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Old 22nd June 2009, 11:20 AM   #1
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Default Common-mode choke discussion please

Hi,

Just finished "researching" this topic by actually looking back a few years on this forum - and not finding anything. So here are my questions:

1. I've seen reference to common-mode chokes used in the INPUT to transistor power supplies and tube filament/plate supplies. Could someone point me to a good discussion of their use and specification (size, current ratings) in this application AND suggest a type(s) and manufacturers.

2. Cello actually used what I'd call a common mode choke in the OUTPUT of their Performance amp +/- 80volt main power supply. It is an L-C configuration with a giant choke that actually has two windings so I'd say it qualifies as "common-mode" - WHILE is handling AMPERES; like 10 to 15. Has anyone used this technique and can share their experiences?

THANK You

Charles
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Old 28th June 2009, 01:32 PM   #2
MRupp is offline MRupp  Germany
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Quote:
It is an L-C configuration with a giant choke that actually has two windings so I'd say it qualifies as "common-mode"
If it is used in an LC input filter or a pi filter (CLC) it would be a "differential-mode" choke, with the normal supply currents adding up rather than cancelling out as in a common mode choke. It is a means of combining two large separate chokes in an LC or CLC configuration into one. I have no info at hand but I know there is info on this forum and elswhere in the WEB.
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Old 30th June 2009, 11:58 AM   #3
marce is offline marce  United Kingdom
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We use them for EMC reasons, here some links to theory etc

http://www.butlerwinding.com/inductors/common_mode.html

http://www.murata.com/emc/knowhow/pd...-1/26to28e.pdf
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Old 30th June 2009, 01:12 PM   #4
infinia is offline infinia  United States
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1) Common mode chokes primarily provide a high series reactance between the source and the load in relation to ground. This series reactance or inductance in conjuction with added shunt capacitance to ground forms a 2nd order lowpass filter. The load current as it is increased will reduce the inductance and Q affecting the filter response. Look at the Corcom site for typical fitering (attenuation) charts. http://www.cor.com/Series/IEC/EF/
To be considered a common mode choke the 2 windings have to be balanced. The forward and the return currents must be from the same source yet opposite in phase ie they must cancel each other. Since the flux of the desired signal ie mains frequency cancel the net difference inductance looks very low yet the common mode inductance is high.
Leakage inductance (much lower in relation to common mode) determines the residual filtering in the transverse mode. Leakage inductance is related to how (un)closely coupled the two windings are. Leakage is good in these applications for dual purpose filtering.

Tutorial
corcom http://www.brainshark.com/brainshark...p?pi=809828417

2) can't comment... any more info? sch. dia?
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Old 1st July 2009, 08:42 PM   #5
MRupp is offline MRupp  Germany
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Quote:
2. Cello actually used what I'd call a common mode choke in the OUTPUT of their Performance amp +/- 80volt main power supply.
I think everything said on common-mode chokes is correct, except that the Cello amp seems not to have the usual common mode choke as stated above, but instead much larger chokes that are used in differential mode.
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Old 1st July 2009, 08:50 PM   #6
infinia is offline infinia  United States
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Post a schematic please
any info & an image of said choke would be very helpful
I'm not going to research all this and do the work for you

it's possible... could be a common mode for RF isolation as well a part of a zobel, dual purpose.


edit>
I would assume these audio amp output (common mode) choke applications are more benificial to class D and to lesser extent class AB.
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Old 5th July 2009, 11:18 AM   #7
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Default Thank You VERY much

for the information so far. I will digest it and respond - will also try and scan the schematic.

Charles
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Old 24th July 2009, 10:21 PM   #8
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Default Re: Common-mode choke discussion please

Hello stellavox,

Quote:
Originally posted by stellavox
...Has anyone used this technique and can share their experiences?...
I use this configuration on my preamplifier (not amplifier like Cello). The inductor is used in LC configuration. It have many advantages (see my article). Model 4 preamplifier

Capacitor is not the only way to obtain continuous tension (if current crossing inductor is constant ).

Eric
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Old 25th July 2009, 12:00 AM   #9
infinia is offline infinia  United States
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Hi Eric
Your comment for your pre amp is interesting... but this discussion is for using common mode chokes on the output of an amplifier. Yours is a SE choke in a small PS.
FYI a common mode choke uses balanced equal windings.
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