Well here it is..... well here is the 240 volt version anyway
and everyone always wants a circuit.... and this is the one I madeup /used:
and no I don't know what I was doing either now i look back at it, no wonder I never keep my scribblings.
However, the circuit board is what most folks need, and that is clear.
Here is the finished board: note the NTC's on the left on the spade connectors... only pin heads, just use a long cable to extend them up to the roof and the tank.
You can calibrate them on the board, as their characteristics won't change when you lift them off and re house then in the panel and the tank
The top one goes to the panel.
The second one from top goes to the tank for the differential.
The third one goes to the tank to read for the max temp.
And the underside: ... warts and all...
Ok, it uses three ntc thermisters, that are rated in this case at 5k@25C.... any ones will do really, as they need to only give us an idea of change, not absolutes until we decide what we want the settings to actually represent, but three of the same type makes thinking easier.
It consists of a comparator to find the difference between the sensor (NTC1) in the solar panel on the roof, and the sensor in the tank (NTC2).
When ever the roof has more temp than the tank, we pump, if the roof drops to same or below the tank temp.. turn off. It's that simple, and then add some hysterisis, and were done.
On this version, I also take a reading of the tank and put that up against a preset voltage, and this turns the pump off if the tank exceeds whatever temp you desire.... mine turns off at 65C. If I go any higher, the freezer carcass I use as the insulated water reservoir starts to leak for some reason, and thats hot enough for me anyway. This stops boiling on hot days, as when it turns off, it drains the panels, and so does not boils and make lots of unnecessary noise.
So we have three NTC's. The first comparator differentiates between NTC1 and NTC2, and turns the pump on and off dependent on the differential of the tank to the panel.
A second comparator simply senses the NTC3 against the tank temperature, and over rides the first comparator if the temp is over the set point.
Yes it's simple I know, but works very very well.
The third comparator checks to see if the second one is turned on or not. If it finds the second one (o/temp) is on, then it shuts down output of comp3. If comp 1 wants the pump on, but comp 2 says no, then comp3 over rides comp 1, else if comp2 is off (below max) then lets comp 1 talk to the relay without interference.... clear as mud really....
We turn on and off a big relay (30 amps) which is about 29 too many, but I had them in the junk bin. The transformer is a one dollar special I picked up somewhere in the past, and is about 14vac 7w. Pump and tranny are fused,and there is not much more to say.
Any questions?
..............oztules
Oh, and this is the 12v pwm version. It runs a 12v water pump (diaphram style) and controls the pump speed dependent on the differential of the panel and the tank. If it is a poor day, the pump will run slowly, if it is hot, and the differential increases, it will speed up the pump to get the maximum from the panel it can. (keep the panel as cool as possible is the best use of solar ( but still above tank temp))
Here it is after a few years (under spider webs and all):
