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DiscoverCircuits.com -- Hobby Corner
"Hobby Circuits for
your Hobby Projects"
Last Updated on:
06/19/2009 02:49:16 PM
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CANDLE POWER
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. |
| The second
method I experimented with used an array of solar cells, positioned around the outside
of a candle. As shown in the illustration, an array of eight 2.0cm X 2.5cm solar
cells could generate about 20 milliwatts of power from the light produced by a single
candle flame. This is only half that produced by the thermopile method but does
produce a much higher voltage. Someone suggested that some calcium oxide (lime)
held in the candle flame might increase the amount of light produced and thus improve
the energy conversion. This method had been used back in the 1800s to boost the light
from gas powered lamps. The term “lime light” came from this technique. |
| 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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DiscoverCircuits.com -- Hobby Corner
"Hobby Circuits for
your Hobby Projects"
Last Updated on:
06/19/2009 02:49:16 PM
|
Automatic DC Motor Brake
designed by David Johnson, P.E. |
| In many DC motor powered
systems you would like the motor to quickly come to a full stop. This circuit
provides an automatic electronic braking action to any DC motor ranging from 6v to 24v
up to 1 amp of current. Some of the component values could be changed for larger
motors. |
| When power is applied to
the hobby circuit, Q1 is turned on. This routes zero volts to the gate of Q2,
turning off Q2. Current flows to the motor through the diode D2. When power
is removed from the circuit, Q1 quickly turns off which routes some stored charge from
C1 to the gate of Q2. This turns on Q2, which provides a heavy current path for
the free spinning voltage emerging from the motor windings. The value of R3 can be
selected for any desired braking action. The fastest braking time occurs when the
resistance of R3 is zero ohms. |
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Click on Schematic below to view PDF
version of this Circuit |
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