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Solar charger after a Siemens circuit

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 FAQ.