Hi
I know I am going against standard calculations and the correct way. But I am in hurry to set up a 12V UPS supply for my wifi router. I am hosting a meeting online and my internet should not be interrupted. So as a quick setup, I am planning to use this type of arrangement. Since the supply is 15V regulated SMPS, I thought a resistor acts as a good current limiter for the battery. However I am not good at designing circuits. The 12V router can get 13.2V in some condition through my circuit. To stop reaching 13.2V to the router, through R1 and battery, I will add another diode at the output of the battery. So please suggest me if making a great mistake.

 

I admire the clever use of diodes but worry that you have to rely too much on assumptions about the battery to be safe.

Short digression: My first microprocessor cost US $25, which was a huge chunk of money at the time (growing family to support). I had the "computer" on the workbench powered by a small benchtop power supply. Everything was fine until I bumped the pot on the power supply and caused too much voltage to be fed to the circuits. From then on that processor could only count to two, which made it worthless.

That's what I think about when I see somebody running a expensive piece of equipment from a power supply may be capable of accidentally sending excess voltage to the equipment. Do you have any 12V regulators that you can use. Even using a 7805 with an 8V reference voltage to the ground terminal would be safer, especially in the case of accidental overloads.

Lacking that a fuse and a suitable Zener diode (with emitter follower if the fuse is a large one) would give a degree of safety.

 

Do you have any 12V regulators that you can use. Even using a 7805 with an 8V reference voltage to the ground terminal would be safer, especially in the case of accidental overloads.

Lacking that a fuse and a suitable Zener diode (with emitter-follower if the fuse is a large one) would give a degree of safety.

Hi Dick, I have 7812 but I am worried about its large voltage drop. I think LDO is needed there.

 

What kind of rectifiers do you plan to use?

 

Probably the router has an internal power supply, you can open the case and verify that. In addition, if you have the original supply for the router, you can check the voltage that it supplies.

 

I am glad that my electricity utility company has backups on almost everything in their system so that power failures are very rare.
Today I read that 58% of the electricity here is produced by nuclear reactors. Windmills and solar power are rare.

 

What kind of rectifiers do you plan to use?

Probably the normal router does not consume more than 1A (I will test). So that I am planning to use 1n4007 easy available 600mV drop diode. If I used 19V SMPS and used 7815, around starting stage of the circuit, I will not suffer from voltage drop problem. If diodes are safe to droping voltage to the router, then I will use it.

Probably the router has an internal power supply, you can open the case and verify that. In addition, if you have the original supply for the router, you can check the voltage that it supplies.

The original power supply of the router has exact 12V output. I dessambled the router and did not noticed any voltage regulators there.

I am glad that my electricity utility company has backups on almost everything in their system so that power failures are very rare.
Today I read that 58% of the electricity here is produced by nuclear reactors. Windmills and solar power are rare.

I guess, many of the countries are now not allowed to do nuclear activities. We have great water resources falling from our tallest Himalayas. But electricity stability is very poor!

 

I have seen some of those mountain streams that could easily power a small turbine to drive an alternator or generator. But do they run all year?? I don't know. There is so much involved with typical city electrical distribution that it is hard to know all the details.
AND, for the UPS, it seems that the stable power arrangement would be to use an adequate battery with the power supply providing just enough charge to compensate for the router current draw.

 

@Willen, are you interested in making a low-dropout regulator? You would need a P-channel MOSFET or PNP transistor as the pass element, rated at 150% or more of the load current.

If you want to try your diode circuit, please try with a dummy load first!

 

Below is a circuit for a very low dropout 12V regulator using a TL431 reference, two NPN transistors, and a P-MOSFET: