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More  Battery Charger Circuits

designed by David Johnson, P.E.

I was challenged by a Discover Circuits visitor a while back.  He wanted to know how to generate some electricity from the heat produced by a common candle.  He further added that he didn’t want the heat to electricity converter to use any moving parts.  That meant a steam or Stirling engine would be ruled out.  I put my thinking cap on and came up with two different methods to produce a very modest amount of electrical power.
I first purchased a common “millivolt” thermopile voltage generator from an eBay auction.  These are often used to monitor the pilot light of a gas fireplace.  The typical unit generates about 0.8v open circuit.  As the illustration shows below, with the thermopile placed in a good hot candle flame, I measured an open circuit voltage of about 0.8v and a short circuit current of 200ma.  The DC resistance of the device measured 4 ohms.  With a quality DC motor attached to the device, the voltage dropped from 0.8v to 0.4v but the motor spun nicely.  With a 4 ohm series resistance, the peak power for this device would therefore occur when the load resistance equaled the source resistance or about 4 ohms.  Under these conditions the thermopile would pump about 100ma into a 4 ohm load or about 40mw of power. 

If an 85% efficient DC to DC converter were used to boost the voltage to a higher level, this would be enough to light a 2 volt red led at a current level of 16ma.  Such a current would be enough to produce a nice output from the LED.  If the candle power circuit were used to charge a 1 Amp-hour lithium ion cell phone battery, it might take about 5 days to charge it up.  Although this device works, it clearly would not be practical for anything but a way to power a pilot light indictor lamp or some other low power load.

For a short movie showing a candle powered motor, double click here.  (download of movie is slow, please be patient)

Some time later, I will post a circuit which could take the low voltage from the thermopile and boost it to a more useful voltage.

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