Solar Powered Joule Thief

After building the Wearable Joule Thief project I became quite intruiged by the circuit and wanted to play around with it a bit more. I particuarly wanted to make it solar powered with my objective being that it should be able to run through the night even during the dark winter months. A Joule Thief will easily run for a week on a fully charged battery so I figured that one night shouldn't be a problem even with a partially charged battery. To achieve a good runtime the circuit must consume the least amount of current possible and have the highest rated charging components possible (solar cell and battery) within the size restraints of the enclosure used. As battery life was a major factor in the building of this project, I knew that I needed to spend some time on the design of the transformer. Although the type of core, size of wire and the number of turns used are not too critical for the circuit to work, they can affect the current consumption (measuring the current isn't an exact art as the circuit is actually pulsing on and off at high frequency but getting it as low as possible is still a good indicator). The classic way of making a Joule Thief transformer is to use bifiliar windings where two wires are wound around the core together and then the end of one is connected to the beginning of the other, effectively forming the centre tap that goes to battery positive. I found though that simply winding the coil linearly around the core with a tapping point along the way works just as well, plus it's much easier to wind and you don't have to worry if you've got it phased correctly!


The size of toroidal core isn't so important but smaller types will fit easier onto the veroboard, which is my favourite standard size of 9 x 25 holes. Although there isn't a strict colour standard for these things (Red may mean one thing to one manufacturer and something completely different to another), I've found that Green ones always seem to work and can usually be salvaged (as mine was) from a scrap mains adaptor. When winding the coil, it's worth finding the optimum number of turns as there is a 'sweet spot' where you get minimum current drain. By experimenting I found that 9 turns for the primary (base) winding and 15 turns for the secondary (collector) winding worked best. I also tried a very small core and found it needed an extra turn on the secondary, so there's always room for improvement. Although you can get away with using pretty much any insulated wire, I used 0.46mm (26SWG) enamelled copper wire which again was salvaged from a scrap power supply. Current consumption can be further reduced by changing the value of the base resistor. The original Joule Thief circuit uses a value of 1K but here it has been increased to 10K which cuts the current drawn by about half, albeit at the expense of slightly reduced light output. I think it's a good trade off though. Usually AA batteries are employed in this type of application but I decided to use a AAA instead as they are now available with fairly large capacities and being smaller, take up less space on the veroboard allowing room for a mounting hole to be drilled at each end if desired. I doubt that the amount of daytime sun in the UK would ever be able to fully charge a high capacity AA battery anyway! A PCB mounted battery holder was obtained with long wires that could be folded horizontally under the board and soldered to the appropriate points. Although not shown on the diagram, remember to cut the tracks underneath the holder or else the battery will short out! An extra little addition to the board was added in the form of a Molex KK plug used in reverse to make a PCB socket that the LED can be directly plugged into. This is handy if you want to quickly try different shapes and colours. Incidentally, if you want to make the light from a clear LED less directional, it can be diffused by 'roughing up' its surface with sandpaper. The KK socket and 'frosted LED' can be seen in the photo

Veroboard layout

I won't go into the operation of the Joule Thief circuit here as there is a ton of information on the internet already, so I'll just explain the three extra components required (4 including the panel) to make it solar powered. Originally I considered using a constant current charging circuit but then reminded myself that the beauty of the Joule Thief is its simplicity, so kept it basic (anyway there's nothing constant about the British weather!). Charging is carried out by a standard 1N4001 silicon diode placed between the solar panel + and battery +. This allows charging current to flow from the panel into the battery during the day but will stop the battery discharging back into the panel at night. A silicon diode will drop 0.6 volts across it and so a single NiMH battery can be charged using a 2 volt solar panel (2V - 0.6V = 1.4V). I've found that Schottky diodes are unsuitable in this application as they allow some current to pass in the reverse direction. Voltage from the solar panel is monitored by a 4K7 resistor and when there is enough daylight to produce 0.6V, the first transistor will turn on effectively grounding the base of the Joule Thief transistor which stops it oscillating. The charging diode also prevents battery voltage from activating the first transistor. The type of solar panel used should ideally match the battery, so if you have a 750mAh AAA then the panel chosen should be able to supply approx 75mA to fully charge it in the usual 'rule of thumb' 16 hours but... as we never get 16 hours of sunlight here, a panel with a slightly higher current can be used. I used a 2V 130mA panel coupled with a 1100mAh AAA battery which is fine as in practice the panel never achieves its rated current anyway

Details of the transformer:

Green toroidal ferrite core (the exact type is anyone's guess!)
0.46mm (26SWG) enamelled copper wire
24 turns tapped at 9 turns on a 11mm OD x 6mm ID x 6mm Ht toroid or
25 turns tapped at 9 turns
on a 9.5mm OD x 3.5mm ID x 3mm Ht toroid
Tapping point goes to battery plus, shorter coil goes to base and longer coil goes to collector

Using the above transformer specifications you should get a current consumption of around 14mA at 1.4V (with a 10K base resistor)