On May 25, 22:05, Steve Leach wrote:
> If you could humor my electronic ignorance, what exactly IS a
tantalum
> cap and how does it differ from a normal capacitor? I was never
before
> aware that there could be more to a capacitor than plates (or foil)
and
> an electrolyte. How can a capacitor have a polarity?
Almost all electrolytic caps are polarised.
In principle, a tantalum capacitor is just like an aluminium
electrolytic, only using a more exotic metal (tantalum pentoxide has a
dielectric constant about 4-5 times higher than aluminium oxide) and a
solid electrolyte. The anode is the foil (or bead, if it's sintered
tantalum tantalum), the oxide layer on it is the dialectric, and the
electrolyte is the cathode.
Passing current from cathode to anode will build up the oxide layer,
the other way will break it down. From a chemist's point of view,
putting electrons into the cathode and taking them out of the anode
makes some of the metallic aluminium (Al) ionise into Al+++, and if
there are oxygen ions (O--) adjacent, it forms alumina (Al203) --
standard RedOx reaction, exactly the same as anodising aluminium for
protection. You have to maintain the polarity to maintain the oxide
layer; if you reverse it the electrons will break down the oxide layer.
The aluminium ions would gain electrons, the oxygen ions would lose
them, the alumina becomes aluminium and oxygen, and then you have no
more highly-insulating aluminium oxide dielctric.
Think of the aluminium-alumina junction as a junction diode, with a
very high resistance in one direction and a very low resistance the
other way.
There's a brief description of construction at
http://www.chipcenter.com/eexpert/akruger/akruger006.html
--
Pete Peter Turnbull
Network Manager
University of York
Received on Mon May 26 2003 - 04:56:00 BST