# Electrolytic capacitace and different voltages



## nonreverb (Sep 19, 2006)

I was checking my other favorite techie discussion group today and an interesting discussion came up which I'm going to ask the techies here for their opinion...If an unformed electrolytic cap (let's say 100uF X 500 Volts) is installed in a circuit with only 100 volts applied to it, will its capacitance be more or less than it's specified rating?....the ball is now rolling....


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## Ripper (Jul 1, 2006)

Here's a pretty good article on that
http://www.cliftonlaboratories.com/capacitor_voltage_change.htm


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## Wild Bill (May 3, 2006)

nonreverb said:


> I was checking my other favorite techie discussion group today and an interesting discussion came up which I'm going to ask the techies here for their opinion...If an unformed electrolytic cap (let's say 100uF X 500 Volts) is installed in a circuit with only 100 volts applied to it, will its capacitance be more or less than it's specified rating?....the ball is now rolling....


There's some assumptions here that I just can't share. For starters, where do you find an "unformed cap" in modern times?

50 years ago the materials available to make electrolytic caps were such that they tended to "stiffen" if they were left without voltage for a prolonged period of time...like a couple of years! This meant that there was a chance (NOT a certainty! Just a statistical chance.) that the cap would lose a bit of it's voltage rating. NOT lots of it, just a bit. 

In any good design you always had some extra rating as a safety factor. Good engineers understood the nature of the parts they used.

So if a radio had not been turned on for some years the filters might have lost a bit of voltage rating but when you dragged the old radio out of the basement and plugged it in it didn't usually fail. It just reformed itself to the original voltage rating.

The major source for the mojo about reforming capacitors originally came from the ham radio world. Hams were and are notoriously cheap and often scrounged electrolytics that might have been VERY old! With these caps it was good practice to put them on a variable DC supply and slowly bring the voltage up to the original rating. They believed that this would allow the electrolyte paste inside to "reform" and work like new.

Not that many real engineers believed this. They understood that the real problem was that the paste had dried somewhat, since electrolytic caps all have to have a tiny "breathing hole" to allow any heated trapped gas inside to escape without blowing up the cap. This hole is where the moisture can slowly seep out over the years. "Reforming" is NOT going to put moisture back inside the cap!

So what was more likely happening is that bringing up the voltage slowly on the cap would quickly show if it was punched or not! Caps were not being rejuvenated but rather damaged caps were being identified.

Modern caps are "formed" during manufacture and have a shelf life of some years. No real tech today bothers to reform modern caps. As for old ones, why use them at all? There is no technical benefit to using an old cap. Filters are not a tone-changing device, except for their actual capacitance value.

For the price of new caps, if the filters are old then YANK 'EM! Why on earth would you keep them? If they're old they're just failures waiting to happen! 

Yank them out BEFORE you even power up that old amp! You're going to have to anyways so why take a chance on having a mess to clean up if one of the old ones pops?

Would you keep the original tires and wiper blades on a 57 Chevy you were restoring?

It's the audiophiles who are really to blame for keeping this reforming idea alive. In that market few users actually know how to put a new plug on a lamp cord. The name of the game is to brag about how much money things cost! Besides, "reforming" is a simple task that anybody with hair in their ears can learn to do but it impresses those who never learned!

As for the link about caps changing with voltage, there was very little in that article about ELECTROLYTIC caps! Here's the only bit I found:

*"The graph below shows both the electrolytic and tantalum capacitors change capacitance and dissipation factor very little with applied voltage. The tantalum is somewhat more constant than the electrolytic, but both show relatively little variation with bias voltage, unlike the high capacitance ceramics we've looked at so far."*

Considering that in the old days electrolytic caps were made to a -20%/+80% tolerance and even today after 60 years of improved technology are made to +/-10% of their labeled value, even this article's graph shows we're once again talking "mice nuts"!

IMHO, this premise describes such a trivial change as to be a non-issue. Yes, a change can be measured. It's just that you need super sensitive and accurate lab equipment to do it and it makes no difference to the circuit in the real world.

:food-smiley-004:


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## Ripper (Jul 1, 2006)

I knew there wasn't alot on electrolytics in there but I thought it would at least show that it really isn't an issue regarding voltage vs capacitance.


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## nonreverb (Sep 19, 2006)

Well, my understanding is that once the cap is formed it's value is pretty well fixed for good. Although some on this other discussion board claim that at a lower voltage, the capacitance is larger by a small amount. The person making the claim also noted that the cap was new and had a constant voltage applied that was significantly lower than spec. Apparently they were experimenting to debunk the idea that capaciance varies widely over large
differences in voltage.


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## greco (Jul 15, 2007)

Cool thread....lots to learn :thanks5qx:


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