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Old 23rd June 2005, 04:30 PM   #1
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Default isolation transformer as common mode choke?

Hello,

Once I bought in a dumpstore some isolation transformers (230V/230V at about 240 VA) that donít seem meet the requirements for their purpose. They draw some 25 mA current at no-load, which is quite high. They already get very warm being at idlle. Iím not able to analyse what is wrong (like wrong iron, not enough windings. Thanks John).
Then Iíve thought about an alternative application: I could use them as a common mode choke in a 230V filter. Just like the one in Sound Practices volume I, issue 4. In this article they use the two secundairy 6,3V windings of a mains transformer as a common mode choke. The filter with such a transformer wiring works great in my experience.

Question for me is: would it be possible to use the primairy and secundairy winding of an isolation transformer in a common mode wiring? Iím aware that it probably cannot serve for a high current load like a power amplifier; the inductance will be much higher than the 2 x 6,3V windings. Also the dc-resistance for instance is about 11,5 Ohm. It might serve though as a filter for a CD-player or a turntable or even a modest current drawing preamplifier. One could add a nice capacitor before and after such a common mode choke, like 2-4 uF/1.500V. And a resistor in parallel for discharge after use to make it complete.

Another application might be to use it as a common mode choke in the secundairy of a high voltage power supply of a tubed preamplifier before the rectifier.

Any opinions?

Best regards, Arjen.
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Old 29th June 2005, 07:03 AM   #2
cerrem is offline cerrem  United States
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It may be possible to make a common mode filter with this beast, by connecting the primary and secondary in opposition.....
And since the leakage inductance is probably very high you can make a differential filter at the same time...
For a common mode choke you would need to add 2 Y caps on the outputs of the choke each cap to ground...
Rule 1 with any filter design... KNOW your source impedance and load impedance....SInce these have a dramatic effect on the filter...Also damping is important... I have seen many poorly designed CM filter that were under damped and actually made the situation WORSE....
First order of business is to measure the inductance of the winding.... the quick and dirty way is...
Put AC 120 volt mains across the winding with an ameter in series and a AC volt meter across the primary...
E/I = inductive reactance... Then divide that by 377 and you get L.... Inductance is incremental so you would need to plot this over a range of voltage increments to find the L curve...
The L will peak about half of it's saturation flux density when using EI laminations ....
Check the excitation current draw withthe secondary unloaded...
Once you find the L curve..I can help out with calculating other values for a filter...
Chris
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Old 30th June 2005, 03:01 PM   #3
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Default isolation transformer as common mode choke

Hello Chris,

Thanks for your helpful response.
After my holidays I'll carry out the measurement you indicate
and report it.

Best regards, Arjen.
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Old 4th July 2005, 11:33 AM   #4
dhaen is offline dhaen  Europe
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Hi Arjen,

I think the reactance would be too great using the high voltage windings, very little current could be drawn at all. Also although the MF would be filtered, the HF may pass due to self capacity.

I haven't looked at the article you mention, but effective filter design is not trivial. Have a look at some mains filter spec's on the manufacturer's websites.

Sorry to seem rather negative,
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