[AMRadio] How to ID a swinging choke?

John Coleman jc at pctechref.com
Sat Dec 8 14:51:57 EST 2007

	Saturable reactors "SWINGING CHOKES" and saturable transformers are
strange in the way they operate.  A choke does not exactly lose it reactance
as the DC current is increased thru it but instead it loses its ability to
react over a certain period of AC cycle and AC current level as the DC
current is increased.  The result in XFMRs is similar to clipping of lower
frequency AC/audio.  

In a swinging choke input power supply, the result is to pass large amounts
of current to charge the filter capacitor during the saturation period.
This saturation period is only a part of the period of the input ripple of
the power supply, perhaps only the top 10 degrees at the peak.  But as the
DC current drain becomes higher by demand from the load then the core is
saturated for a longer period of time.  A greater amount of the peak in the
ripple is passed unimpeded to the filter capacitor causing it to charge to a
higher voltage in hopes to compensate for the resistance losses in other
components.  The change in reactance is apparent change over short periods
of the input cycle ripple.  It is actually possible, with certain component
choices, for the output voltage to rise higher than the average voltage at
peak current.  For this reason the output capacitor of the power supply
needs to have a long time constant to absorb slow rises and falls in voltage
as discussed in previous messages.

So, in short, it is difficult to measure the effects of saturation with out
a scope, a variable AC and variable DC supply, as well as having some known
examples to work with as well.

Don, K4KYV, had come up with the idea of taking the "I" section of iron from
two identical 12 Henry chokes.  He did this and placed the two "E" section
together, series connected the two windings in the proper phase.  The idea
was to make modulation reactor of about 40 Henry.  The gap between the two
sections had to be adjusted for the maximum reactance at the DC current
level that final amp was using.  We did this by series connecting a known
choke with the experimental choke and actually using the two of them as a
modulation reactor.  Using a insulated VOM to measure the AC voltage drop
across each and adjusting the gap we were able to come up with a very nice
modulation reactor but is was tedious and dangerous working with the HV
stuff and making sure the DC was all gone before adjusting the gap or
changing the meter probe points.  Thanks still to Don as I am still using

I am sure some type of test facility could be constructed for this with some

John Coleman, WA5BXO

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of jeremy-ca
Sent: Saturday, December 08, 2007 8:16 AM
To: amradio at mailman.qth.net
Subject: [AMRadio] How to ID a swinging choke?

Is there a simple bench test or an eyeball way to tell the difference 
between a fixed and swinging choke? Something about an air gap if I remember

but not all my chokes are open frames.


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