# Automatic switching between two power sources

#### Jennifer016

Joined Feb 17, 2019
6
Hello everyone, I am new to this forum and I am a total noob when it comes to electronics and circuits (means, I know nothing about them)
So here is what I m doing :
I'm looking for a way to switch between two 12 volt power sources
First source or the main source is (12v, 1 Amp) DC adapter and the second source is 12v, 4 Amp) battery
So here is the scenario, the charger is giving output to a device , so the moment adapter's power is cut, the device should start taking power from the backup battery.
The battery should only be used when the main adapter's power is down
And when the adapter is ON, battery's connection should be cut off
So any suggestions ?

#### LesJones

Joined Jan 8, 2017
3,707
If you can be sure that the voltage from the 12 DC adapter is always slightly more than the battery voltage then you can just use two diodes.Assuming the 12 volts battery is a lead acid battery it can be close to 13 volts when it is fully charged so the output of the DC adapter would have to be at least 13 volts when supplying the load. Other things to consider. Can the load tolerate a few tens of mS break in the supply if a relay was used to switch over. What is the lowest voltage the load will work from. (This is to see if it was permissible to add a few diodes in series with the battery to make the diode method work if the DC supply is exactly 12 volts.

Les.

Joined Mar 10, 2018
4,057
Something like this -

Diodes could be Schottky to minimize V drop across them.

Regards, Dana.

#### Jennifer016

Joined Feb 17, 2019
6
If you can be sure that the voltage from the 12 DC adapter is always slightly more than the battery voltage then you can just use two diodes.Assuming the 12 volts battery is a lead acid battery it can be close to 13 volts when it is fully charged so the output of the DC adapter would have to be at least 13 volts when supplying the load. Other things to consider. Can the load tolerate a few tens of mS break in the supply if a relay was used to switch over. What is the lowest voltage the load will work from. (This is to see if it was permissible to add a few diodes in series with the battery to make the diode method work if the DC supply is exactly 12 volts.

Les.
The load is a wifi router and it works on 12v 1 amp
I already had damaged 2 routers before once by using lower voltage than 12 v
And second one was damaged after I use 13v maybe 14 v

#### Jennifer016

Joined Feb 17, 2019
6
Something like this -

View attachment 170334

Diodes could be Schottky to minimize V drop across them.

Regards, Dana.
What if I wanna charge the battery while the adapter is supplying the power to the load

#### LesJones

Joined Jan 8, 2017
3,707
If you want to keep your 12 volts battery (Assuming it is an SLA battery.) on float charge it will need to be supplied with 13.8 volts. If the brand of router you are using is damaged by an under voltage input then you would need to add a cut off circuit to protect it as the battery discharged. (You also need a cut off circuit to protect the battery from over discharge.) To design the power system you will need to find the input voltage specification of the router so you know what to set the cut off voltage to and how high the input voltage can be allowed to go. It may work out easier to base the system round a 24 volt float charged battery with a switch mode step down converter to 12 volts.

Les.

Joined Mar 10, 2018
4,057
If the routers you are buying are not themselves internally protected,
and you must stay sourcing them, I recommend you consider an
embedded control solution to handle the switching and the charging.

That way also would handle brownout conditions for the router, eg. when
AC line is down and battery discharged. Plus the UP would handle proper
intelligent battery charging for you. Something like an Arduino or ATTINY
would suffice.

Not only that I would recommend a crowbar fuse circuit as part of the pro-
tection for over V on the router.

Curious, you sure you hosed a router with under V ? That would say any
AC power failure, no battery present, would damage it and that would
be an unusual design flaw.

Regards, Dana.

#### Jennifer016

Joined Feb 17, 2019
6
If you want to keep your 12 volts battery (Assuming it is an SLA battery.) on float charge it will need to be supplied with 13.8 volts. If the brand of router you are using is damaged by an under voltage input then you would need to add a cut off circuit to protect it as the battery discharged. (You also need a cut off circuit to protect the battery from over discharge.) To design the power system you will need to find the input voltage specification of the router so you know what to set the cut off voltage to and how high the input voltage can be allowed to go. It may work out easier to base the system round a 24 volt float charged battery with a switch mode step down converter to 12 volts.

Les.
Well that all went over my head, have I mentioned I am a total noob (like I don't know anything more than v stands for voltage kinda noob) sorry for my ignorance

#### Jennifer016

Joined Feb 17, 2019
6
If the routers you are buying are not themselves internally protected,
and you must stay sourcing them, I recommend you consider an
embedded control solution to handle the switching and the charging.

That way also would handle brownout conditions for the router, eg. when
AC line is down and battery discharged. Plus the UP would handle proper
intelligent battery charging for you. Something like an Arduino or ATTINY
would suffice.

Not only that I would recommend a crowbar fuse circuit as part of the pro-
tection for over V on the router.

Curious, you sure you hosed a router with under V ? That would say any
AC power failure, no battery present, would damage it and that would
be an unusual design flaw.

Regards, Dana.
Well I think so

#### LesJones

Joined Jan 8, 2017
3,707
First, sorry for using the acronym "SLA" without saying what it stands for. It stands for sealed lead acid. Float charging is for applications when the battery needs to be on permanent charge so it is in a fully charged state when required. An under voltage cut off circuit monitors the power supply voltage and if it falls below a set voltage it disconnects the load. So for example to protect a 12 volt SLA battery from over discharge the load would need to be disconnected when the battery voltage dropped to 11.8 volts (That value is from memory so it would need to be checked.) So as the battery slowly discharged from its fully charged state of 13.8 volts the load would see the voltage slowly drop from that value until it reached 11.8 volts at which point the load would see the voltage suddenly drop to zero volts. A switch mode step down converter would be able to accept an input voltage from about 30 volts down to about 16 volts and still give an output of 12 volts.

Les.

#### Jennifer016

Joined Feb 17, 2019
6
Well one thing for sure it's cheap but reliable battery
Its old but still woks like a charm
It has transparent casing and the battery needs to be filled with distilled water every now and then.

#### J12345

Joined Jul 1, 2019
6
Something like this -

View attachment 170334

Diodes could be Schottky to minimize V drop across them.

Regards, Dana.
What exactly is that resistor for?
Is that necessary for switching between the power supplies or is it just to limit the current that flows into the device that we are powering. (Router in this case)

#### J12345

Joined Jul 1, 2019
6
Also, would the 1N5822 Schottkey diodes be good for this type of project. I know you mentioned 1N4148 diodes in your schematic before, but those arent Shottkey diodes so the voltage drop is much to high.
Sorry for all of the questions, I'm a bit of a noob when it comes to electronics. Thanks for your help in advance.

Joined Mar 10, 2018
4,057
What exactly is that resistor for?
Is that necessary for switching between the power supplies or is it just to limit the current that flows into the device that we are powering. (Router in this case)
Thats a representation of your load so the simulation can be run.

Regards, Dana.

Joined Mar 10, 2018
4,057
Also, would the 1N5822 Schottkey diodes be good for this type of project. I know you mentioned 1N4148 diodes in your schematic before, but those arent Shottkey diodes so the voltage drop is much to high.
Sorry for all of the questions, I'm a bit of a noob when it comes to electronics. Thanks for your help in advance.

Yes, those look to b e quite usable for this design.

Regards, Dana.

#### Zaid111

Joined May 19, 2021
7
Something like this -

View attachment 170334

Diodes could be Schottky to minimize V drop across them.

Regards, Dana.
I am also having a similar problem but instead of a 12v source i have a 3v battery pack and a 3v ac - dc adaptor as the other source. Is this circuit still applicable ?