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Currently I have this UPS power setup for my Router with the help of a DPDT relay so that the router works when mains power supply fails. The working principle of my project is real simple. Two separate power supplies are connected to a DPDT relay where one the power supply powers the relay coil and a battery. When the mains power fails the relay coil is no longer powered and switches to the battery supply. I used a relay mainly because it provides full isolation between the 2 power supplies.

My concern is that in my current setup the relay coils are always powered on when the mains in available which according to Omron, might lead to a contact failure or coil burnout in the long run.

enter image description here

So is there any a way to replace my relay setup with a more reliable component? maybe a Mosfet or SCR? Will the coil really get damaged?

I'm attaching the diagram of my setup enter image description here

Edit:

So I went through the below suggestions and decided to change my setup into this new one which might be better than my first one. These are the changes that I've made.

  1. I got rid of the 9V wall wart and now it's just a single 12V 2A wall wart that powers everything.
  2. Now uses a CC CV Buck converter with Li-ion charging feature to charge batteries instead.
  3. Added a 2 Series Battery Management Module with Cell balancer feature so that I could now use 2x18650 cells for longer runtime and better voltage to boost
  4. Replaced the Boost converter with a Buck Boost converter.
  5. Scrapped the Relay.

enter image description here

So, will my new setup work as intended? or is there any problem? or maybe place to improve?

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  • \$\begingroup\$ I also am going to build a same UPS project for router. Can you tell me the final block diagram that you finally end up building with any suggestions for improvement? That would be great.Thanks \$\endgroup\$
    – Dinesh Bhardwaj
    Dec 6, 2021 at 4:48
  • \$\begingroup\$ @DineshBhardwaj This all in one module will work with a USB mobile charger. link \$\endgroup\$
    – Kokachi
    Jan 3, 2022 at 7:13

2 Answers 2

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I can't understand why you have such complexity built in to your design. Surely, this will work and is far simpler; requiring no relay and no change-over to a 9 volt supply (now redundant): -

enter image description here

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  • \$\begingroup\$ I've tried that it doesn't work if the battery is charging since the 5V supply can only give 2A of current and the battery charger draws 1A leaving insufficient current for the Router which needs 600mA after the conversion loss. \$\endgroup\$
    – Kokachi
    Apr 9, 2018 at 12:56
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    \$\begingroup\$ Then why not use a slightly more powerful 5 volt supply OR, if you are insistent on your original design find a way of directly connecting the 9 volt DC to the boost regulator output OR find a more suitable relay. As a last resort you should go down the MOSFET switching. I'm trying to guide you to the most effective solution. \$\endgroup\$
    – Andy aka
    Apr 9, 2018 at 13:10
  • \$\begingroup\$ 5V 3A supplies are a bit pricey, however 2x 5V 2A supplies are an option. Connecting the 9V DC supply to the boost regulator and you still have some loss in current from an already limited power supply. \$\endgroup\$
    – Kokachi
    Apr 10, 2018 at 13:32
  • \$\begingroup\$ The boost converter would become reverse biased and no appreciable power would flow into it. You set the booster to produce 8.95 volts and the 9 volt supply to produce 9.05 volts and, hey-presto, it works (small print - this works with boost converters I've seen but you may have something that is non-standard so detail checking and measurement would be needed). \$\endgroup\$
    – Andy aka
    Apr 10, 2018 at 13:56
  • \$\begingroup\$ See this: learningaboutelectronics.com/images/… and notice that the diode blocks any significant power. \$\endgroup\$
    – Andy aka
    Apr 10, 2018 at 13:58
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No need to use a relay, just a couple of diodes.

enter image description here

The only thing to be aware of is that the router will now be powered from whichever 9V supply has the higher voltage output so you need to make sure it's the one that's powered when the mains is on - so you may need to adjust the boost controller output slightly. If necessary, add an extra diode in series with the output to add a bit more drop.

This has the advantage of reducing the chances of the router resetting due to a dropout when it switches to battery power and back again.

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  • \$\begingroup\$ I had this option in mind when first designing the setup but then a slight problem was that the diode would drop the voltage from the 9V wall wart and the router will become unstable. \$\endgroup\$
    – Kokachi
    Apr 10, 2018 at 13:29
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    \$\begingroup\$ Then use Schottky diodes for a lower voltage drop. \$\endgroup\$
    – Finbarr
    Apr 10, 2018 at 14:05
  • \$\begingroup\$ I've updated my setup and have uploaded new diagram. I have made use of diodes in my setup. Is it fine or will there be any problems? \$\endgroup\$
    – Kokachi
    Apr 10, 2018 at 16:56

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