Joule Thief
So I came across the circuit for the Joule Thief on Evil Mad Scientist. Such a fun website. Its amazing how much a YouTube video and a little hand waving can get you excited over an academic topic. Bill Nye, your the guy. SCIENCE!
The name is a fun play on words an another example that a catchy name will sell just about anything. The purpose of the circuit is that you can drive an LED on a 1.5 AA battery to the point where the battery is incredibly dead. (Jewel - Joule).
This is pretty cool because already takes more than 1.5 volts to make just about any color of LED light up. You cant light a blue LED with a AA because LEDs are diodes and require a minimum voltage to light it up. 1.5 volts will not do it. Just break open any blinky shamrock or pumpkin you get with a Coors or Bud logo on it and you will see it takes TWO button batterys to operate. (most all button batterys are 1.5 volts.) Ok thats a strange example, but not for me considering that I smash every blinky alcohol-ad related widget I come across just…well..because…..umm…..SCIENCE!
So it takes more than what a AA can provide to to see an LED light up. How does it work and what kinda science can I walk away from this post with?
Its all about inductors. Ever wonder how a “converter” can plug into the wall and drop 120 volts AC to the 9 volts that you can charge your ipod works? Inductors! You take a piece of metal (or metal like substance) and wrap wire around it. Bill Nye would show that this makes an electromagnet..but it also can be used to bring up or drop down a voltage swing.
Take a ring of metal and wrap a pair of wires around it. If the number of loops of the “primary” outnumber the “secondary” you will drop down the voltage swing. Likewise, if the secondary loops outnumber the primary, you will step up the voltage.
This works well with Sine waves which are the “Steady Eddys” of nature. Its a nice ebb and flow. It ramps up and ramps down and all is good. Its the delta in the voltage that translates to higher voltages.
With the Joule Thief, the oscillations are a bit more spikey. With fast spikes (fast deltas) come wider voltage swings and the ability to step up voltages to the point where you can light up the LEDS.
Anyways, this little circuit was the focus of my amusement over Christmas vacation.
I found it interesting because I have never really dealt with inductors outside of power supply applications. In the old days, capacitors and inductors were kinda the Yin and Yang of electronics. Transformers are still around, but most filters are constructed as active filters. Capacitors are cheaper to make and op amps have eliminated the need for inductors. I never dealt with them in College and I graduated with a EE degree!
I had a blast rooting around the garage with my dad over Christmas looking for torrids to play with this circuit. Pretty educational.
Back to the circuit.
This guy seems to claim to have coined the term, but the circuit is nothing new. Here is a guy has a blog that deals with the circuit at an obsessive level. Another website here does a pretty good job of explaining the whole thing as well.
Of course…its not any fun without pictures. Here are my completed circuits.