EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: joseph nicholas on March 29, 2018, 10:37:22 am
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Twenty years ago I came across a repair video of a tech measuring electrolytic capacitors by grounding his negative test probe to the case of an audio amplifier and measuring the voltage when he touched the top of the can.
How can this be since both pins appear to be isolated from the case?
I tried this myself with the circuit yes I was getting dc voltage values. Is this a valid test technique, and what is it showing?
Anybody have this experience?
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Some, not all, capacitors have the can connected to one of the terminals.
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Some, not all, capacitors have the can connected to one of the terminals.
Cool....I didn't know that. I learn loads from just reading random posts.
Thanks :)
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Some, not all, capacitors have the can connected to one of the terminals.
Yes, that was very common back in fire-bottle days when you would see those tall, slender aluminum capacitors mounted to the chassis between the tubes/valves. Since the tubes circuits used only positive voltages, it was common to use the entire metal chassis as the negative/ground point. So the outer shell of those capacitors was the ground/negative node. And those cans often had multiple capacitors inside them. The photo below shows a vintage chassis-mount and PC board mount varieties of these capacitors. The outer can is the common negative terminal of the multiple capacitors inside. These are becoming rather rare with the demise of the vacuum-tube.
(https://www.tubeworld.com/mallorycan.jpg)
But here in the solid-state (and integrated circuit) era, large electrolytic capacitors are typically isolated from the outer can. Because it is frequently the case that the positive side of the capacitor is connected to circuit ground. The photo below shows a modern electrolytic can capacitor with isolated terminals. Neither the positive or the negative side is connected to the outer can. (Or if one side IS connected to the can, the can is insulated with heat-shrink tubing which you can see as the blue outside of the can below)
(https://www.tedss.com/stock/images/1175/1175003111.jpg)
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Not only that, but once in a while an electrolytic can can be mounted to an insulating bakelite flange, which is isolated from the chassis, and at some negative potential. Could be a problem if unfamiliar. :scared:
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The can is aluminium, and is connected via the electrolyte filling of the capacitor to some voltage which roughly relates to the voltage on the capacitor. Depending on the design and which foil is the outer layer on the surface, this will tend to be the negative terminal, but leakage current will raise this voltage somewhat. The case is also an unwanted connection, unless it is one terminal or is firmly bonded to the one, typically negative. As an unwanted connection it also form a very small value capacitor, not a very good one, but also one that is very leaky and thus it is never intended to be used.
Thus the case being isolated with the sleeve, and on high voltage capacitors also by robust insulation, and on types with a fixing stud as part of the case the mounting hardware either is designed to provide robust isolation but having plastic parts that need to be installed, or the case is meant to be connected to the chassis ground direct, and is also internally connected to the negative terminal by a tab.
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I kind of though this was some kind of leakage but wasn't too sure. But it does give some values which could aid in quick trouble shooting a circuit if it somehow can be interpreted. Thanks for all the input.