Solar Radiation Sensor
The solar radiation sensor is small photovoltaic
element. When it is exposed to the sun it produces an open circuit voltage of
about 0.5 V. The characteristics are given in the figure below. When a
potential of -5 Volts is applied to the sensor, the current that passes through
the sensor will be linear to the solar radiation.
This quality is used by the controller to
determine whether there is enough energy in the suns rays to heat the water.
The pump has to be switched on when the radiation is between 150 and 175
Watt/m2. The current through the sensor will then be between 140 and 165 mA.
Taking in account the light transmission losses of the cover the pump needs to
be switched on between 125 and 140 mA.