Electrolytic cap reverse voltage?

What is the maximum reverse voltage a polarized electrolytic capacitor can withstand? I have a 1uF electrolytic cap rated at 50 volts. Is it safe to reverse charge this cap to 1.5 volts?

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Reply to
billbowden
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Google "reverse voltage on electrolytic capacitors" and see if that helps.

Reply to
John S

One old ROT is 10% of the forward voltage rating. I'm sure -1.5 is OK.

I did some serious testing on polymer aluminum caps, posted here somewhere. They seemed OK at -80% of rated voltage. I'm going to use them at -20%.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

I was thinking that about 3V would be OK. But why do you have to do this ? If you do, the lifetime of your device is likely shortened as well as its reliability.

What is the reason for this ? There is no way to design it right ?

Reply to
jurb6006

For a regular "wet" aluminum electrolytic then 1.5V should be absolutely fine provided it still spends most of its time polarized the right way. I don't know about solid Al types.

I also don't know the time constant for this "most of the time" but from what I read is certainly hours-days, possibly even much much longer.

piglet

Reply to
piglet

n
o

I accidentally put a 16v lytic backwards on a 23v rail. I discovered the er ror many years later, never had ahy problem with it. I'm pretty sure that r ail was low current limited so it would have had a chance to form. On the m ain rail I suspect it would have been confetti.

NT

Reply to
tabbypurr

The reason is space. The circuit works fine with a large non-polarized cap but most everything else is SMT parts. So the one capacitor occupies 30% of the space and looks ugly.

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Reply to
billbowden

n
o

I have use -10% as a ROT too without failure. Quality of source will affe ct leakage which allows some to be better. When failure occurs it causes s elf-heating leading to thermal runaway with explosive results if enough ene rgy at source is available. If high R series was used, that would limit he at energy such as in filters. THen thermal resistance vs electrical R defi nes the runaway breakpoint using I^2R.

The rule is safe at room temp and one loses margin at elevated temp where t here is more leakage.

So no problem

Reply to
Anthony Stewart

Problem solved. I found a real of 1000 SMT 0.47uF caps at a swap meet for $2. They are rated at 50 volts but didn't have any polarity markings, I charged one to 10 volts both ways and there was no leakage current. I soldered one into my circuit and it works fine. It's amazing this little SMT part can replace an older through-hole part about 100 times larger. But the older cap is probably rated at 200 volts, so it's big.

Reply to
billbowden

Why would you expect polarity markings? THey're undoubtedly ceramic caps. Even X7R 0.47uF, 50V, caps come in 0805s.

Reply to
krw

All of the electrolytics i have ever used including the old wet electrollytics were useable and low leakage in that region. The Sprague TE series were good to almost full rated voltage (reverse mode).

Reply to
Robert Baer

The size is a grain of rice, so I expected a polarity for a 1/2 microfarad cap at 50 volts. Turns out it doesn't work. I had the thing running for a couple days and then spilled a glass of coke on it and it quit working. The cap had shorted and measured 40 ohms both ways. I put the big mylar cap back in and everything is ok.I have no idea why some coke water woujld destroy the cap. It's tied to an inductor so the voltage may have exceeded the 50 volt limit, but I doubt it. These SMT caps are only good for bypass applications, not for timing applications.

Reply to
billbowden

applications, not for timing applications.

There's only so much electrostatic energy you can store per unit volume for a given dielectric, and the makers all lie about it in various ways.

Murata seems to be about the most nearly truthful of the lot.

Cheers

Phil Hobbs

Reply to
pcdhobbs

"Grain of Rice" doesn't tell me much. That could be an 0805 or, at most a 0603. 1/2uF isn't shocking.

At 40-ohms, they're not much good for bypassing, either. The cap was probably cracked. You may have cracked it when you soldered it. The normal failure mode for MLCCs is shorted.

They aren't usually used for timing applications because the ceramic dielectrics used in the lager value caps are so poor. Bypassing isn't their only use, though.

Reply to
krw

You probably grew a solder dendrite across the surface of the board underneath the cap.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Reply to
Phil Hobbs

I found a 0.47uF silver mica cap in my inventory that measures 5/8 by 3/4 by

1/2. I think silver mica is the best choice, but it's too big to fit on the board. Buit it sure is a nice looking cap.
Reply to
billbowden

Those nice brown ones are pretty, I agree. I have a few dozen in the drawer myself. They have decently high Q, but their soakage is poor.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

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