Auto switching between power sources using SPDT relay causes delay

Question

I have a circuit with two DC Power sources, one is a 12 V, 1 A adapter and the other is an 18 V, 150 W solar panel.

I have a DC buck converter LM2546 which turns that 18 V into 12.4 V. The 12 V power source isn't always available due to power cuts, mostly during daytime. The solar panel is available for most of day. I would like to build a circuit that will use the 12 V if it's there, and 'fall back' to the solar power source when the electricity isn't available.

I'm using it for my router, I have made something that works 90% for me. There's one issue: switching delay. I'm using an SPDT 12V relay and a diode. When the adapter power is cut, the router restarts. However, when the adapter comes on, it doesn't restart. Below is the diagram (along with measured voltages input and output) I created in MS paint.

It sounds simple, but I'm guessing it isn't. How can this delay be eliminated? I don't want my router to restart when power source switches.

Answer 1

Your problem would be solved by using 2 low-Vf Schottky diodes instead of the relay.

Answer 2

I don't think it's a switching problem. I think there's a delay from the solar - I think it doesn't like suddenly going from no-load to full-load. One way to find out - try vu2nan's Schottky diode method and see.

However if that doesn't work, you have to take a look at the method being employed here, which is e[xtremely sub-optimal][1]. Buck converters are not designed for solar panels.

And attaching regular loads directly to solar panels doesn't work well, as their output is variable.

The right stuff here for this application is an actual solar charge controller and a modest sized battery. I know you want to avoid the battery - everybody always wants to avoid the battery for some reason - but it corrects solar's shortcoming of often being able to supply too little to sustain a given load.

Note that loads are not static. I suspect the router has internal capacitors to "coast it across" the transition time of the relay. That would mean it has a significant inrush current when it does connect. That doesn't faze the wall-wart. However it may be giving the solar+buck some trouble. Again, the battery simply smooths right across that.

The battery will also do a vastly better job at your REAL objective, which is having stable power availability during power outages.

The only issue is that if power outages are likely to last long enough to entirely drain the battery, you need some sort of discharge protection or BMS on a lead-acid or lithium battery. One way around that is use nickel-cadmium, which while it is sensitive to charge isn't harmed by being discharged flat.