A semi-practical capacitor-powered flashlight


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.
                capacitor-powered flashlight

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:
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:
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:
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.
Diagram of the
                capacitor-powered flashlight voltage converter

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.
Circuitry of the capacitor-powered flashlight
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.

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!   


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.


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.  

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