Forum - Recent Threads

12VDC UPS Project

Atalantia — Thu, 17 Sep 2026 09:07:33 GMT

Hello People,

I try to outline a 12V UPS for my router. The commercial things I see are either to complicate, means have to much functions or they use a relay. I want to build a solid state switch that is faster than a relay. I have a first sketch. What do you think thats the right direction?

The battery has her own BMS it's not in the circuit here. The comparator is the watchdog who activates the switch in case of a power outage. There are a few diodes to prevent return current. A few resistor to adjust the components margins. R4 is a trimm-pot. Works as a pull-down resistor and adjust the threshold for the comparator.

PS: I know. The input voltage has to be higher than 12V.

RE: 12VDC UPS Project

geralds — Fri, 18 Sep 2026 16:37:43 GMT

I understand you perfectly. I’m a tinkerer too—someone who always wants to improve everything, and I mean *everything*.
My hammer is always within reach, "pangs!", and I already have new ideas.Bit by bit, a drop of knowledge...

RE: 12VDC UPS Project

Atalantia — Fri, 18 Sep 2026 05:44:19 GMT

OK, now I understand why all the USV I came along have a relay. Maybe I shouldn't reinvent it. 😅

RE: 12VDC UPS Project

geralds — Thu, 17 Sep 2026 13:59:30 GMT

Solid-state relays are okay—a perfectly viable option. Solid-state changeover switches are a bit komplexer.

But please don't forget about the leakage current (return current in µA) inherent to semiconductor switches; the battery will feel that.
True isolation (via a mechanical contact) would be better; that way, you avoid equalization currents, which are also highly temperature-dependent. Leakage currents drain the battery without providing any benefit.

Don't be afraid to use a relay.

The "only" potential issue is:
the contact sticking shut even after the relay has de-energized; the contact joint can melting, current switch off lightning.
With a DC relay, the iron core can become permanently magnetized if the relay remains energized by direct current for extended periods.
One possible solution for monitoring this is to use a second contact with an LED to indicate whether the circuit is closed or open.

A more complex approach involves applying a short negative pulse to the relay coil upon switch-off to collapse the magnetic field.
This is achieved using a second winding around the iron core (utilizing back-EMF).

->> One more point regarding your circuit:
Make sure to include deep-discharge protection and a charge cutoff for the battery; otherwise, the battery will eventually be ruined.

RE: 12VDC UPS Project

JWx — Thu, 17 Sep 2026 13:56:39 GMT

I wonder if putting another diode between battery anode and output wouldn't be beneficial in this case? Than way, big C wouldn't be needed - output would be powered using this additional diode during relay toggle, and in a steady state diode would be shorted by the relay contact, eliminating power loss on diode's voltage drop?

RE: 12VDC UPS Project

JWx — Thu, 17 Sep 2026 13:49:49 GMT

I think there should be also some current limiting during battery charging - now, when battery voltage is < (12V - Vd4), it would be charged through D4 with possibly big current (even creating fire hazard in case of - for example - battery cell shortage and powerful PSU), but maybe BMS would protect against it?

RE: 12VDC UPS Project

Atalantia — Thu, 17 Sep 2026 13:24:45 GMT

The goal is to do it with a solid state switch. No relay. I'll try to modifier this circuit to a solid state.

RE: 12VDC UPS Project

shabaz — Thu, 17 Sep 2026 13:18:33 GMT

I did near-identical circuit with my home router a few years ago. I just used 8xAA cells (non-rechargeable) since it is very rare here to have outages. I would possibly use Ni-MH if I were to do it again today.

RE: 12VDC UPS Project

dang74 — Thu, 17 Sep 2026 11:35:06 GMT

I think you will need second resistor connected to R1 to form a voltage divider. Otherwise I think your comparator will always select the battery. The comparator should also have a bypass capacitor connected to its supply pin.

RE: 12VDC UPS Project

geralds — Thu, 17 Sep 2026 11:18:59 GMT

Hi look here, an idea:

This is a very simple "UPV".

The switching time is not the big problem if you make a buffer (Elko-accu) during toggling the relay.

It's just an idea.

