
• http://fstewart.ne.mediaone.net/DaEtiCpesIdcosEtchedNews.shtml a simple example:
• http://nickel.arai.riec.tohoku.ac.jp/yamaguti/general/j_papers.html papers on inductors and various methods of construction
• http://members.tripod.com/rclindia/trans.html There is an FM transmitter (100MHz) with three etched inductors on its PCB here: (PCB inductors aren't necessarily spiral or even wound about themselves, every track features inductance)
• http://solutions.sun.com/catalogs/now/Electronic_Design_Automation/Design_Analysis/34633.html If you have a SUN workstation and lots of money you could use SPIRAL: A 3D inductor design package meant to deal with PCB inductors specifically.
• http://fstewart.ne.mediaone.net/DaEtiCpesIdcosSpiral.html#Spiral You can find a few other messages (including journals) to spiral inductors here:
• http://www.ednmag.com/reg/download/analog.asp Here is a demo analog simulator that has a model for spiral inductors:
• http://members.aol.com/marctt/Technical/Inductance_References.htm Someone wrote a bibliography of inductor calculations, including references to 22 works which include impedance calculations relating to spiral inductors specifically:
• http://members.aol.com/marctt/CV/Abstracts/inductance.htm Here's a short "Guesstimating inducatance of wire loops"
• http://www.pcim.com/articles/1999/art0006/art1.html And "Inducatance Calculation Techniques - Part II: Approximations and Handbook Methods" quote "Included are inductance calculations for polygons, disk coils, finite-length solenoids and flat planar spirals"
• http://www.marblehead.com/business/034/electron.htm This company appears to distribute shareware, one program (spirlind #2259) does the calculations for spiral inductors:

See:

• /techref/microchip/dttwist.htm
• /techref/piclist/projects.htm

Questions:

• spamnajeeb_qKILLspam 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:

• http://www.rfcafe.com/references/electrical/inductance.htm
• http://ludens.cl/Electron/trafos/trafos.html Practical transformer winding.+
• http://ludens.cl/Electron/Magnet.html How to design electromagnetic parts+

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