I’ve been wanting to do this for many months. I finally got some scrap parts together and made my first Joule Thief. This circuit milks the last bits of energy out of your “dead” batteries.
Circuit diagram, video, pictures, and observations included…
Got a “dead” AA alkaline battery out of my scrap collection, connected it to the circuit, and watched it power up (2) white LEDs. This circuit uses common parts and was basically free to make since I used salvaged parts from scrap. This little circuit gave me a feeling of euphoria when I saw it working. It’s like magic!
VIDEO: Joule Thief from Scrap Parts
This circuit is also a kind of “boost converter”. I originally found a version of the circuit from a video by morpher44 titled “Joule Thief Circuit – Driving CFLs with 1.5v“. Then I found the website where the term “Joule Thief” originated. [ http://www.bigclive.com/joule.htm ] The original circuit was apparently first published in a magazine dated 1999.
- Transistor – I used On Semiconductor BC547B. I suspect many different kinds will work, including: BC547B, BC549, BC337, 2N3904, and 2N4401.
- Capacitor: 1,200 pF (marked “122″) [ Capacitor Markings ]
- Resistor: 960 Ohms
- Toroidal inductor [ home-made toroidal inductor with 20 turns ]
- LED - any color (I tested with red, white, and blue)
- Battery – So far I have seen this circuit run with a battery voltage of 0.45 V ~ 9 V.
Here’s a setup where a single AA battery is powering (9) LED’s.
The transistor I am using, On Semiconductor BC547B, has a maximum continuous IC of 100mA. That’s not much, but plenty for a few LED’s. But what is the current draw while powering (9) white LED’s from a single AA battery?
I thought I better get the meter out and measure the current draw to make sure I’m not frying the transistor.
No problem. The current draw is well within the transistor’s specifications. I realize that a multimeter does not measure the current spikes, as an oscilloscope could. What this reading tells me is that even if the current were 4 times higher during the ON cycle, the transistor would still be OK, as the transistor I am using can handle 100 mA continuously. This transistor can handle a peak current of 200mA, as long as it is not continuous and as long as the transistor does not stay ON for long. [ Source: BC547B data sheet (PDF)] [ Edit, Dec-31-2012: The transistor is still holding strong after nearly two years of use. ]
Based on these two measurements, we can calculate the total power draw from the AA battery.
(9) LED’s lit up by this Joule Thief consume 30mW.
How much current does the Joule Thief provide?
From my tests, it doesn’t matter how big the battery is, whether it’s a tiny button cell or a big D cell. The current appears to be determined by the voltage of the battery.
When I connected a 9V battery just for a second, the LEDs were super bright and blinding. To safely connect a load to the Joule Thief, I think it’s wise to make sure it oscillates, which means the battery voltage should be less than what the load requires. This will protect the load from getting too much current. A 9V battery is a bad choice for an LED. A single cell battery at less than 2 V is good for most LEDs.
One thing I like about the Joule Thief is there is no wasted power on a resistor limiting the current flow. Compare this to a voltage booster that supplies a constant voltage higher than is needed, which would require a resistor to limit the current flow. The Joule Thief requires no resistor to limit current flow.
What kind of LED could be used in the Joule Thief?
The toroidal inductor acts like a transformer to boost the voltage to whatever the LED needs.
“The magnetic field collapses, inducing however much voltage is necessary to make the load conduct”. [ Source ]
To me, this means ANY LED could be used, no matter how small or large the forward voltage is, since the field collapses once that path in the circuit is opened.
LED Color Affects Current
As the color of an LED varies, usually the forward voltage varies. The forward voltage of my collection of LEDs are all different, mostly depending on the color. As the forward voltage changes, the current changes.
(2) white LEDs used 11mA.
(9) white LEDs also used 11mA. The color is still white, and these LEDs run at the same voltage, so the current is unchanged in the Joule Thief.
(1) red LED used more: 12mA. Red LEDs have a lower forward voltage than white LEDs. It still seems ironic to me that a single red LED uses more current in this circuit than (9) white LEDs. The red LED also makes much less visible light, so it seems like a waste of current.
(1) IR LED used the most: 31mA! Infrared is nearly invisible to the naked eye and yet uses the most energy, because the forward voltage is the lowest of all the colors I tested. (My IR LED has a forward voltage of 1.5V.) A lower forward voltage results in more current making it through the Joule Thief’s oscillations to turn on the LED more times per unit of time. The LEDs with the lowest forward voltage have the most ON cycles, allowing more current to pass through the circuit.
Buzzer works with a diode
When I connected a buzzer as the only output of the Joule Thief, it would not make a sound. I verified that the buzzer worked with the batteries alone, but when the buzzer is in the Joule Thief circuit it remained quiet.
I fixed this by adding a diode and capacitor. The buzzer made sound if I altered the circuit so that there is an LED (or diode) in front of the buzzer and put the buzzer in parallel with a capacitor. The capacitor is probably not necessary but the diode is important to rectify the current.
Lowest Battery Voltage
When the battery gets down to about 0.45V, it is still lighting the LED’s, but at a much dimmer level. If I disconnect that battery and immediately reconnect it, the LED’s will not turn back on. It seems the voltage is not high enough to get the oscillation started. (This circuit is an oscillator.) If I set that battery aside for the day and come back to it, the voltage in the cell increases again, maybe to about 0.65V. Now the same battery will start the Joule Thief circuit oscillating again and will light the LED’s. So, if you want to keep the LED’s running, don’t bump the circuit and disturb the oscillation.
This is an amazing little circuit and has great potential. I’ll be exploring this further as a way to make use of low voltages.
Related Topics and Further Study
Handmade Toroidal Inductor Here at ElPerfecto.com
Wikipedia page on Toroidal Inductors:
Forum #1 – Tons of comments at overunity.com:
Forum #2 – Useful comments at energeticforum.com:
Forum #3 – Interesting comments:
Websites with Joule Thief experiments: