A conundrum

The digital timer ordered from China arrive several days ago.  No instructions were included; which made for an interesting time.  Why buy a timer?  I’ve decided to alter the operation of the pool solar water heater.  You may remember the heater pump is activated by a thermocouple switch, that in turn is activated by the temperature of the water in the heater panels.  This setup works, but relies on the water reaching 85°C.  Whilst that happens it isn’t circulating water as frequently as I would like.

The idea is the digital timer will complete a programmed timing loop and activate for a set time at the end of each loop.  During this activation period the pump will push water through the heater panels.

Why is it that when you look at the product photos on Aliexpress the item turns out to be much smaller than you anticipate?  My timer is ¾ the size of a cigarette packet.  Two water resistant 12V 40A relays where purchased at the same time because I (correctly) didn’t trust the timer maximum current rating

With no instructions the conundrum was identifying what each of the timer terminals did.  Eventually I took it to my brother who has an oscilloscope and several better quality multi-meters than my el-cheapo. 

After an hour of experimenting we established the three terminals on the left were the 12V pos & neg with the 3rd being a signal terminal that triggered the timer.  The three on the right were a common in the middle and a Normally Open at the top and Normally Closed at the bottom.  Essentially the right terminals are a switch activated by the timer. 

I want the timer to only work during the day when the sun is on the water heater panels and there is power to the pump from the photo-electric panel.  Why pump water that isn’t being heated and why flatten an old 12V battery in the process?

It was then a case of drawing out a wiring diagram

The MPPT Solar Controller will receive 27V DC power from the solar panel on the shed roof. 

The MPPT battery terminals will feed the battery, timer and water pump.  The latter through a relay

The Timer will be activated by a signal (12V power) from the Load Terminals on the MPPT controller.  The Load Terminals only work when there is sunlight on the solar panel.

If my logic is there will always be power to the timer but the pump will only be activated via the Timer when there is sunlight.  

A new control box is required as the original is now too small