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Conventional silicon transistors just
can't operate at voltages less than about 0.7v. Old germanium transistors could be used,
but those are hard to find these days and most are rather large in size. Some new
n-channel MOSFET devices with very low gate-source threshold voltage can operate at
quite low voltages. I've been experimenting with various devices and came up with one
electronic circuit (shown below), which demonstrates how to boost the low voltage from a
single solar cell to a higher voltage. The key component in the circuit below is a cheap
single logic device from Texas Instruments. It turns out that TI's 74AUC family of parts
can work down to about 0.45 volts. I tried one of their single schmitt trigger parts and
found I was able to make on oscillator function nicely at 0.5 volts. I then used a
charge pump technique and a cheap NPN transistor to form a low power flyback converter.
The circuit can produce about 6 volts at the output from a 0.5v input. The idea is to
use this boost circuit to generate the higher starting voltage needed by a much more
powerful DC to DC converter. Once started, part of the converter's output could then be
feed back to the input, to sustain converter operation. This is known as a "bootstrap"
technique. In the future, I hope to post a circuit which can supply several watts of
power from a 0.5v input voltage. This would be ideal for charging a battery using power
from a single large solar cell or several smaller cells wired in parallel. |
