john@hp-pcd.UUCP (john) (07/22/85)
<<<< Back when I first sat down to learn electronics (we won't mention the year or even the decade) they told us about capacitors and how they were measured in Farads."Of course the Farad is actually a ridiculous unit of measure because a one Farad capacitor would be about the size of a railroad tank car." Recently I have seen data on a new type of capacitor called the Maxcap(tm) double layer capacitor. A 1 farad maxcap is 1.1 inches wide and .55 inches high. It looks like its some sort of a very weak battery that can be fully discharged or even reverse charged without harm. Has anyone ever used these? It would seem that the initial surge current if you connected one to a 5 volt supply could easily overload most supplies. Any experiences and or horror stories would be appreciated. John Eaton !hplabs!hp-pcd!john
gnome@olivee.UUCP (Gary Traveis) (07/27/85)
> > Has anyone ever used these? It would seem that the initial surge current > if you connected one to a 5 volt supply could easily overload most supplies. > > John Eaton > !hplabs!hp-pcd!john There are a few problems with the 1 farad caps - their internal resistance is pretty high (ie: if you short one out, it won't spot-weld itself to the screwdriver) because the high-surface area material is (i think) carbon. Their leakage current could be better too. There are some newer ones on the market now that have lower leakage and internal resistance. Really they were meant for use in CMOS short-term power fail situations. Gary (hplabs,ihnp4,allegra) oliveb!olivee!gnome
larry@kitty.UUCP (Larry Lippman) (07/29/85)
> Recently I have seen data on a new type of capacitor called the Maxcap(tm) > double layer capacitor. A 1 farad maxcap is 1.1 inches wide and .55 inches > high. It looks like its some sort of a very weak battery that can be fully > discharged or even reverse charged without harm. > > Has anyone ever used these? It would seem that the initial surge current > if you connected one to a 5 volt supply could easily overload most supplies. > Any experiences and or horror stories would be appreciated. I have on occasion used some large capacitor arrangements (around 1 Farad) on process control instrumentation to provide a simple solution to short duration power line voltage drops (like starting motors). I simply use a low resistance (0.5 to 1.0 ohm) between the rectfier diode bridge and the capacitor to limit the charging current and thereby avoid transformer and diode stress. My loads are only in the couple hundred milliampere range, so the 1 Farad capacitor gives me pretty decent protection without the maintenance worry and added circuitry required to charge a nicad or gel cell. Actually, the current limiting resistor is not even necessary unless the transformer is vastly oversized with a very low impedance and high short circuit current. If you consider the total amount of the charging energy going into the capacitor (0.5*C*V**2) and figure out the watt-seconds, it presents a miniscule momentary overload for the transformer and rectifier diodes. Larry Lippman Recognition Research Corp. Clarence, New York UUCP {decvax,dual,rocksanne,rocksvax,watmath}!sunybcs!kitty!larry {rice,shell}!baylor!kitty!larry syr!buf!kitty!larry VOICE 716/741-9185 TELEX {via WUI} 69-71461 answerback: ELGECOMCLR "Have you hugged your cat today?"
karn@petrus.UUCP (Phil R. Karn) (07/29/85)
I've never understood the fascination some people seem to have with "large" capacitors like 1 Farad. What really matters is the ability of a capacitor to store energy. For a capacitor of a given size, this is generally much greater at high voltages (and correspondingly lower capacitances.) This is because as you increase the thickness of the dialectric, the capacitance decreases linearly, but the working voltage increases linearly. Since the energy stored in a capacitor is 0.5 * C * V^2, the net result is a linear increase in energy storage capacity. One of the reasons switching power supplies are so small for the amount of power they carry is that the AC line is directly rectified and filtered at relatively high voltage, before conversion to the output voltage. For example, I have here a Lambda 10A @ 5V switcher. The first cap is 330 uF @ 200V. When rectifying the AC line, this cap will store 4.36 J, but it is only half the size of a 35mm film can. On the other hand, storing this much energy in a 1F cap would require a working voltage greater than 3V, and it would certainly be much larger than this. Phil