On May 25, 22:05, Steve Leach wrote: tantalum before and 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