The "undervoltage protection and dusk light switch" has two purposes:
- It switches the load off as soon as a critical (low) voltage is reached.
- It disables the output as long as the solar panel still provides more than an adjustable voltage (e.g. by day it disables the attached LED lights).
There is no charge controller in here as it is a separate function. There are no fuses drawn here, too. Always use fuses if dealing with stuff that can provide enough power to burn or damage something... Also, there is no (required) flyback-diode for the relais coil drawn.
The circuit is designed with low parts count and bin parts reuse in mind, so it is by no means a perfect by-the-book circuit and may show bad design technique. Think of diode forward-voltage and its temperature dependency, relais hysteresis depending on part variation etc.
Here is a quick sketch of the schematic which shows the working principle:
The output from the charge controller is connected to the car battery by a power diode. A 10 volts zener diode (with current limiting 2k2 resistor in series) drives the base of an BC547B npn transistor. A second 2k2 resistor is in parallel with the base-emitter-path of the BC547B npn transistor. This reduces the sensitivity of the circuit to noise, but also reduces hysteresis (which could lead to oscillation). The base capacitor of a few µF dampens oscillations which could occur by the non-zero series resistance of the car battery, a fuse etc. as the transistor circuit provides negative, phase shifted (relais inductor coil) feedback with gain.
If the battery votlage goes high enough, current flows thru the zener diode and the base voltage of the BC547B npn transistor rises high enough that there is enough base current to drive the relais strong enough to switch on. Load connected.
If the battery voltage falls low enough, base voltage and current of the BC547B transistor goes down so the relais will switch off again. Load disconnected.
automatic dusk light switch
Assuming that the battery is fully charged, there is no or only a low load on the solar panel. This means it provides approx 20V (at least mine does). A solar panel is (within limitations) a current source with current depending on light level.
By driving this current thru a current-sensistive amplifier/switch (BC547B again...) and preventing the relais-driving npn transistor from switching on by stealing its base current, the logic function "if light level is above a certain limit, disable output" can be achieved.
To make the switching point adjustable, a potentiometer can be used to set the base current.
As the load on the solar panel increases (charge circuit tries to kick in as soon as battery voltage drops), the output voltage of the solar panel drops further (now by darkness and load). This mechanism provides positive feedback, so this time no oscillations should occur.
First test testup of undervoltage circuit:
...built on prefboard:
...provides the following Uin (CH1/X)/Uout(CH2/Y) relationship (sorry for the screenSHOT, I had no fat16 usb stick):
Hysteresis is just about 240mV with this circuit, which may be too less to avoid oscillation, based on the series resistance of the battery.
I've decided to leave the dusk sensing out of this circuit part for now, as the LED modules provide this function.