High-energy capacitors of the type discussed can produce
hundreds of amps when short-circuited, possibly
causing severe burns, fire and/or property damage! Please be
aware of these hazards when using these capacitors and take
precautions to avoid accidentally shorting them out.
Other "capacitor-powered" flashlights:
Figure 1: The capacitor-powered flashlight. The white PVC
portion at the end contains the inverter circuity to run the
LED and the LED itself as well as shielding the positive
terminal of the capacitor to prevent accidental
short-circuits. Click on the image for a larger version.
You probably remember seeing advertisements for those flashlights
whose claim was that they would never need batteries
and one simply shook them back-and-forth to generate all the power
that was needed. As it turns out many of these same
flashlights actually do contain batteries and that
while they still worked if they were removed, it took several
minutes of shaking to get any usable light and that it was quite an
effort to maintain a useful light output with these same
flashlights.
In a nutshell, one could vastly improve these flashlights if they
used:
A better capacitor. The cheap flashlight had a
rather small (0.22 Farad) capacitor for energy storage - not
very much energy, really, approximately 6.6 Joules maximum or
less than 1/1000th of what a single AA alkaline cell
contains! Being a standard "super cap" its internal
resistance was quite high (10's of ohms) which meant that a
large percentage of the energy dumped into it during charging
and that extracted from it to run the LED was lost as heat - not
much heat, but heat just the same.
Switching converter to run the LED. The LED
didn't even begin to light until 2.7-3.0 volts or so
appears across the capacitor and it isn't usefully
bright until there is 3.6-4.2 volts available which meant that a
significant portion of the energy in the capacitor (all of that
at voltages of 3-ish volts and below) was unusable. A
simple switching converter would allow both extraction of that
additional energy as well as regulate the LED's current so that
its brightness was more consistent over the entire charge range
and, in theory, could also be adjusted upwards or downwards as
necessary.
As it turns out, the back-and-forth shaking motion isn't really a
very efficient means of generating electricity in terms of expended
muscle energy and compared to a conventional crank-type generator,
it would necessarily be larger and heavier in order to be made
reasonably efficient. By gearing up rotational speed, one can
more-efficiently spin a smaller magnet faster in a larger number of
generator poles with a motion that requires less human effort and
what's more, a crank-type generator is quite "scalable" in its
operation: You could crank it gently for a long time or do so
vigorously for a shorter time and get roughly comparable results in
terms of total energy output - with reason, of course.
What is more likely in a practical situation is that one actually
has a source of power somewhere else(e.g. an
already-charged battery, solar panels, a plug-in power supply,
etc.) that can be used to charge the flashlight and that it's
unnecessary to actually bring along the means of charging the
battery with you.
Such devices are already available in the form of batteries -
particularly rechargeable - so having a capacitor-powered rechargeable
flashlight is more of an intellectual exercise rather than one of
practicality, but being practical has not usually been much of a
deterrent to the experimenting nerd!
A flashlight using a capacitor that actually stores
a useful amount of energy:
Some time ago The
Electronic Goldmine in Arizona had a large quantity
of Maxwell
Technology BCAP0010 "BoostCaps" (tm) available. These
were obtained for just $6 each had a rated capacity of 2600 Farads (yes,
that's 2.6 kF or kiloFarads!) at 2.5 volts with a "surge"
voltage of 2.8 volts - whatever that means... (I noted that
at other times they had models that were closer to 3 kiloFarads at
2.7 volts, but these were sold for far more than $6 each.
Alas, as is the nature of surplus, the supply was limited and they
sold out fairly quickly. Sometimes these types of capacitors
will show up elsewhere on the surplus market so if you want some,
it would pay to look around!)
Unlike the 0.22F capacitor in the original "shake" flashlight, these
units, with their 10000+ times larger capacity (albeit lower
voltage) have very low internal resistance - in the
milliohm area - as their intended use was to provide a large burst
of current for a short time, say, in an electric vehicle.
To demonstrate their high-current ability to myself I charged one of
these capacitors to 2.5 volts and then I carefully shorted out the
terminals with a length of #14 AWG bare copper wire, holding it with
a pair of pliers. Within a second or so the current from the
capacitor had burned this wire open and in so-doing, it only lost
about 0.1-0.15 volts! For these particular capacitors the
maximum rated current is on the order of 600 amps so
I have no doubt that I could have repeated the same trick (not
recommended!) with larger gauge wire!
What this means is that resistive losses of this type of capacitor
are negligible when it comes to its being charged
from power source and by being discharged by an LED.
As an illustration, let's assume that we need to draw 100 milliamps
from two different types of capacitors:
A standard "supercap" with an internal resistance of 10 ohms -
an approximate, typical value.
A "boostcap" power system with an
internal resistance of 100 milliohms - a resistance value being
mostly that of thin wires connecting to the
capacitor: The internal resistance of the capacitor
itself is actually much lower!
