I am considering to augment the power from my solar cell with a dc adapter whom will take up the slack when the sun can not provide, and feed it to my solar charger/regulator and led-acid setup. Will a diode to prevent back feeding to my solar cells be enough or do I need some thing else? I am thinking that the dc supply will act as a second solar unit in parallel, am I wrong? The reason for this setup is a 12v circulation pump and led light for my fish tank, with a battery backup. The heater will have to remain ac for now. Hope you can help.

  • \$\begingroup\$ Please add a schematic, even if it's very simple \$\endgroup\$ – MaximGi Feb 15 '16 at 16:25
  • \$\begingroup\$ I would use a diode from each source to your load to be safe! \$\endgroup\$ – user1582568 Feb 15 '16 at 17:09
  • \$\begingroup\$ Haven´t made one yet, it´s still on my mental drawing board \$\endgroup\$ – Anders_gbg Feb 16 '16 at 18:21

Yes, a diode is what you need, preferably a Schottky since presumably the voltages are low.

You do have to pick a DC power supply with just the right voltage. This should be the lowest voltage your solar charger/regulator can handle. You want the power to come from the solar cell when it has sufficient illumination, so the power supply voltage needs to be lower.

  • \$\begingroup\$ Yes I wish to have the sun give me as much as it can and then for the dc supply to take over. My pv will be at 14.5v and my dc at 12,5(max) the battery is just to sit there being maintained till needed. \$\endgroup\$ – Anders_gbg Feb 16 '16 at 18:09

Personally, since you said you're using a PV solar, PLUS battery that you want to function as primary power, with the dc power converter/supply as a "second backup" to the battery, it might be a good idea to hook up the PV panel+battery (with protection diodes between the PV panel and the batt. always) on one "subsystem," with a "low voltage trigger" set at ~11.5V that "turns on" the DC power supply until the battery voltage reaches back up to a 12V "cut-out". This would be easy to implement with either a voltage reference & comparator, or a microcontroller, & a transistor to "turn on/off" the DC back-up source.

Alternately, here's one circuit you could build from discrete components, which would allow the DC power source to 'kick in' to stabilize load voltage whenever Vbatt drops below about 11.5V. This circuit doesn't include any hysterysis, so it won't meaningfully charge the battery & (unless you substitute a comparator/schmitt trigger for Q1) will keep the P-MOSFET in its 'linear region' much of the time, so it'll need a heatsink to dissipate the energy from the voltage drop.
backup-power trigger
(You could also, depending on your specific requirements, get rid of Q1, Q2 & all of the resistors, then connect the Gate of Q3 to the Vbatt+ trace (before the 'output' shottky diode) & use a P-MOSFET with Vgs(th) suitable to cause it to 'bring online' the DC supply @ your chosen voltage differential.

  • \$\begingroup\$ I am not good enough to build microprocessor controls but maybe a nand gate can ensure that the pv is the first choice when sufficient. \$\endgroup\$ – Anders_gbg Feb 16 '16 at 18:12
  • \$\begingroup\$ @Anders_gbg answer updated with 3 possible solution circuits (without using any uC). However; I'd be willing to bet you can find an IC or uC that's already set up do accomplish the same thing. \$\endgroup\$ – Robherc KV5ROB Feb 16 '16 at 21:50
  • \$\begingroup\$ It may not have been obvious but I do have a charge/over/under volt protecting "solar-charger" with separate connectors for fv batt. and load. I was planning to use a diode on my dc (and perhaps a resistor) and connect it in parallel with my FV cell. Is this not a functional way? \$\endgroup\$ – Anders_gbg Feb 18 '16 at 0:58
  • \$\begingroup\$ Sorry a diode will of cause be on the fv cable also. \$\endgroup\$ – Anders_gbg Feb 18 '16 at 1:00
  • \$\begingroup\$ @Anders_gbg if you use the diode+resistor on the dc in approach, be sure to select components which can handle the power they'll be dissipating (voltage drop across diode & resistor [times] load current). As long as your DC supply voltage is below your PV charger's "load" output voltage, this should work for you; but if your dc charger's output voltage is >= PV voltage, then the dc supply will always be providing (at least some of the) power to the circuit. \$\endgroup\$ – Robherc KV5ROB Feb 18 '16 at 1:42

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