johnbl@tekig5.UUCP (John Blankenagel) (11/30/84)
> So it was with great expectation that we charged this thing up. Got a piece > of 16 gauge wire. Taped it to a stick. Sloowly approached the terminals > with it ... and ... nothing happened. Apparently these things have a fairly > low internal resistance so they discharge shortly after the current is > removed. Sigh. Where did you learn your physics? These capacitors have a large internal resistance. That is one of the trade-offs that had to be made in order to make a capacitor with 3.3 F of capacity. They certainly would not be too useful if they discharged shortly after the current was removed. The main use of these capacitors is for such things as battery backup. They will produce a small current for a long time, but don't try to filter the high frequency ripple out of a power supply with them. It won't work. If you want to melt wires, get a large computer power supply capacitor. That is what they were made for (and you thought it was to filter power supplys). John Blankenagel
kpmartin@watmath.UUCP (Kevin Martin) (12/02/84)
>> So it was with great expectation that we charged this thing up. Got a piece >> of 16 gauge wire. Taped it to a stick. Sloowly approached the terminals >> with it ... and ... nothing happened. Apparently these things have a fairly >> low internal resistance so they discharge shortly after the current is >> removed. Sigh. > > Where did you learn your physics? These capacitors have a large internal >resistance. That is one of the trade-offs that had to be made in order to make >a capacitor with 3.3 F of capacity. They certainly would not be too useful >if they discharged shortly after current was removed. The main use of these >capacitors is for such things as battery backup. They will produce a small >current for a long time, but don't try to filter the high frequency ripple >out of a power supply with them. It won't work. If you want to melt wires, >get a large computer power supply capacitor. That is what they were made >for (and you thought it was to filter power supplys). I think we are getting into confusion over leakage resistence vs. series resistance. The former determines how fast a capacitor discharges when its terminals are left open. The latter determines how fast it discharges when you short it out. leakage ______/\ /\ /\_____ | \/ \/ | | | | +-- | | | # | series | | # | __________/\ /\ /\___________| #____________________ \/ \/ | # | # +-- Electolytic capacitors, in general, have a lower leakage resistance than other types (e.g. mica or mylar), since the insulator is just a chemical film between a piece of metal (usually) and some semi-liquid gunk. If you apply reverse voltage, the film gets electrolysed out of existance, which is why the electrolytics are polarized. By varying the composition of the electrolyte (the gunk), the thickness of the film can be adjusted. A thicker layer gives a higher voltage rating and higher leakage resistance, but a lower capacitance. To increase the capacitance without reducing the voltage rating, you have to increase the (metal) plate's surface area. You can make it bumpy by pressing a pattern into it; someone else mentioned coating the plate with carbon particles; you can make the plates themselves thinner (and thin out the layer of electrolyte too). The first two solutions are the ones where big technological improvements are being made. The latter solution increases the series resistance, since the conductors via which the charge leaves the capacitor are thinner. If you want to blow up a wire, you need a low *series* resistance, so the capacitor will discharge rapidly. You should have no problems with the capacitor discharging through its leakage resistance. Kevin Martin, UofW Software Development Group
keithd@cadovax.UUCP (Keith Doyle) (12/04/84)
> So it was with great expectation that we charged this thing up. Got a piece > of 16 gauge wire. Taped it to a stick. Sloowly approached the terminals > with it ... and ... nothing happened. Apparently these things have a fairly > low internal resistance so they discharge shortly after the current is > removed. Sigh. Maybe you didn't give it enough time to charge up? Lets see... charging with 1 volt thru 100 ohmns would take 100 seconds to charge to .63v in a 1 F capacitor.
paulb@hcrvx1.UUCP (Paul Bonneau) (12/06/84)
[Out Vile Jelly!] >> So it was with great expectation that we charged this thing up. Got a piece >> of 16 gauge wire. Taped it to a stick. Sloowly approached the terminals >> with it ... and ... nothing happened. Apparently these things have a fairly >> low internal resistance so they discharge shortly after the current is >> removed. Sigh. > Where did you learn your physics? These capacitors have a large internal >resistance. That is one of the trade-offs that had to be made in order to make >a capacitor with 3.3 F of capacity. They certainly would not be too useful >if they discharged shortly after the current was removed. The main use of these >capacitors is for such things as battery backup. They will produce a small >current for a long time, but don't try to filter the high frequency ripple >out of a power supply with them. It won't work. If you want to melt wires, >get a large computer power supply capacitor. That is what they were made >for (and you thought it was to filter power supplys). I stand corrected! My thinking on the matter was that they had a small PARALLEL resistance (ie across the plates) not a large series resistance. However I am by no means an expert on the subject (I was only making a guess). However if there was a such a beast (low resistance across the plates) the time constant would restrict its use to such things as power regulation. -- I'm a man! I'm not a horse! Paul Bonneau {decvax|ihnp4|watmath}!hcr!hcrvax