# Re: Choke size?

```Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Steve,

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You will need the cross sectional area of the core, the number of turns, the length of path thru the core, and any gap width. If no gap, you will need the permeability of the material. However, you can measure the inductance by powering the choke up slowly while monitoring the current using a variac and current meter. Plot the current vs voltage across the choke. The V/I ratio will be the impedance (ZL) of the choke at your line frequency. ZL = 2*pi*freq*L. So,
```
L = (V/I)  / (2*pi*freq)

Gerry R.

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```Original poster: "Steve Studer" <reddykguy@xxxxxxxxxxx>

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Does anyone know of a formula for calculating the inductance of a choke with an iron core? I have 4 chokes the size of large mot's they have about 30 turns of .250" flat copper wire. I cannot seem to find any formulas except for air core formulas. I would like to use two of these for a DC resonant power supply. I just don't have any idea what the inductance might be.
```Thanks,
Steve

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