January 2016 65
least 300 mcd. The LEDs above 1CD
typically use a clear lens, so can’t be
seen off axis. As an alternate, if you
only need it to run a couple of days,
then just solder a L-36BSRD flashing
LED inside a 9V battery snap. I make
these up for cavers; they leave them
behind like breadcrumbs to find their
way back.
Bob Turner
Salamander Bay, Australia
[#11151 - November 2015]
Super Capacitor Comparison
I need some help understanding
what’s going on inside a super
capacitor. I did some experiments
using regular capacitors as a backup
power supply to a real time clock
chip. Mathematically, the amount of
time an electrolytic capacitor (1500,
2200, and 4700 µF) would power the
chip became predictable once I came
up with a formula. However, when I
connected a super capacitor, the math
broke down.
The real time clock should have
exhausted the stored power in the
super capacitor after exactly three
days. Instead, it is still maintaining the
correct time after four months! Clearly,
something is physically different about
a super capacitor. It acts more like a
battery than a capacitor.
Can someone explain to me how
the chemistry of a super capacitor
differs from electrolytic capacitors?
Why does the amount of charge
stored seem to far exceed the capacity
indicated by its Farad value, under an
extremely light load?
#1 “Super caps” are, indeed,
electrolytic capacitors. Their
construction typically uses tantalum
(not aluminum) plates to obtain a
large capacitance value in a relatively
small package compared to standard
aluminum electrolytics. Plus, their
electrolyte formulation differs enough
from aluminum capacitors to allow a
larger charge vs. size capability.
Because of their large charge
capability, they are ideal for use in
short-term backup applications where
low currents (typically, less than 100
microamperes) are required (i.e., real
time clock chips). However, they
are not the same as a battery (i.e.,
lithium coin cell) since — like regular
electrolytics — super caps do self-discharge over time and they can not
deliver a large supply current (i.e.,
>1 mA) for longer than a couple of
seconds.
Ken Simmons
Auburn, WA
#2 For any given capacity rating,
an electrolytic capacitor should
behave very much like a super
capacitor, at least in a low current
circuit like a RTC. However, super
capacitors provide much higher
capacity in a given package size. So
much so, that they are rated in millifarads (1 mF = 1,000 µF) or Farads (1F
= 1,000,00 µF) rather than microfarads (µF).
Are you sure you took this
into account when you did your
calculation? If not, you may be off by
a factor of 1,000 or one million. The
other possibility is leakage current
causing the electrolytic capacitor to
drain faster than you expect. This
presents itself as if there was a resistor
connected across the capacitor,
constantly draining away its charge.
The leakage current value is listed
on the datasheet for the capacitor
and must be taken into account
when you’re using it for long storage
applications like a real time clock.
Mark Lewus
Denville, NJ
#3 Your calculations are probably
correct. However, you may have
used the worst case current from the
datasheet; if you use the nominal
current, it should be good. A couple
of months is the expected retention
at room temperature. Just be careful
if you use Schottky diodes in your
circuit (to power the RTC from
normal 5V). The leakage current in
a poorly selected diode can exceed
the standby current. (Schottky diodes
are made with three doping levels,
producing forward drops of 200, 300,
and 400 mV; the 400 mV has the
lowest leakage current. It will usually
have an H in the part number.) A
super capacitor is a electrolytic
double layer capacitor (EDLC), and
each electrode is coated in very fine
carbon granules. The total surface
area is about 1,000 times higher
than just aluminium foil, with a much
thinner dielectric; hence the increase
in capacitance (and drop in maximum
operating voltage). There is no actual
oxide layer for the dielectric. An
EDLC has charged layers of ions a few
molecules thick instead, and charge
and discharge just moves the ions
back and forth across the layer.
Bob Turner
Salamander Bay, Australia
[#12153 - December 2015]
LiPo Smart Charger
I’m looking for a smart charger
for LiPo cells that can run from
a solar panel. I’ve heard that the
combination is incompatible because
of fluctuations in output from the
solar panel. Is this true? If so, is a
workaround a larger panel?
There are a lot of solar to li-ion
battery charger ICs. You might get
one on a breakout board, like the
following: www.ladyada.net/make/
solarlipo
I made a small solar powered
solar tracker that has a similar IC, and
charges two li-ion batteries to about
8.0 volts. It works well. It’s been in
operation over a year now.
John McCullough
La Habra, CA
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