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Simple 2 Transistor RE circuit

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(Thanks Bryan for moving this to the correct place, I have added some text so it makes sense as its own thread). :)

Description. This is for people who want to make a shunt controller for small(ish) solar power setups. A shunt controller goes between a solar panel and the 12v battery, to stop the battery from being overcharged once it is full.

How it works. Whenever the battery voltage is greater than X the battery is considered "fully charged" and the shunt controller diverts the excess power from the solar panel to a "dump load". The dump load might be a resistor or car headlight that just wastes the power as heat, or some people use a water pump etc that does something useful with the excess power.

This design is a bit "rough and ready" because it uses very few parts and is easy to build, but is still adjustable to set the desired voltage limit for your battery and works quite well for such a simple circuit. The circuits below may need a change to the value of the "HYST" hysteresis resistor, this controls the "gap" between the high voltage threshold where the dump load turns on, and the low voltage threshold where the dump load turns off again. Generally you set the high threshold at about 13.8 volts and the low threshold at about 13.3 volts for sealed 12v gell cells suitable for a 10W or 20W solar panel.

Schematics. Here is a simple 2-transistor version I've used for years for a hysteretic controller, it can be used as a solar regulator, but it can also be used for temperature control (switch a fan on when heatsink gets hot) or light control (LED comes on at dark etc) or automatic 12v battery chargers (motorcycle battery maintainers).

Here's a couple simple versions; the first is good for small setups; ie solar panel up to 20W. The second should be ok for larger setups, depending on choice of output FET.

Note with the first circuit the threshold voltage is affected a little by heat in the TIP122 Darlington output transistor (which is also the regulator). The sense voltage will vary by 0.05 or 0.1v as the transistor gets hot, normally this is not a problem.
The second circuit is not affected by heat in the FET.

The hysteresis resistor may need to be changed to give the required hysteresis. Larger resistor values give a SMALLER voltage difference between the high and low thresholds.


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