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Redundant UPS Working and Design Details

The post explains a couple of straightforward redundant UPS circuits for providing a secured uninterruptible power to crucial gadgets such as computer ATX or modems etc. The idea was requested by Mr.  Shayan Firoozi.

Technical Specifications

There are many products which has 2 input for different power supply,for example one for normal mains,one for generator or other mains,like servers,routers,and some critical equipment,we call it redundant power supplies

I have an equipment which consumes 3 ampere in 12 volt dc,if I use 2 transfer with 12 volt,3 amp output which one take responsibility and which one is waiting for first loss?? Both are same on voltage and amperage,I don,t want them to work together,I want second power supply to be standby

Just a simple question: What would happens if I replace battery with another 12 volt power supply ?? Will it work as a redundant or standby power supply ?? Thanks for your answer in advanced And if it's possible tell us about model of diode and other components for 12 volt 3 ampere

 The Design

As per the request, the circuit discussed in the above link can be modified to work with another DC power supply by eliminating the battery and associated stages as shown in the following form of redundant UPS circuit:

Redundant UPS Working and Design Details

Using Two Power Supply Inputs

As we can see, the circuit is intended to work with a couple of power supplies having identical specs, such that whenever the primary power supply fails, the relay instantly changes over to the secondary power supply source ensuring an uninterruptible power supply to the connected load.

The diode D1 makes sure that while the primary power source is active and the relay in the deactivated position, it connects in series with D3 creating a greater forward drop than the primary supply diode D4...thus allowing the primary voltage to be in command and powering the load.

However as soon as the primary source goes through an outage, D4 is disabled, and for that split second D1 and D4 takes over powering the load, until the relay has changed over bypassing D1 and enabling the full rated power to the load.

The next diagram shows a method which allows a battery to be included within the proposed redundant UPS circuit, and the primary power source replaced with a solar panel, making the system a 3 way protected UPS circuit

Solar Panel based Redundant UPS Working and Design Details

Using Power Supply with Battery

Referring to the diagram, as long as the solar energy is available, the relay stays activated keeping the mains derived 14v supply cut off from the system.

The solar power in the meantime charges the battery and also the connected load via D1.

The battery power being slightly subdued than the solar panel power keeps D2 deactivated such that only D1 is allowed to carry the solar energy to the attached load at the output.

Using TIP122 for CV Battery Charging

The TIP122 ensures a regulated and safe over charging protected supply for the battery which charges solely through the panel voltage during day time.

As night sets in, the relay deactivates at some of time when the solar supply gets too weak to hold the relay activated.

The above changeover instantly switches the mains operated 14V into the system enabling the load to switch to the mains derived voltage without an interruption.

The battery power makes sure that while the relay is transferring over from the solar to the mains adapter supply, it compensates the split second changeover lapse in power by supplying its own power to the load, and inhibiting even a microsecond break of supply for the load.

The battery also forms the third "line of defense" in case both the primary and the secondary power happens to fail together, and is always positioned in the standby mode for the recommended redundant uninterruptible power supply circuit operation.

The first redundant UPS circuit incorporating two power sources can be better modified in the manner shown below, here the relay N/C can be seen directly connected with the load, thus enabling zero drop in the supply line:

Need Help? Please leave a comment, I'll get back soon with a reply!


  1. Swagatam , is this easy to invert 12 volts DC to 12 Volts AC ?? without converting to 220 Volts

  2. yes it's easy but the waveform will be rectangular not sine wave..

  3. does it matter for an AC load ? if not please tell us how to do that :) thank you

  4. what is the AC load, please explain in detail for getting a correct answer....

  5. It's an electric door lock use a 12 volt 2 amp AC motor to open or lock the door,it's not a continuous load,just 2..3 seconds each time we use it

    Thank you

  6. OK, you can try the following circuit:

  7. Hey Swagatam, thanks for this schematic! I wanted to know if this circuit can be used with a 9V DC adaptor as the primary supply and a 9V DC rechargeable battery(Li-ion cells with a charging circuit) as the backup supply?

  8. Thanks Nisha, yes it can be used with a 9V input supply and a 9V battery, however the input must be always 20% higher than the battery voltage in order to enable proper initialization of charging

  9. Thanks for the swift response Swagatam. I am not good with electronics but calculating for 20%, 12V should do but then I will have to step down the output voltage. I think. I want to power a 9v 600mA wireless router.

  10. Hi Nisha, actually it should be slightly less than 20%, for 9V it should be around 10.7V, and this level is supposed to be your battery's full charger level.


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