# Buying/winding inductors

Are inductors usually hand-wound or bought pre-wound? For the former, how is the inductance measured? Is there some test instrument that can be used or is it done by measuring the length and radius (What about non-solenoid shapes?)? If it's the latter, where are they normally purchased? I've only ever seen choke coils being sold, in stores. I don't know if those are the same.

## 3 Answers

You don't say what your application is, what current and frequency.

If you wind them yourself an RLC meter will tell you what the inductance is. This will depend on the frequency, so make sure you set the meter for the right measuring range. (Mike rightly notes that RLC meters don't always support HF measurements.)
If you don't have an RLC meter, you can measure it by applying your AC signal to the coil in series with a resistor. On a scope you can measure the phase shift between voltage and current, from which you can derive the inductance/resistance ratio.

For air coils there are online calculators which give you the inductance as a function of inner and outer diameter, length, and the number of turns. The formula is

$L (\mu H) = \dfrac{0.315 (N A)^2}{6 A + 9 B + 10 C}$

Where N = number of turns, A = average coil radius, B = coil length and C = coil thickness, all in cm.

I've used coils from Coilcraft, but IIRC they're not cheap.
Vishay is another manufacturer.
Sumida is very inexpensive. I always had to have their SMD power catalog close at hand.

But actually there's dozens of them. Look for specific values at DigiKey, Mouser or Farnell.

• I didn't realize that there are different methods depending on the application. My main concern is radios, so that would be between the MF and VHF range (Mostly VHF). As for current, I have no idea. When reading schematics, I'd always shove them aside the moment I see an inductor because I don't know where to get them, and that happens long before I have a chance to take into consideration the amount of current it uses. Jul 4, 2011 at 0:54

Commercial inductors are pre-wound, and are generally measured with an impedance analyzer or RLC meter at a specified AC frequency and amplitude. You can get inductors in all shapes and sizes: surface-mount, toroidal, bobbin-wound, the list goes on and on. Most large component suppliers stock a variety of inductors for various purposes (DC/DC conversion, EMI filtering, etc.) - it's easy enough to Google and find.

Depending on the application, you can use ferrite cores which have a inductance factor $A_L$. The number of turns n, required for a particular inductance is given by $L=n^2A_L$

For lower inductances, sometimes free wound (air cored) coils are wound and the length & radius would be chosen to give the required inductance as you say.

LCR meters/bridges can be used to measure inductance as stevenvh says but the commonly available handheld units measure at low frequencies (kHz). Very high frequency LCR bridges are available but are also very expensive though!.

• $A_L$ is actually called inductance factor AFAIK, but the rest of your answer is spot-on. Jul 1, 2011 at 15:50
• @Madmanguruman - You're allowed to correct it in the answer (I've done it for you). If Mike would object he can always rollback. Jul 2, 2011 at 6:32