For this example we really don't care about the actual voltage
or capacitance involved - only resistance and current.
If we take the formula: P = I^2 R (that is, power equals the
square of the current multiplied by the resistance) and ignoring
other practical losses we get:
A loss of 100 milliwatts from the super cap.
A loss of 1 milliwatt from the "boostcap" and its connecting
wires.
Now, if that LED were running from, say, 2 volts at 100 milliamps,
the total power for the LED in each case would be 200 milliwatts,
but you can see that the super cap would be losing 100 milliwatts in
heat while the boost cap would be losing just 1 milliwatt - a
considerable difference!
Clearly, the use of a boostcap offers superior efficiency when
discharging, but it also works in reverse: One could dump many
amps into the capacitor (if you used thicker connecting wire) and
charge it very quickly and efficiently.
We still have the problem of running the LED, however. The
boost cap capacitors that I obtained were designed to be charged to
just 2.5 volts or so and this is too low to run a standard white LED
which requires 3.6-4.2 volts so a circuit is required to bring up
the voltage and one of the simplest of these circuits is a variation
of the ubiquitous "Joule Thief"
circuit - a topology that can effect 75%-85% energy conversion
without using exotic components.
Figure 2: Schematic diagram of the circuit driving the LED. Click on the image for a larger version.
While there are more efficient circuits out there, there are almost
none that are simpler and adaptable to parts that might be found in
scrounging around.
What I came up with is the circuit to the right. At it's heart
(Q1, T1, R1, LED1) is the Joule Thief circuit comprising a "Blocking
Oscillator" that, using inductive "kick", will
produce a voltage higher than that of the power supply itself,
sufficient to light the LED.
While the simplest version of the circuit using the aforementioned
components did work, it was very bright at the higher
capacitor voltage (above, say, 1.8 volts) but it got noticeably
dimmer - but still useful - at lower voltages. Since the
intent was to provide only a "useful" amount of light, I decided
that I didn't need "maximum brightness" at the higher voltages and
that I'd be happy with a much dimmer - but consistent - brightness
at a much wider range of capacitor voltages. This also had the
obvious side-effect of allowing a much longer run-time since,
overall, the power consumption was reduced to a
fairly steady level over the entire voltage range.
To regulate the current a simple circuit was added consisting of T2,
D1, R2, R3, C2 and Q2. The way this circuit works is that the
AC current through the LED goes through the primary of T2 and the
voltage on its secondary is integrated by D1, R3 and C2 and if this
resulting voltage is too high (correlating with higher average LED
current) Q2 would conduct, "pinching" off the drive to Q1.
Originally, a circuit consisting simply of a series resistor along
with a transistor like Q2 was tried in which the current through the
resistor - if it exceeded the 0.6 volts required to turn on the
transistor - would be used to turn off the oscillator and regulate
it, but this added resistor required that a bit of the LED's current
to be lost as heat - plus, it just didn't work very well!
Using a simple transformer arrangement to transform the current into
voltage reduced the efficiency losses one might have in a
current-sense resistor while still being fairly simple. Being
simple also meant that there was still a fair amount (say, 25% or
so) of LED brightness variation across the target 1.1-2.5 volt range
but that was considered to be acceptable for a simple circuit.
This circuit is also somewhat affected by temperature owing to the
fact that not only do the various current gains of the transistors
change, but so do the threshold voltage of the transistors and D1.
In this circuit there's really only one critical component and
that's Q1, an NPN transistor that was specifically designed for use
in photoflash inverters and as such it can switch several amps of
current, this being many times that of the more ubiquitious 2N3904
or equivalent. While a standard NPN like the '3904 will work,
it will not work nearly as well and it will be much less
efficient. Fortunately, this transistor is pretty cheap and it
(and similar types) are readily available from suppliers like Mouser
- or you could scrounge one from the photoflash unit of a discarded
disposable film camera. Similar transistors to the KSD5041 are
the 2SC695 and NTE11.
An even better alternative for Q1 was suggested by Brooke Clarke (a
link to one of his web pages analyzing the Joule Thief may be
found here) and that is the Zetex ZTX1048A,
available through Mouser and Digi-Key for approximately $1 each in
small quantities. This device - like the KSD5041 and 2SC695 -
offer increased efficiency by its very low collector-emitter
saturation voltage - an important consideration when one has
conflicting needs of both high current and low voltage in a
circuit such as this and according to the specification sheets, the
'1048 offers the possibility of even lower saturation voltage than
the '5041!
Figure 3: Inside the flashlight, showing the LED and inverter
circuitry. Click on the image for a larger version.
The two inductors were toroids salvaged from a defunct computer
power supply and even some of the original wire was salvaged!
In this particular power supply - and several others that I have
seen - it's common to see several different-sized toroidal inductors
and I happened to choose the larger one for T1.
