Low Current Dump Circuit

I know solar regulators are cheap, but I have been experimenting for fun.  I have a 40 Watt Solar panel and a 12v battery for my dinghy to operate a light and a radio for emergencies.  I created a low current dump circuit, that triggers only when voltage trigger has been met (set by Zener plus .6v to activate PNP Transistor).  Based on 18v max voltage, divided by 680 Ohms, and multiply a gain of 75 for TIP42C PNP Transistor, the circuit will dump about 2 Amps, until battery voltage drops below trigger voltage.  The Transistor forms a short and dumps the battery.  At 2 Amps, the Transistor is within the 6 Amp limit of the Transistor.


  • BB.BB. Super Moderators, Administrators Posts: 31,438 admin
    edited December 2015 #2
    Dump controllers are not great at charging Lead Acid Batteries... They need variable voltage/time (high voltage/current at first for X amount of time, then lower voltage for float).

    Generally, it is not a great idea to design a circuit running the transistor in the the active range asumming a fixed Beta value... Beta usually varies quite a bit.

    Also, you will need some good heat sinking on the transistor (2 amps * 4 volts drop = 8 watts).

    Transistors (and drive circuits) sometimes fail... You should have a fuse/breaker between the battery and transistor to prevent a battery dumping 100's of amps into a dead short.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • plasmahunt3rplasmahunt3r Registered Users Posts: 18 ✭✭
    Thanks for you advise BB.  I redrew the circuit adding a fuse and a dump resistor.  The transistor gets the heat but with a small heat sink, it was adequate.  I did the circuit just to see if it would work.

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