# What's with this Joule Thief thing?

• sophiecentaur
In summary, the Joule Thief is a circuit that makes use of a battery's energy to generate more light than would be possible with just the battery itself. It's a simple converter that boosts a battery's voltage to a level high enough to run an LED, and it's been seen mostly in domestic lighting applications. There may be more to the circuit than just boosting voltage, as demonstrated by the blue curve on wikipedia.
sophiecentaur
Gold Member
It's amazing that a name like that can grab the attention of so many people and trigger such a flurry of constructing.

Is looks to me as if it's a way of driving a non linear device like an LED is such a way that it produces more light output than it would from a battery. Is it not just, effectively, a switch mode inverter to produce pulses of more current than the little battery, on its own, would provide?
The Joule Thief doesn't claim to be an over unity device - it just seems to make LEDs look brighter. Why is that a "Thief"? Surely it's just proving to be more efficient (or is it?) than an approximate voltage source (a battery). Novel, but anything more?

Has this been done on a large scale? Domestic lighting is so much more efficient using LEDs that it's hardly worth the bother but there would be many situations where PV / battery combinations could be cheaper for a given light output.

Yeah, it's just a boost DC-DC circuit to let you use up almost all of the energy in a battery cell:

http://www.instructables.com/id/Make-a-Joule-Thief/

In the general case, you would use a buck/boost or SEPIC converter to let you get the LED current from a voltage that can vary from over the LED drop to less than an LED drop.

But nearly all the energy in a battery is available at its fully charged voltage. Most modern cells are pretty well empty of energy once the volts drop. Is it not more to do with the LED characteristic?

sophiecentaur said:
But nearly all the energy in a battery is available at its fully charged voltage. Most modern cells are pretty well empty of energy once the volts drop. Is it not more to do with the LED characteristic?

It has very little to do with the LED characteristic, apart from the LED requiring a higher voltage than a near-dead battery can deliver...it could be used to drive other circuitry as well. The Joule thief is just an extremely simple switching converter that can produce a few volts from very low voltages. That little energy remains in a mostly dead battery compared to its fully charged condition is rather irrelevant...a few joules do remain, and the Joule Thief makes use of them.

If you must have a practical, non-educational use for the circuit, you can drive LEDs or LED strings with batteries that don't produce sufficient voltage even when fully charged, or with supercaps that have an exponential discharge curve.

Rechargeable cells mostly give a constant voltage until they run out of charge and then the voltage collapses quite quickly.

However this is not the case for dry cells like alkalines.

See the blue curve for Zn/MnO2.

If you wanted to run a 1.2 volt LED off this, you would not be able to do this after the cell had used up 40 % of its capacity. This means that the battery would be thrown out with 60% of the charge you paid good money for still in it.
All that lovely power going to waste.

As James suggested, if you have a single solar cell, you can't get more than 0.6 volts out of it, but garden lights use white LEDs that need 3.5 volts or so.
So, these have a very efficient DC=DC converters to convert from this voltage, stored in a NiCd cell, up to about 3.5 volts to run the LED.

But there must be more to it than just saying you're producing a DC-DC boost. If it were as simple as that, you would be putting more current than you needed through the LED when the battery volts were high - which would just be wasted as heat in the LED or series components. If there really is something in it then you would actually get More total light energy out but you'd have to measure it over the life of the battery charge. Merely showing brighter LEDs doesn't demonstrate any advantage.

I'm sure there must be something about the LED V,I,light-output characteristic that self regulates this oscillator to produce a more optimal current supply to the junction. If yoiu look at the data sheets for typical low power LEDs, they specify typical and max current for a nominal voltage and people have more or less left it at that. It isn't too surprising that a slightly different way of supplying an LED (with current pulses) produces more light.
I haven't the facilities (and can't be arsed) to investigate it but I'd bet an enthusiastic and slightly more technical than usual constructor could get some interesting conclusions from an in depth study.

sophiecentaur said:
I'm sure there must be something about the LED V,I,light-output characteristic that self regulates this oscillator to produce a more optimal current supply to the junction.

If you look at the circuit in http://en.wikipedia.org/wiki/Joule_thief it is fairly clear that taking more current through the LED reduces the Q factor of the inductors. The wiki page explains two other nonlinear effects that tend to limit the current.

This is a nice example of nonlinearity being a good thing. The circut will work pretty well for any reasonable component values. The only critical feature is that there must always be a load connected.

## 1. What is a Joule Thief?

A Joule Thief is a simple electronic circuit that is used to power small electronic devices or light emitting diodes (LEDs) using a low voltage power source. It is essentially a voltage booster that can increase the voltage output from a low power source, such as a dying battery, to power a device.

## 2. How does a Joule Thief work?

A Joule Thief is made up of three main components: a transistor, a resistor, and a toroid (a type of inductor). When a low voltage power source is connected to the circuit, the transistor acts as a switch to rapidly turn the power on and off, creating a pulsing current. The toroid then stores this energy and releases it in short bursts, boosting the voltage output.

## 3. What are the applications of a Joule Thief?

Joule Thieves have a wide range of applications, including powering small electronic devices, such as LED flashlights or small radios, from low voltage sources like batteries or solar panels. They can also be used to extend the life of batteries by draining them completely, and they have potential uses in renewable energy systems.

## 4. Are there any risks or safety concerns associated with Joule Thieves?

Joule Thieves are generally considered safe to use, but as with any electronic circuit, there are potential risks if not used properly. It is important to use the correct components and follow safety precautions when building and using a Joule Thief, such as wearing protective gear when soldering and ensuring proper insulation to prevent shocks.

## 5. Can I build a Joule Thief myself?

Yes, Joule Thieves can be easily built using common electronic components and basic soldering skills. There are many tutorials and guides available online that provide step-by-step instructions for building a Joule Thief. However, if you are not familiar with electronics or do not feel confident, it is always best to seek help from someone with more experience.

Replies
13
Views
3K
Replies
15
Views
2K
Replies
6
Views
2K
Replies
4
Views
1K
Replies
27
Views
5K
Replies
3
Views
2K
Replies
22
Views
3K
Replies
6
Views
2K
Replies
54
Views
8K
Replies
17
Views
5K