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Old 20th January 2009, 05:04 PM   #1
Atilla is offline Atilla  Norway
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Join Date: Jul 2008
Default Determining CM Choke values

I've been working on my own power-line filter, like the ones you see in typical commersial equipment - pi-filters with X caps and CM choke. I'll throw a couple of extras there for surge suppression as well, as I've nowhere else to mount them. I've a few working examples here and the required safety guidelines printed on my desk

I've got access to quite a lot of scrapped electronic equipment, so I've desoldered a decent amount of those items. Quality X1 and X2 caps from anything between 100nf and 470nf.

I've also got a good amount of CM chokes, in various shapes and sizes, however only one of those ever had its specs printed on it. Since they all probably vary in values, I was wondering which one(s) to use in my filter. My equipment right now is limited to a standard multimeter, which doesn't measure capacitance or inductance values. Is there any way to figure them out right? Some are salvaged from low-power equipment, some from ATX power supplies and one from a CRT monitor, which is also the biggest and best-looking (quality-wise) one. Some are wound on a toroid core, some look like dual cable-reels.

It should be rated for something more than 2A, since that's the value of my mains fuse.
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Old 17th February 2009, 08:10 AM   #2
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Join Date: May 2008
To find out the approximate inductance you need two voltmeters, a frequency generator and a known resistor.

First measure the DC resistance Rl of the choke.
Then connect the choke and resistor in series to either the frequency generator. Measure the voltage across both and across the choke. Now you can use Ohm's law to calculate the resistance of the choke at that frequency.

U = Ur + Ul

Xl + Rl = R * Ul / Ur

Then you subtract Rl to isolate Xl and get the inductance with

L = 2*PI*f*Xl

Instead of a frequency generator you can also use a transformer that you connect to mains. f will be 50 Hz then and the result will not be too exact. Make sure that the secondary voltage is low enough to neither destroy the choke nor the resistance, but high enough to achieve useful measurements.

The current rating is something you can only estimate according to the wire thickness. The more windings the choke has, the more you have to derate the current rating for a given wire gauge.

Or google for CM chokes and see, if you find the types you have or similar ones to get the specs.
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Old 17th February 2009, 10:28 AM   #3
Atilla is offline Atilla  Norway
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Join Date: Jul 2008
Thanks for the wonderful idea. I hadn't thought of using transformer and 50-60hz mains wave to measure it. I just need approximate values, so some margin of error is fine.
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