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[Phys-L] Re: Polarity of a 1F (super) capacitor

(1) Super capacitors generally are not polar. If there is no marking,
it is a non-polar capacitor. On the other hand, capacitors requiring a
specific polarity should be clearly marked on the body of the capacitor.

(2) Do not associate lead length with polarity with capacitors, LEDs, or
any other components. The difference in lead length is to make
insertion easier. You put the longer lead through its hole first, then
put the shorter lead through its hole. That is significantly easier
than trying to get both leads aligned with the holes simultaneously.
There is no uniform standard as to whether the longer lead is positive
or negative, and you'll only get yourself into trouble if you act as if
there is a standard.

(3) Be a bit careful if charging a super capacitor from a transformer
and rectifier circuit. When discharged, any capacitor present a near
short-circuit to the rectifier. The high initial current during
charging of a capacitor from its discharged state can exceed the current
rating of the rectifier diodes. With normal-sized capacitors this high
surrent doesn't usually last long enough to hurt the diodes, although it
can. As the capacitance increases, the risk of damage to the diodes
increases. The super caps create a reasonable risk of diode damage
unless a resistor is placed in series between the diodes and capacitor,
or some other current limiting circuitry is used. In some applications
you might want to use the special high-inrush resistors that limit high
currents while maintaining low resistance to low currents.

Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
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