Sep 172017

So we’ve got a free weekend where there’ll be two of us to do a solar installation… thus the parts have now been ordered for that installation.

First priority will be to get the panels onto the roof and bring the feed back to where the cluster lives.  The power will come from 3 12V 120W solar panels that will be mounted on the roof over the back deck.  Theoretically these can push about 7A of current with a voltage of 17.6V.

We’ve got similar panels to these on the roof of a caravan, those ones give us about 6A of current when there’s bright sunlight.  The cluster when going flat-chat needs about 10A to run, so with three panels in broad daylight, we should be able to run the cluster and provide about 8A to top batteries up with.

We’ll be running individual feeds of 8-gauge DC cable from each panel down to a fused junction box under the roof on the back deck.  From there, it’ll be 6-gauge DC cable down to the cluster’s charge controller.

Now, we have a relay that switches between mains-sourced DC and the solar, and right now it’s hard-wired to be on when the mains supply is switched on.

I’m thinking that the simplest solution for now will be to use a comparator with some hysteresis.  That is, an analogue circuit.  When the solar voltage is greater than the switchmode DC power supply, we use solar.  We’ll need the hysteresis to ensure the relay doesn’t chatter when the solar voltage gets near the threshold.

The other factor here is that the solar voltage may get as high as 22V or so, thus resistor dividers will be needed both sides to ensure the inputs to the comparator are within safe limits.

The current consumption of this will be minimal, so a LM7809 will probably do the trick for DC power regulation to power the LM311.  If I divide all inputs by 3, 22V becomes ~7.3V, giving us plenty of head room.

I can then use the built-in NPN to drive a P-channel MOSFET that controls the relay.  The relay would connect between MOSFET drain and 0V, with the MOSFET source connecting to the switchmode PSU (this is where the relay connects now).

The solar controller also connects its control line to the MOSFET drain.  To it, the MOSFET represents the ignition switch on a vehicle, starting the engine would connect 12V to the relay and the solar controller control input, connecting the controller’s DC input to the vehicle battery and telling the controller to boost this voltage up for battery charging purposes.

By hooking it up in this manner, and tuning the hysteresis on the comparator, we should be able to handle automatic switch-over between mains power and solar with the minimum of components.