Thu May 14 19:02:04 PDT 2015
The Joule Thief Nouveaux
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The traditional Joule Thief circuit employs an oscillator to generate an increased voltage from a 1.5 battery in order to drive a LED. The Joule Thief circuit is very widely circulated on the web, it contains few components, and can light a LED with a single battery for many hours.
Why is the Joule Thief circuit popular? A white LED needs something like 3 volts in order to light up. A normal battery cell provides 1.5 volts, so some circuitry is required to light a LED with only one cell. The Joule Thief circuit is simple (it uses 4 components) and efficient.
The Joule Thief increases the voltage seen by the LED by allowing a magnetic field to induce a current in a transformer. By suitably choosing the coil a significant step up in voltage can be obtained.
An alternative voltage increasing strategy is provided by using a capacitor to store charge, then switching the capacitor to be in series with the battery, effectively doubling the battery's voltage. Given appropriate switching controls, this method for increasing voltage can be very efficient, but it requires significant circuitry in order to carry out the switching.
However, as an experiment, I recenly created a circuit which lights a white LED using a single cell and a capacitor based voltage doubler network. The circuit is shown above, if you click on the image you should see a larger version. This circuit simply feeds a voltage doubler network using an astable multivibrator. The diodes in the voltage doubler must be germanium diodes, because these diodes have a forward voltage drop which is much smaller than a silicon diode.
Although the circuit works, it uses many more components than a Joule Thief circuit, and it is not as efficient! Nevertheless, I will probably make myself a flashlight based on the circuit at some point - I simply want to have a device which uses a voltage doubler network in my pocket!
The component values are not especially critical. However, it is essential that the diodes in the voltage doubler network are germanium. (I am calling it a voltage doubler network - but actually it quadruples the voltage supplied by the multivibrator which just gets the voltage up to the level required by the LED. This is because when a transistor switches on it does not conduct perfectly and there is the 100 ohm collector resistor which drops some of the voltage, so only about +/- 1 volt is presented to the voltage doubler network. Nevertheless, it does work and uses relatively few components, so that it is accessible for simple experimentation.
So, if you are ready for an interesting conversation piece flashlight (or torch depending on you linguistic predalictions) - build a Joule Thief Nouveaux!