2011-12-30 Joule Thief, High Efficiency, Supercharged

My watsonseblog dated 2009 Feb 19   These may be dupes or unfinished.

2009-02-19

Joule Thief, High Efficiency, #01 Supercharged

See the schematic and original blog here.

I have built several of these high efficiency ‘supercharged’ Joule Thiefs (SJTs) to try to optimize the parts values, especially the coil. I’m posting these SJT blogs to allow the reader to get better results when building a duplicate SJT. I’ll give info on winding the coil, wire size, etc.

This SJT used a toroid core from All Electronics, part number TOR-54 (they have since sold out). I put a single turn (piece of straight wire) through the hole in this core and it measured 0.08 uH (I measured several of them to get an average). This means it is a low permeability core, much lower than the usual ferrite RFI suppressor sleeves that I’ve used (compare that to the charcoal TOR-23 cores they sell, which have 0.48 uH per turn). It also means that I have to use more turns to get the inductance up to 100 uH.

I jumble wound three solid enameled wires, each 36 inches long, wound trifilar, i.e. all three at the same time. Two were 28 AWG, connected in parallel for the primary winding. The third was 30 AWG, for the feedback winding. I cut the three wires to length, and on one end I twisted all three together to allow me to thread the wires through the core. Afterwards I cut off the twisted part. Each winding measured 124 uH.

For Q1 in the Fig. 2 circuit shown in the schematic (see link at top), I used a BC337-25. The R2 was 820 ohms, D1 was 1N4148. C2 was 1000 pF. The LED was a cheapo white LED removed from a 9-LED flashlight. A 47uF tantalum was across the supply rails.

MEASUREMENTS
I connected it to a 1.5V supply, and measured the supply current at 50mA. The LED current was 21 mA. The freq was 224 kHz. The LED was very bright. I calculated the efficiency at 89 percent, which seems too high, however the circuit did have very low power consumption.

Observations
This coil was wound using .335mm wire for the two primary windings. This is actually slightly larger than 28 AWG, about 27.5 AWG. It’s taken from a fan motor winding made in Switzerland. I had to scrape the very tough insulation off the ends with a single-edged razor blade, and it took quite a bit of scraping.

I am trying to keep the coil inductance above 100uH, and with the TOR-54 this means using solid enameled wire. If I used plastic insulated telephone wire, I wouldn’t be able to get a long enough length on the core to get 100uH.

Back to experimenting…