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Q & A: Joule thieves

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Most recent answer: 03/12/2015
Q:
Joule Thiefs or "unregulated voltage boost converter" http://en.wikipedia.org/wiki/Joule_thiefHi, You were kind enough to help answer a question about how Peltier Tiles work for generation of electricity from "waste" heat. This has now moved onto an exploration of energy harvesting. Online, we have found a device called a Joule Thief. I was hoping that you could explain - in more simple language than we've encountered so far - how and why this device works to boost voltage.We know there is a torroid that is wound to be a transformer. And there is a transistor.Can you explain what these two do to "boost the voltage"? I found this comment by steveastrouk on an instructables.com post that seems to highlight the activity: "Energy is stored in the magnetic field in the coil, and then the collapsing field induces a higher voltage than created it in the load." What is the torroid into transformer doing? How and why does the number of turns on the torroid make a difference? We have seen on other websites that people have success with "starting" their joule thief at different minimum voltages between 1volt to 25mvolts. People not that it varies with the number of turns and even un-even number turns between the transistor base and collector.Thank you very much,Phil
- Phillip Torres (age 42)
Chicago
A:

These "Joule thieves" are very clever ways to squeeze the last bits of free-energy from low voltage sources. I hadn't heard of them before, so this is fun to explore.

Here's the key physics. A changing magnetic flux through a loop induces an electro-motive force (EMF) around the loop. The sign of the EMF is the one that opposes the change of magnetic field, i.e. it tries to keep the current through the loop constant. That all follows directly from Maxwell's equations for electromagnetism. That EMF depends only on how the field is changing at the moment, not on how it was built up in the past.

The basic idea of the devices is that you can gradually build up the magnetic field energy in a coil using a low-voltage source, so long as the electrical resistance of the coil is very low so that the small voltage can drive a big enough current. If the circuit is then interrupted, the inductance of the coil makes an EMF to keep the current flow from changing rapidly. The voltage produced is proportional to the rate of change of the current. A sudden switch in the circuit can make a huge voltage, even causing sparks of current to jump across the break in the circuit. The voltage is also proportional to the inductance, which increases for bigger numbers of coil turns. 

The transistor is used to switch the circuit open, since the collector-emitter impedance of a transistor depends on its base-emitter current. The device described in the Wikipedia article is actually rather complicated, using two separate coils. It relies on a property of the transformer core material, that it quits getting more magnetic when the field reaches some saturation level at which its spins are nearly all lined up. In principle, one could use simpler methods, e.g. a magnetic relay that would pull open when some field was reached. I guess that a capacitor should be included in parallel with the output to keep the voltage from getting too big just as the switch opens.  Some diodes may be needed to make the outupt dc and to avoid having current flow back into the power source when it's reconnected.

Mike W.


(published on 03/12/2015)

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