DIFFERENTIAL CONTROLLER 
with
INDICATORS

Are you searching for a differential controller that’s not only easy to install but also easy to adjust? If so this is your lucky day. After dozens of controller designs I finally came up with an enclosed controller design with a Differential Controller Indicator that facilitates the adjustment used to automate the solar heat collection process. The differential adjustment is made through a tiny hole in the top cover plate. The red LED turns on when the pump comes on and the green LED glows in proportion to the differential temperature.

A net solar heat gain takes place when the collector is hotter than the storage tank and the pump comes on. If the pump stays on too long stored solar heat will be lost. Also there is a point of diminishing returns as the storage temperature approaches the collector temperature. The single differential pot adjustment allows the user to easily strike a balance between heat gain and energy conservation. Heat transfer only takes place when there’s a difference in temperature. As the temperature differential increases the heat transfer also increases. Sooo we’re left with the question what is the best differential adjustment for our system…

As you can see the connections and the adjustments have been greatly simplified. As a matter of fact the pump connections to a PV system with a battery back up may be further simplified by placing a jumper between J1 pin 2 and J1 pin 3. If you are using this controller to drive an AC pump the connections illustrated are the ones you will use. All you'll need to power the controller is a small 12VDC wall wart DC adapter. The actual power consumption of the controller is less than 0.1 Watts per hour.

The most important thing to remember when connecting the DC power supply is to get the polarity correct. Reversing the polarity could damage the controller even though the damaged parts are easy to replace it is best to make all the connections with care. The Collector probe and the Storage probe have a common junction at J2 pin 2.

This is where the green differential indicator LED comes in. Rather than calculating the differential you’ll be able estimate it directly by observing the intensity of the green LED. The red LED comes on when the pump is activated and intensity of the green LED indicates the difference in temperature between collector and storage. If the green LED is bright the pump should turn on. if the green LED is dim the pump should turn off. It’s as simple as that. How bright and how dim will depend on the differential adjustment. A moderate counterclockwise adjustment will maximize solar heat collection and a clockwise adjustment will maximize electrical power conservation.