The circuit itself was built with components hanging in free space,
soldered to each other's leads with the entire assembly being
eventually "potted" in thermoset ("hot-melt") glue to stabilize
them. As can be seen from the pictures a small piece of PVC
pipe was used to not only contain the circuit, but also to shield
the positive terminal of the capacitor so that it was not possible
to accidentally short it out - something that could conceivably
start a fire!
The LED itself was a 3-watt Luxeon III Star that I had kicking
around but it's not being run at anywhere near its maximum ratings
so about any 1-3 watt white LED that you might find would
suffice: A standard epoxy-encapsulated white LED would not be
recommended in this circuit at the current drive levels obtained,
although several (say, 4-6) in parallel would probably be fine.
Comment:
It is required that the LED's "on" threshold be
above the supply voltage for this circuit to work
properly. This means that only blue, white and some
times of green LEDs may be used. If a lower-voltage LED
is used (e.g. red, yellow or infrared) it will likely be
destroyed immediately since it will conduct current through
the coil since it will be "turned on" by the voltage on a
fully-charged capacitor! Putting two such LEDs in series
will permit them to work in a circuit like this one at and
above 1.8 volts.
Originally, I considered putting a lens on the LED to concentrate
the light, but I soon realized that without using a special lens
designed specifically for this LED I'd end up with less
light overall since it would likely not focus efficiently.
Even with the LED being "bare" its light output is more than enough
to be useful - even walking along a mountain trail in the dark - and
its beam is broadly cast so that one isn't as subject to the
"spotlight effect" of some LED flashlights where you can see only
that which is in the beam and everything else around you disappears!
Charging the flashlight's capacitor:
To charge the flashlight I set a variable-voltage bench supply at exactly
2.60 volts and then connect it on the connector (not visible in the
pictures) which is connected directly across the capacitor.
From a state of complete discharge (0 volts) it will take several hours
for a 1 amp bench supply to fully-charge the capacitor!
Whatever you do, do not allow the capacitor's voltage
to exceed its maximum ratings or else it may be damaged! I
have no idea what actually happens if you do this, but I wouldn't
recommend trying!
The power supply I use to charge is a linear type
which means that it's extremely inefficient in its charging of the
capacitor, burning up most of the energy in heat. For the
highest in efficiency, one would ideally use a switching-type supply
to optimally convert the input power to whatever the output voltage
happened to be at that point in the capacitor's state-of-charge,
stopping when the maximum capacitor voltage was reached. Such
a supply would not only waste relatively little in heat, but it
would be the optimal solution if the capacitor needed to be charged
from a battery source or solar panel.
I've used this flashlight for more than a year, now - both around
the house and at night while hiking in the mountains and in that
time I have only charged it once!
While this may sound like a lot of energy storage (and it is!) it's
worth noting that the total amount of energy stored in one of these
capacitors when it is fully-charged (approximately 8200 joules) is
in the same ballpark as the amount of energy contained in a single
AA alkaline cell!
Comments:
For an AA Alkaline cell, given an average of about 1.25 volts
and capacity of 2.2 amp/hours at that voltage, this correlates
with a usable energy storage capacity of 9000-9500 joules,
depending on load, temperature, etc.
These calculations also ignore the fact that some of the
energy being stored in the capacitor or battery at low voltage
is not usable as the LED's converter circuit will not operate
below approximately 0.9 volts to extract energy.
So, does this flashlight actually work? Yes, it
does!
Is this flashlight practical? No, not really.
As it turns out the capacitor itself is not only fairly heavy -
about 525g (1 pound) - but it is quite big - 60mm (2-3/8") diameter
and 172mm (6-3/4") long - not including the circuity or bolts on the
end connectors: I have fairly large hands and I find myself
moving the flashlight from one to the other as I hike along owing to
a bit of muscle fatigue from its diameter and weight. Again,
the capacitor itself only cost about $6 from a surplus seller but
that was just a fraction of its original cost (perhaps $150-$200)
and one could buy an awful lot of AA cells for its original price!
I suppose that as time goes on capacitor technology will improve and
eventually its power/size/weight will approach (and even surpass!)
that of conventional battery technology, but until then a flashlight
such as this is a bit of a nerdy novelty!
"Boostcap" is a trademark of Maxell Technologies.
Again, PLEASE NOTE:
High-energy capacitors of the type discussed can produce
hundreds of amps when short-circuited, possibly
causing severe burns, fire and/or property damage! Please be
aware of these hazards when using these capacitors and take
precautions to avoid accidentally shorting them out.
Do you have any comments or questions? Send an email. Please note that
the information on this page is believed to be accurate, but
there are no warranties, expressed or implied. The
author cannot take responsibility for any damage or injury
that might result from actions taken (or not taken) as a
result of reading this page. Your mileage may
vary. Do not taunt happy fun ball.