RE: 12VDC UPS Project

Atalantia — Thu, 17 Sep 2026 10:28:50 GMT

[quote userid="121623" url="~/challenges-projects/project14/f/forum/57274/12vdc-ups-project/238218"]It is a good idea to disconnect the battery from the load if its voltage drops too low but your circuit doesn't do that.[/quote]

The dropout voltage of the battery is managed by the BMS. It's not in the circuit.

RE: 12VDC UPS Project

michaelkellett — Thu, 17 Sep 2026 10:08:40 GMT

The comparator and switch are not really doing anything useful.

Two diodes will do the business quite easily.

Both cathodes to the output, one anode to the 12V DC in and one to the battery.

It is a good idea to disconnect the battery from the load if its voltage drops too low but your circuit doesn't do that.

RE: Need more time for Make a Connection Competition

rsjawale24 — Tue, 08 Sep 2026 17:39:51 GMT

Thank you so much!

Need more time for Make a Connection Competition

redcharly — Mon, 07 Sep 2026 13:46:56 GMT

Hi, I'm Carlo and I worked these weeks on "Show & Tell" project and on "Make a Connection" project. I've completed the first but I need more time for the second, the two competitions were too close together in time!! Is it possible to postpone by 2 or 3 weeks the end of competition?

If the competition end date cannot be moved, that won't be a problem; I will complete the project and submit it to another competition.

Regards.

Carlo Russo

RE: Need more time for Make a Connection Competition

JoRatcliffe — Tue, 08 Sep 2026 14:54:24 GMT

Thanks for feeding back! I have extended the project submission deadline by two weeks to midnight (UK time) on Sunday 27th September and I have updated the Make A Connection competition page to reflect this.

RE: Need more time for Make a Connection Competition

arvindsa — Tue, 08 Sep 2026 12:57:35 GMT

Looking forward. Write Write. No wait, Type type

RE: Need more time for Make a Connection Competition

embeddedguy — Tue, 08 Sep 2026 12:40:33 GMT

Well not to discuss much But I am fine with admin's decision. The thing is make a connection was there since two months...extending it now may be bad idea for some of the participants. Still I would say one week should be enough.

RE: Need more time for Make a Connection Competition

embeddedguy — Tue, 08 Sep 2026 12:32:00 GMT

No, I am fine, My project is ready. 😁

RE: Need more time for Make a Connection Competition

Qbit — Tue, 08 Sep 2026 11:17:22 GMT

Really helpful!

RE: Need more time for Make a Connection Competition

arvindsa — Tue, 08 Sep 2026 10:34:58 GMT

embeddedguy might be helpful to you.

RE: Need more time for Make a Connection Competition

JoRatcliffe — Tue, 08 Sep 2026 10:20:16 GMT

Hi redcharly I would be happy to extend the competition by two weeks. Would anyone else also find the extra time useful if I did this?

RE: Need more time for Make a Connection Competition

arvindsa — Tue, 08 Sep 2026 05:16:47 GMT

The date extention is upto the admins, so im not speaking for them , still 3 weeks is nearly one third of the entire competition duration. but Id suggest you post your post your project even if its not completed. But on the other side, time is a limited resource, prioritise and plan accordingly. all the best

Reminder - Show & Tell Competition Closes This Sunday!

JoRatcliffe — Thu, 27 Aug 2026 14:24:25 GMT

Hi everyone, just a quick reminder that the Show & Tell competition closes midnight (UK time) this Sunday 30th August.

You can submit any electronics project you have not previously shared on the community - whether that is something you created at work, at university, or in your spare time. There are 25 Arduino Uno Q's up for grabs so have a go and share your project if you haven't already!

Click here to check out all the competition details and submit your project.

RE: Reminder - Show & Tell Competition Closes This Sunday!

e14phil — Wed, 02 Sep 2026 09:31:37 GMT

Hi, this project got auto moderated and was released. This will be included in the judging.

RE: Reminder - Show & Tell Competition Closes This Sunday!

arvindsa — Sun, 30 Aug 2026 06:52:09 GMT

My Project got flagged for moderation. :( Appeal filed for review.