[Amps] Filter Chokes

Jim Barber audioguy at q.com
Fri Mar 9 11:34:20 PST 2012


You could determine the light-current inductance by hooking it in series 
with a suitable capacitor, then using an audio generator and a scope to 
find resonance.

It's most likely they are indeed swinging chokes. If not, they'd again 
most likely be wired up to resonate with a fairly small capacitance, 
something like 0.1uf. (that makes a resonant choke filter at 159hz, a 
little above the 120hz ripple frequency as cussed and discussed on this 
list)

So:

. Find a decent-quality 0.1uf capacitor. (electrolytic will work, but 
not my first choice)

. Hook the cap up in series with the choke

. Set your audio generator to 40hz or so to start with

. Put your audio generator and scope (or AC voltmeter) across the pair

. Slowly increase the frequency of the generator until you see the 
voltage "spike", then tune the generator for maximum voltage.

The resonant freq is then 1 / (2 * PI * SQR(L * C))

Where:
"SQR" means Square Root

PI = 3.141592654 or thereabouts

and L and C are in equivalent units. Farads and Henries, microFarads and 
Microhenries, etc. Doing a little algebra will derive the inductance 
given the other two, *or* here's a list of approximations for 0.1uf:

159hz	10 Henries
225hz	5 Henries
355hz	2 Henries
500hz	1 Henry

Note:
The only way to know the saturation current inductance of a swinging 
choke (that I know of) is to saturate it, sorry.

Online resonance calculator:
http://www.deephaven.co.uk/lc.html

My .02, adjust for your local currency as appropriate.

73,
Jim N7CXI


On 3/9/2012 10:55 AM, Mike Lucas wrote:
>       I'm trying to characterize two filter chokes from 4x1 amps, other
> than big and heavy. Can someone
> suggest a procedure, literature, or visual methods to identify what I
> have? Am I safe in assuming that
> they are swinging chokes? Originally from W4GD supplies, weight is about
> 25 Lbs each, although they
> are of different construction.
>
> Thanks, Mike W5CHR Memphis
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