Original Joule thief, by Z. Kaparnik. The history and basics

[Kangsteri]

I'm making a small study on the Joule thief. Please let me know if there is something to fix or add here?

Description:
The most influential "free energy" consept that opened people's eyes to look electricity from untraditional perspective, was started by Joule Thief circuit. It was originally published in electronic hobby magazine called Everyday practical electronics in year 1998, under section where people could post their own circuits. Here is the original article from the section called Ingenuity unlimited:

One Volt LED-A Bright Light

Illuminating a LED from a very low supply voltage is difficult as most devices have a forward drop of at least 18V. This excludes their use in products operating from a single 12V or 15V battery. However, by applying techniques used in DC- to pulsed DC converters, a very compact, economical and efficient solution can be produced. The circuit diagrams shown in Fig. 1a to Fig. 1c will brightly illuminate a LED from a supply as low as 750mV and as high as 15V, i.e., most single cell batteries available including nearly dead ones.

In the Micro-torch circuit Fig. 1a, transistor TR1, transformer T1 and resistor R1 form a current-controlled switching oscillator. Each time TR1 turns off, the collapsing magnetic field in T1 generates a 30V (off-load) positive pulse at TR1's collector (c). This, in series with the supply, is fed directly to the LED.

Switching occurs at a very high frequency and with a low duty cycle, which results in an average LED current of about 18mA, sufficient to illuminate most LEDs. Current, and therefore brilliance, can be increased by reducing the value of resistor R1 and vice versa. A value of 2 kilohms produces 30mA, which is more than enough even for hyper-bright devices.

Conversion efficiency depends on transistor TR1. Although any transistor can be used, high performance devices with very low VCEISAT) yield the best results; for the ZTX450, efficiency is 73 percent. A ZTX650 increases it to 79 percent whilst a BC550 reduces efficiency to 57 percent. Even at this value it still out-performs conventional circuits using higher voltage supplies where efficiency rarely exceeds 50 percent.

A micro-toroid center- tapped transformer, T1, is constructed using an anti- parasitic bead 6mm by 4mm in diameter with a 2mm hole. Fold 90cm of 38s.w.g. enameled copper wire in half, press the crease tightly together and then thread the folded wire repeatedly through the bead hole until 20 turns are wound. Trim protruding wires to 25mm.

The bead now contains two sets of 20 turns with two starts at one extremity and two ends at the other. Join an appropriate start and end together to form the tap (CT). If the circuit fails to oscillate, check the tap is correctly formed; otherwise, it's most likely a shorted turn.

The simplest application, Fig. 1. Three drive circuits for operating LEDs from supply voltages below 1.5V. Fig. 1a is a Micro-torch, power- on indicator or simple infrared transmitter. Indicators for use with other circuits are shown in Fig.1b and Fig. 1c, the latter consuming no power when off. Diode D2 in Fig. 1c raises the forward drop as some LEDs leak when operated with a fresh alkaline battery; it is also necessary with infrared devices that have a forward drop of less than 15V.

When used with other circuits, decoupling with capacitor C1 in close proximity to the oscillator is recommended. Also keep lead lengths short, especially to the transformer, as the circuit operates at a high frequency: fortunately using a micro-toroid transformer significantly reduces radiation.

Z. Kaparnik, Swindon, Wilts, UK

Link for the original schematics: https://i.imgur.io/4lJZMpl_d.webp?maxwidth=640&shape=thumb&fidelity=medium

Additional information for the Joule thief:
Joule thief is often confused with other similar circuit called "Joule ringer". The same principles work for both. The difference between these circuits is that joule thief has always two identical bifilar coils and it's designed to be connected on some power source. Joule ringer always has two different size bifilar coils and it's designed not to be connected on any power source.

As stated above on the original description, the circuit is operating at high frequency. This means that many same rules that work with radio frequency, will also work with the Joule thief. The working mechanism in physics terms for it is the "beat frequency", or in mechanical terms it's called "electric ram pump". Resonance amplification happens when two slightly different frequencies form to third louder pulsing frequency.

The best toroid core materials for it are Metglas or magnesium zinc. Mn-Zn cores usually are dark green, dark grey or uncoated toroids. It's possible to test the material with hacksaw. If the blade bites to the core, then it's most likely iron. If the blade slips on it, it's most likely magnesium zinc. Iron core is usually better used as choke, they are often yellow or black, etc. You should cover the core with Teflon tape to reduce the resistance.

Best wire to use is Litz wire, but regular magnet wire works well too. Usually 10 to 20 turns of two wires is most officiant. The wire should be evenly distributed across the whole toroid core, not tightly on one side or overlapping on other wires.

With low voltage input the resistor is not needed. It's used to protect the transistor. But with more voltage 1 – 10k is mandatory. You can use potentiometer too.

Transistor should be fast switching (RF, UHF, VHF, etc). Germanium will work well too, but they are not cheap. For common testing purposes 2N2222 is often used. Metal core transistors can be opened and used as solar panel too. It's possible to use LEDs with multiple unused legs for different colors to solar capturing too.

If it's used as step up transformer, the same things also apply to the diode. It should be fast switching and low forward voltage drop. Germanium will work better. Just make sure it is clear glass with color stripes, not the orange fake Schottky from eBay! Diodes, LEDs and transistors will have less voltage drop if you use two / three of them in parallel. Using 4 or more will reduce the voltage drop more, but they will be exponentially less effective. So it's usually not worth it to use more.

It's possible to boost the current flow more by shorting one wire, or low uf ceramic capacitor through the coil. Or adding magnet to the core to increase the inductance.

The circuit is producing pulsed DC. It can be converted to regular DC with cone coil, or smoothing capacitors. Or adding bifilar iron core choke between positive and nagative, low value ceramic capacitor and resistor across at the end (search for EMI, CLC or Pi circuit).

If you just need something fast or to test it, any scrap parts will most likely work. It can be done on rod core or transformer core too. Or if you want to make it very small, it's possible to replace the core and wires with two same size surface mount inductors. For more current, it's also possible to add multiple joule thief's in parallel.