Tantalum cap marking

[Hupla]

I have Tantalum cap here and the marking on it is .22 and then under it it has 35 and to the right shows it's polarity.

Im pretty sure the 35 is its voltage rating but what does the .22 indicate? 22pf? or .22pf?

[Dan N]

220nf 0.22uf etc...

[Hupla]

220nf 0.22uf etc...

I was under the impression that tantalum were in the pf range?

[bumblebee]

If it was 22pF it wouldnt have the  . before the 22.

[amptramp]

220nf 0.22uf etc...

I was under the impression that tantalum were in the pf range?

Tantalum can be anodized, so it is used to make an electrolytic capacitor.  The value you quoted is 0.22 uF at 35 volts.  The tantalum pentoxide dielectric has a dielectric constant of 27, so these capacitors are quite small physically and the dielectric is almost pinhole-free, so the leakage current is small.  However, dry-slug tantalum capacitors should never be used as power filters because any local heating will turn the tantalum pentoxide into tantalum dioxide which is conductive.  If any part of the dielectric does turn into the dioxide, the local heating will cause the local area to turn into dioxide as well, so you can get a firestorm happening in the dielectric unless the current is limited so that the heat required to transform the pentoxide to the dioxide cannot be delivered to the capacitor.  The dielectric film cannot be made very thick, so tantalum electrolytic capacitors are usually limited to 75 volt sizes in most types with maybe a few 100 volt types.

[PRR]

> I was under the impression that tantalum were in the pf range?

No. They are Electrolytic, and we only use that when we need more C than Film can give us, and can tolerate a little leakage.

0.05uFd and down, we can usually find a happy film-cap, so we rarely find Electrolytic that small.

Tantalum is EXPENSIVE. Above 500uFd the cost gets killer. Designers try to make-do with leakier Aluminum Electrolytic because the price is so much lower. (And improved Aluminum Electrolytic, and the occasional BANG from Tantalum, are pushing Tant off the market altogether.)

So the main zone for Tantalum was 100uFd to 0.1uFd. ".22" is surely 0.22uFd.

> 22pf? or .22pf?

22pFd is a very small ceramic; you rarely find them that small as a standard part. In Tantalum Electrolytic it would be microscopic. And cost more to make.

0.22pFd is fabulously small. A tenth-inch of twisted-pair is ~~0.22pFd. The leads on any standard cap exceed 0.22pFd. The parasitic capactiance on any node in most circuits is ~~10pFd, 50 times higher; another 0.22pFd changes nothing. You don't buy a 0.22pFd. If you need such a tiny amount, you space your parts to get that much stray. If you look at very old radio, sometimes a dead-end wire came off the plate to within an inch of the grid: that might be a few-tenths pFd feedback (neutralizing) "capacitor".