Hi,
I wonder if someone with this knowledge can help me? I’ve noticed on my tube preamplifier the designer has taken a wire from the casing of the small power transformer to one of the adjacent reservoir capacitor clamps. Presumably a handy place to pick up an Earth. Would this be for transformer screening or electrical protection as the transformer is suspended on rubber for anti-vibration purposes. Also that reservoir capacitor has half of its plastic insulation removed so that the clamp can earth its body. Its fellow reservoir capacitor also has half the plastic removed where it is clamped to the chassis. Is that for screening which I would have thought a little unusual? Any advice about this design feature would be much appreciated as I’m wondering if I need to remove the plastic coating when I replace the electrolytic? Thanks.
Case and (-) terminal are not always connected, especially on PC-mount modern parts.
Many capacitor companies go to the effort to insulate the aluminum can that houses the capacitor contents. If you observe a circular piece of plastic insulator installed on top of a vertical can capacitor, and it's retained there by the plastic sleeving that has the capacitor markings.
If the aluminum cylinder has been insulated, it was done for a deliberate reason, and someone (outside the capacitor) assuming the can was supposed to connected to the chassis, and doing so, raises the question of what effect did this have on the capacitor. You'll have to read on a website of a capacitor manufacturer what the status of the case is. It's been a while and I don't remember exactly why, but I think it has something to do with the chemical formation process of the dielectric in electrolytic types.
Sometimes people post online that they discovered, unpleasantly, the aluminum case of a capacitor was not at 0 V DC. The can may not have been at the same electrical potential as either the (+) or (-) terminal. It may have produced a shock, or just a disconcerting observation and question.
Unless a capacitor case is conspicuously connected or used as a (-) terminal it is probably not a good idea to 'ground' it. Why not? As implied above, the manufacturer may not have connected the case to the(-) terminal, by design. You better be sure you keep your chassis connected to electrical mains safety ground if you are connecting a 'floating' voltage to your chassis. Is it 'healthy' for the capacitor to have this extra connection 'correctively' made? Will that remain constant forever, or just the life of the capacitor? Is there some electrolytic process evolving over time that will affect the lifespan, or the voltage rating or the capacitance value? Consider that an electrolytic capacitor powered up for the first time at a voltage level much less than it's marked voltage rating, 'forms' a dielectric (essentially an insulative process) and the capacitor may need that reformed if it is subjected to a significantly higher voltage closer to the marked rating. This is a process unique to electrolytic capacitors that other capacitor technologies lack. It's related to the reason people 'reform' the dielectric on capacitors that haven't been powered up for a long time, or as precaution because they are ancient, or believed to possess magical properties (other the imagined internal smoke) based on their vintage.
Electrolytics don't last forever, 'by design', regardless of how long we wish they will, or how close to forever personal experience happens to be on a case by case basis. Read a datasheet and you will see lifetime specs, in hours, for both shelf storage and operation at an internal hotspot temperature.
Capacitor 'failure' isn't always visually catastrophic. A multitude of parameters like leakage current, ESR, capacitance, so on can degrade over time. In applications like hf SMPS, where an audible hum is not an indicator for the end user. Other startup malfunctions or a failure to allow the end product to perform fully can happen long before the guts leak or eject from the capacitor can.
We all have examples of capacitor longevity statistics, but when one does visually fail, it's origin or ethnicity or legitimacy as 'real' or 'fake' are raised before understanding the datasheet. End users don't have a responsibility to do that. Engineers do, but there are many parameters to balance, and Purchasing may have found a less costly capacitor vendor after the design that seems equal. Or an engineer made a decision about certain capacitor parameters, but not others they have not yet had a problem with or learned about yet. They don't all come out of school with gray hair, you know. The decisions are not necessarily bad or negligent. There are many factors. One is that no one but the customer thinks the product will last forever. It may not need to even if someone wishes it does.
Not knowing the electrochemical effects of 'grounding' a floating capacitor case could be an error of small proportion. We can't predict it, but the reason for the isolation does appear on multiple capacitor manufacturer websites.
If you ever connect two electrolytics of 'C' uF in series to produce a composite capacitor of 1/2 'C' at twice the voltage rating, it should be obvious to you that this is an example of the reason why the midpoint connection up the 'lower' C (+) and the upper 'C" (-) must not be connected to the chassis which should be at '0' V level. If this is not obvious, reflect on this a while before you make a mistake.
Sorry, first draft. May not make total sense.