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Capacitor Measurement

Question asked by SimonB on Sep 30, 2009
Latest reply on Oct 6, 2009 by SimonB

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Hi,

We have a new non polarised 1nF capacitor modification fitted to one of our existing PCB circuits and are experiencing problems trying to measure its value (See attached jpg). The capacitor (In Red) is fitted in parallel with the two diodes D71 & D72. Whenever we try to stim on the Capacitor the diodes conduct, does anyone have any suggestions on the best way top tackle this scenario?

Many thanks
Simon

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HansEchterhagen Oct 5, 2009 5:03 AM
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The problem you see here in this circuit is not the diodes, but the very large capacitors which are defacto in parallel to the 1nF capacitor. The coil of the transformer is almost a short, and this is also true for the 1.5mF capacitor. If we now guard this capacitor and the coil, we will get a guard ratio of 1 to 1 500 000. This is far higher then the maximal possible guard ratio of 1 to 10 000 which is defined in the Agilent 3070 specifications. It might be worth to do a try with a high frequency and no guarding, if you use the new Agilent ASRU, which can do a 200 KHz frequency for this type of circuits. Of cource it is still out of specifications.

Sorry that I could not give you a better Answer, kind regards Hans
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SimonB Oct 6, 2009 1:49 AM
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Many thanks for your reply Hans,

We only have old style ASRU cards

Best regards

Simon
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Capacitor Measurement

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