curve tracer and capacitor question

[lokeycmos]

i saw an electronics repair technician today testing for bad electrolytic caps. he was using a huntron tracker 2000 curve tracer. i was kinda confused because he was putting one lead on the leg of the capacitor and the other lead on the metal casing on top of the cap. if it is an ellipse it is good, any other waveform means its bad. the thing i was confused about was why he was putting a lead on the casing of the cap? i just measured from casing to each leg with my ohm meter and im getting double digit Megaohms. this shows that the case is NOT in common with either leg. Please Explain!!TY!!

 

[ronsimpson]

It might be that he is not testing the "capacitor" but he is testing if the capacitor is shorted to the case. (and by shorted I mean connected or partly connected) There are many failure modes.

A ohm meter is not a good test. It tests for leakage at low voltage only.
Passing AC through a capacitor at near rated voltage tests more parts of the capacitor.

 

[unclejed613]

i've used a Huntron Tracker before, and it's an excellent troubleshooting tool. usually, with very few exceptions, the case of an electrolytic cap is connected to the negative lead. there are a few that are not. the ellipse comes from an internal resistor in the Tracker creating an RC time constant with the capacitor, which results in a phase shift between the applied current (the X axis) and the voltage (the Y axis), resulting in an ellipse.