Joule thief workshops

We spent a some time at the first Tekwizz ‘hands-on-workshop’ to identify practical projects that we’d be interested in making. One project is the ‘Joule thief’, so I did a bit of research to figure out how to make it at workshop-scale.

A Joule thief is a kind of DC-DC converter. It steps up the voltage of one non-rechargeable ‘dead’ battery sufficient to light a white LED (over 3V). It’s claimed it lights for 10 to 20 hours. A ‘dead’ battery is one which has run down to a voltage too low to power normal domestic electronics. I think it’s best not to use rechargeable batteries with a Joule thief, because it will run a rechargeable down so low it will be damaged.

The Joule is the SI unit of energy (or work), so it is quite a nifty name.

This is one of the first search hits: http://www.bigclive.com/joule.htm

The Wikipedia Joule thief page has some good information on how it works.

Most (all?) electronic step-up voltage converters use an inductor for energy storage, and some sort of oscillator to drive a transistor switch. The Joule thief is rather clever because it uses the oscillations of the step-up inductor to switch the transistor on and off. So it is beguilingly simple.

The components are:

• white LED, which are easy to get off-the-shelf,
• ferrite core used to wind the inductors,
• wire, which winds on the ferrite core to make the inductor,
• BC549C transistor (or better), used to switch the inductor,
• 1K resistor, used to pull the transistor on.

The resistor and a transistor are relatively low-cost and easy to get, though some authors recommend a better transistor than the original Joule thief BC549 (which is hard to find anyway; places like RS have discontinued them). For example look at http://quantsuff.com/ or http://www.instructables.com/id/Joule-Thief-LED-circuits/step6/The-Usual-Suspects/

The recommendation is to use a transistor with a lower collector-to-emitter saturation voltage, Vce(sat), than the 0.6V of the BC549. The aim is to maximise the (already low) battery voltage across the inductor. This should make the Joule thief more efficient, and run for longer. I don’t believe a Joule thief is as efficient as a specialised integrated circuit, and I think the spirit is we’re getting almost free power, so I don’t feel it is worth fretting too long about this. There are suitable sub-20p transistors with 0.3V Vce(sat) or less, half the Vce(sat) of the BC549.

A significant cost is the 38swg (0.15mm) enamelled/insulated wire. A Joule thief only needs 1 metre, so it is actually quite cheap, but the smallest reel I found was almost 2,000 metres, at about £15+VAT. Good if you’re doing workshops for hundreds but a bit of a waste for dozens.

The other problematic part is ferrite inductors. Low-cost (sub 10p) ferrite cores have a central hole 2mm or less in diameter (and come in packs of 20 or 10).

I wrote to ‘Big Clive’ and he replied, saying the Joule thief is pretty robust, and forgiving of component variation. So thinner or thicker wire would be okay.

I found reels of hundreds of metres of 34/35 swg for under £7 (+VAT), and 42/43swg even cheaper. The extra thickness might make it a bit stronger while winding the inductor, and the thinner wire might be easier to thread through cheap, small hole, ferrite cores. We’ll need to do some experiments.

Clive recommended trying to scavenge a ferrite core from a broken Compact Fluorescent Light (CFL). This would be good because it is free, but IIRC, CFLs contain some quite unpleasant chemicals, so be careful, and if you are at all concerned, don’t try. If a group of folks want to all run a Joule thief workshop, investing in a 10, 20 or even 100 pack of ferrite cores might make more sense.

Another obstacle to running workshops is a ready supply of ‘dead’ batteries. I usually have a few hanging around, but not 20 or so, which might be needed for a workshop.

I spoke to my local Sainsbury’s who have a battery recycling ‘drum’. I explained that we are interested in helping local schools and community make their own technology. I explained what a Joule thief is, and said we’d like a source of ‘dead’ batteries for workshops. They were supportive and quite enthusiastic. They would like a letter for their ‘local support’ notice board to explain the project to the public. This would give them a record so folks in-store could figure out why we are picking through their battery recycling ‘drum’, and anyone who’d just put batteries in might be lured into a workshop 🙂

So it looks like there are possible solutions to all the obstacles to running Joule thief workshops; though the devil is in the detail, and doing it is the main test.

There are lots of web pages showing a Joule thief using large diameter ferrite cores, and ordinary plastic coated copper wire. Those cores are significantly more expensive than the small ones. There are also Joule thief designs with inductors using no ferrite, though I believe that is less efficient. Please use whatever comes to hand.

This isn’t a definitive approach, just initial thoughts and investigation. I hope it might help folks who’d like to run low-cost scalable Joule thief workshops.