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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.

enter image description here

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.

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2 Answers 2

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

enter image description here

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  • \$\begingroup\$ What parameters of diodes do you recommend? Also, what if solar voltages drop due to clouds like to 8v for example? \$\endgroup\$
    – Ali
    Commented Feb 4, 2023 at 7:14
  • \$\begingroup\$ I found a 1N5819 Diode. Will that work? \$\endgroup\$
    – Ali
    Commented Feb 4, 2023 at 7:24
  • \$\begingroup\$ Hi Ali, It depends on the lower limit of the router input voltage. The source with the higher voltage will predominate. 1N5819 should be suitable. \$\endgroup\$
    – vu2nan
    Commented Feb 4, 2023 at 12:34
  • \$\begingroup\$ With that input power and output load of 12v 1A, would this get hot if using 24/7? If, I happen to connect battery after sunshine in place of solar panel? \$\endgroup\$
    – Ali
    Commented Feb 4, 2023 at 14:15
  • \$\begingroup\$ That shouldn't be a problem. \$\endgroup\$
    – vu2nan
    Commented Feb 5, 2023 at 3:21
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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.

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  • \$\begingroup\$ I tried with the battery (13v) too, with and without Buck converter. The problem is with the relay. For the diodes, the wouldn't those make current double, when both are on or cause issues due to for example adapter 12.4V and if solar voltage down to 12v due to clouds etc.? \$\endgroup\$
    – Ali
    Commented Feb 4, 2023 at 8:34
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    \$\begingroup\$ Try the diode approach with battery and without the relay. The battery is feeding the router directly. The battery will need undervoltage protection! \$\endgroup\$
    – datenheim
    Commented Feb 4, 2023 at 8:56
  • \$\begingroup\$ @Ali what datenheim says, the relay is not helping. But see my answer. You should wire it to run off battery at all times, with solar and AC power replenishing the battery whenever available. You need battery management e.g. too-low-voltage shutoff. \$\endgroup\$ Commented Feb 5, 2023 at 7:50

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