Tuesday, September 11, 2012

Solar Battery Charger, AC/DC Adapter Changeover Relay Circuit Diagram

The enclosed circuit of an solar battery controller, AC/DC adapter automatic changeover circuit was requested by Mr.Juan. Let's learn more about the request and the circuit from the below given discussions:

Hi Swagatam,

Your information and circuits are great.

But I want to ask for a special circuit.

I've a little solar panel with a solar/battery controller and a battery.

My load is connect to the load-pins of the controller, so when the battery voltage drops, the controller cuts off the output in load-pins immediately (from 11V-14V to 0V)

As hobby, I want to solar power from this system to a 12V led strip in my kitchen. But in case the light is on and the battery falls off, I want to autoswitch to a 220AC/12DC adapter that I have. So if my light is on, I will notice a little flick but nothing more, the light will be on all the time I want.

I don't want to "auto charge" battery with de AC/DC adapter in that case, because the main utility of my project is to use solar power.

I want to ask you severeal questions/circuits

1. I think I can't put together my controller ground and my AC/DC adapter ground, so I need a DPDT LATCH RELAY ("latch" in order not to waste lots of power from de battery system). And because of I can't put them together, I can't use the AC main switch of the kitchen to control all the system (I mean, the AC main switch of the kitchen will control the light, while the battery/controller powers the light either the AC/DC adapter)

2. What I want is that when my controller's load-pins output goes to 0V, the RELAY will turn to AC/DC power adapter. And when that output returns to 11-14V, the RELAY will turn to battery/controller system in order to waste "solar power" in my lights.

3. It doesn't mind if the relay is a single o dual coil, but the circuit has to be ultra low power consumption. 

4. The ultra low power consumption is the reason for use a latch relay. It will only drain power when it has to activate o deactivate. I expect it not to activate never, so means that my solar system has a good battery capacity.

5. How can I control the light only with the AC main switch of the kitchen ?

Do I explain correctly ?

Before I knew about not to join grounds of the to systems (AC/DC adapter and controller output) I design this circuit with a simple SPDT normal relay. I have attached to you as a guide to understand this long post. but I suposse I can't do this way.


Hi Juan,

I am little confused, I couldn't understand the procedure correctly. There are three parameters:

The solar panel,
The battery,
And the AC/DC adapter.

I couldn't understand how you want to integrate these together.

According to me it should be like this:

During day time the solar panel charges the battery and also stays connected to the LED strip, so that it can be lit through solar panel.

At night, the LED strip gets automatically connected to the battery and uses the battery power for illumination.

And during overcast conditions or in absence of sunlight, if the battery voltage drops below 11v, the battery gets connected to the adapter so that it is able to get charged from the AC/DC source....

Is it that way you want??

First of all, thanks for your help.

Excuse me for my English.

The led strip is NOT always ON. It's a secondary light in my kitchen.

The solar panel is connected to a solar/charger/battery controller (it has 2 inputs and 1 output: solar panel, battery, and load).

The battery is also connected to the controller.

The load attached to the controller is the led strip.

What I want to do is give 2 power supplies to my led strip. The main supply is the one that comes from the controller (it's using solar power or a battery charged with solar power). The secondary supply is the one that comes from the AC/DC source.

I don't want to charge my battery with the AC/DC source (I've found some circuits for that). 

I want to use the solar-battery-controller group to supply my led strip, but, just in case the controller cuts off the output (to protect the battery because of 3 or 4 four cloudy days or whatever), the led strip will be supplied by the AC/DC adapter.

Then, the next sunny day, the battery will be charged again with solar power (solar-battery-controller group).

I've to check the output of the controller, and when that output is 0V, I have to change to the AC/DC adapter. The battery remains "untouched".

May be this picture helps.


There's also a handicap, the switch on the wall has to "control" the led strip (either supplied by controller, or by the ac/dc adaptor).(You will understand my previous post's pdf, the coil was energized by the AC/DC source, in order not to energized it if the wall switch is open)

NOTE: In future, I will also get a USB female in order to charge mobiles, and similar. (I've already get circuits to step down 12 V to 5 V). May be this USB female connector will have the same "AC/DC source as emergency" or not). but this doesn't matter now.

I got it now, the circuit will be very simple, I'll draw it and publish it in this blog as a new post, with the above discussions included....I'll inform you when it's posted....soon.

Thanks a lot, 

Remember that is very important to drain a very "ultra low" power from the battery to make the circuit/relay/or whatever work. The solar system is little, so I can't have a constant drain of 30-50 mA, 24 hours per day. (that is because my first try was power the relay's coil directly with ac/dc source).

I will be using transistors instead of a relay, so the consumption will be negligible....

Done...here's the circuit requested by Mr.Juan, designed by me:

The following circuit goes in response to the added comment by Juan.


  1. Hi Swagatam, i need your help in doing my solar project, im stuck at doing mppt, please help me,guru_ram86@yahoo.com

  2. hi swagatam

    if i would like to replace the bc557 for bc547 in the first circuit can it be used as a changeover circuit...?

    1. Hi JP,

      no it won't work that way.

    2. i tried the second circuit practically for charging my LA battery(6v4.5ah) from panel it charge perfectly upto 6.46V(9hr charging) with using 6v relay. the panel needs to be above 6 to cut off the dc supply from wall.. but the one thing it makes lot of noise(relay keeps clicking) when the panel volatge is around 5v.. what i want to make a relay free circuit.. please suggest me idea or circuit..
      only want to modify the second circuit...


    3. you may try the first circuit given in the above article.


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