charging. The PIC measures the battery voltage and the
solar panel voltage, and takes the difference. If the
voltage is over 1.25 volts (about 20 amps), it goes into
the alarm mode and turns the FET off. The LED will blink
off and on red. It will automatically reset.
FETs will also conduct both ways, thus creating a
potential that the batteries could discharge through the
solar panel. To prevent this, an algorithm is used, and
when the solar voltage drops below the battery voltage
the unit disconnects from the solar panel. This prevents
discharge when the sun goes behind a cloud or the sun
goes down. Once every 10 minutes, the battery is
disconnected from the solar cell and its voltage is
checked. When the battery reaches 14.00 volts, the solar
panel disconnects and the micro continues to monitor
until the battery’s voltage reaches 13. 5 volts. It then reconnects to the solar panel.
A kit is available from the
Nuts & Volts Webstore. It comes
with a preprogrammed chip and
board. If you want to program
your own chips, you will need a
programmer. The printed circuit
board (PCB) files are available at
the article link, along with the
assembly and hex files for the
PIC, plus some usage hints and
tips, and a copy of the Parts List.
The board files are from
ExpressPCB and can be
downloaded with their free
software. If you are a programmer, you can change the
tripping voltages if you feel so inclined. (Refer to the
Hints and Tips file at the article link.)
Place the heatsink with its mounting holes facing
down; rotate it so that its single mounting hole is to the
right. Using the template available with the downloads,
cut the template out and glue it on the top of the
heatsink. Drill three holes using a 9/64”drill and deburr.
Make sure the heatsink is flat with no burrs. Remove the
template with hot water. Take a look at Figure 1.
With Q2 facing you, bend its leads toward you 90
degrees so they are facing up. To prevent the heatsink
from becoming a conductor, locate the insulating pad
and place it on the heatsink. Mount the FET using the
6-32 nylon screw and nut. The screw should be on the
back of the heatsink; the nut should be on top of the
FET. The purpose of this is to allow the removal of the
transistor and board without having to de-solder. Refer to
March 2014 25
■ FIGURE 1.
■ FIGURE 2.