Solar Powered Joule Thief

After building the Wearable Joule Thief project I became quite intrigued by the circuit and wanted to play around with it a bit more. My goal was to make it solar powered with the objective being that it should be able to run through the night even during dark winter months. A Joule Thief will easily run for a week on a fully charged battery so I figured that it should be able to run for one night even if not fully charged. 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 would need to spend some time getting the transformer right. Although the type of core, size of wire and the number of turns used are not too critical for the circuit to work, they do affect the current consumption. The classic way of making a Joule Thief transformer is to use bifilar windings where two wires are wound around a toroidal 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 is much easier and works just as well, with the shorter coil going to the base of the transistor, the longer coil going to the collector and the tapping point going to battery positive


Although the size of core isn't critical, smaller ones will fit more easily onto the veroboard which incidentally is my favourite standard size of 9 x 25 holes. Suitable cores can often be salvaged from scrap power supplies and for some reason I've found that Green ones always seem to work best. When winding the coil there is a 'sweet spot' where you get minimum current drain, so if using a random size core, it's worth experimenting to find the best number of turns. Current consumption can be further reduced by increasing the value of the base resistor. The original Joule Thief circuit used a value of 1K but here it has been increased to 10K which greatly reduces the current drawn, from over 30mA down to approx 14mA at 1.4V depending on how ‘good’ the transformer is, albeit at the expense of slightly reduced light output. I think it's a good trade off though. AA batteries are traditionally used in this type of application but I decided to use a single AAA instead as they are now available with fairly large capacities and being smaller will take up less space on the veroboard allowing room for a mounting hole to be drilled at each end (if needed). 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 connector used as a socket so the LED can be easily plugged in without soldering. This is handy if you want to quickly try different sizes and colours or even a flickering candle type. 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 (first photo shows the 'frosted' LED in its KK connector)

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 (four including the solar 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 and so kept it basic. Charging is carried out by a standard 1N4001 silicon diode placed between the solar panel positive (+) and battery positive (+). 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 about 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 for 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 light to produce 0.6V on the base of the first transistor, it will turn on effectively grounding the base of the Joule Thief transistor which stops it oscillating. The charging diode also prevents the first transistor being turned on by the battery. The solar panel should ideally match the type of battery, so for example if you use 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. The UK never gets 16 hours of uninterrupted sunshine anyway so using a panel with a slightly higher current should be fine. I chose a 2V 130mA panel coupled with a 1100mAh AAA battery, but in practice I doubt the panel will ever achieve its ‘over exaggerated’ maximum rated current anyway! So does it run through the night during the UK winter? Most nights it does... just about!

Details of the transformer

Toroidal Core - 10mm OD x 6mm ID x 5mm Ht (Green ones seem to work best!)
Number of turns - 26 turns tapped at 8 turns (wind 8 turns, make a tapping point, then carry on winding for a further 18 turns)
Type of Wire - 0.4mm enamelled copper (27SWG or 26AWG) or 0.45mm if measured with vernier calipers (due to the enamel)

There's scope for experimentation. A core size of 8mm or 11mm can also be used. 25 turns tapped at 9 turns works well too!

The Solar Joule Thief assembled inside a Kilner Jar with the panel sealed onto the lid with potting compound
(yes a hole did need to be drilled through the glass!)