| This solar charger
schematic was taken from Urs Muntwyler's book "Praxis mit
Solarzellen", Franzis', 1993. It is not extremely
sophisticated, i.e. it doesn't use techniques like Maximum Power Tracking. It
compares accumulator and solar generator voltage at regular intervals,
and switches the series transistors on or off depending on the measured
voltage difference. 
As it is cheap and easy to build, I use this circuit in my camper van.
Its description, as found in the abovementioned
book:
"The 25k-potentiometer R2 limits the maximum
charge voltage to e.g. 14,3V for a lead-calcium accumulator."
(My comment:
You must set this
to the correct value for the type of accumulator you are using. Connect the
fully-charged accumulator to the circuit. Connect a voltage source, e.g. 19 -
20V, to the solar connector. The "charging" LED should light.
Measure the voltage drop across the drains of Q2 and Q1. Adjust R2 so that you
get the difference between the input voltage and the desired accumulator's end
charge voltage.)
"Q2 operates as a diode and prevents the
accumulator from being discharged when the solar cell's voltage becomes low (e.g. at
night). Q1 and Q2 are controlled as follows: If the end charge voltage of the
accumulator is reached, the voltage is held constant (U-charging), allowing
the accumulator to control the charge current, until only the current flows
which is necessary to keep the charge level. This procedure does not only
guarantee a long life-time of the accumulator, but ensures complete usage of
charging capacity.
During charging, the switch (Q1,Q T2) is
periodically opened for a short time (e.g. 1:1000) by the pulse generator
(IC2A, IC2B). This state is held until the solar generator's open-circuit
voltage exceeds the accumulator's voltage. The pulse generator, consisting of
two NORs, creates a query impulse of 15 ms duration every 14 seconds, shutting
off the SIPMOS transistors. During this time IC1A compares the accumulator's
voltage and the solar generator's voltage. If the solar generator's voltage is
lower, the SIPMOS transistors remain shut off because the low signal of the
comparator is dominant. The sampling starts again when the output of the
comparator is high."
Grounding the circuit must be done using the
positive wire, which is not a problem for a camper van charger, however this
may create problems in house installations. You can regulate powers of upto
100W and voltages of upto 50V with this circuit, however take care that Q1, and possibly Q5, are
properly cooled. As far as I know, Q2 does not have a heat problem. You must not connect the heat sinks
of the transistors Q1 and Q2 together (this shorts the two drains which in
effect permanently connects solar cell and accumulator).
During normal operation (I-charging) the
transistors do not become hot. Heat becomes a problem only when the circuit
goes into U-charging mode, i.e. the transistor Q1 is partially "closed" so
that it gets e.g. 18V on one side and 13,8V on the other. With a current of
1A, for example, the transistor has to burn (18V - 13,8V)*1A = 4,2W; probably
that's pretty much already without cooling. The BUZ11
can dissipate up to 90W, though, so this does not really pose a problem unless
you don't take care of proper cooling. While testing this circuit my Q1 became
so hot that the solder which held it in place began to melt! Miraculously,
though, the transistor survived.
Download Eagle
schematic and layout
Please note that building this circuit
occurs on your own risk! If you are not sure what you do here, leave it. I
do not assume responsibility for any damage you or any equipment may suffer
from building or using these circuits. Please respect the common security
guidelines when working with electric equipment. A description of these can be
found at the sci.electronics.repair FAQ.
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