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How to make a double electric circuit? Answered

First things first, I don't think the title is accurate for this question but anyways,

We have frequent power cuts in the place where i live and this is pretty irritating as my modem wont work and i cant use the internet. So I have this backup generator and I got an extension from that to power my modem (The backup generator only powers specific lights). But, the problem i face is that every time the electricity goes, i have to manually remove the plug from the mains and plug it into the generator and vice versa when the electricity returns. 
So, in a nutshell, i want to make a device which can accept both the mains and generator inputs but output only one of them at any time to the modem.
If the generator switched on as soon as the mains went, it would be fine. But thats not the case.
1) As soon as the power goes, the generator takes about 2 minutes to switch on and then my modem takes about 3 minutes to reboot.
2) When the power returns, the generator stays on for about 2 minutes as well.

So if I were to make the device I mentioned earlier, For 5 minutes I would have no internet (generator+modem starting times) and for 2 minutes I would get 440 volts instead of 220 which would practically kill my modem.

Finally the device should have, (I think)
2 inputs.
1 output.
A small battery or a capacitor(For the 2 min generator lag)
A switch to automatically switch form generator to mains.
So can anybody give me some ideas? If you didn't understand what i am trying to say, i can gladly rephrase.

I thought of using a ups or something like that but i don't know how to provide two inputs and not kill the ups itself.




3 years ago

Does your modem need 220V directly (internal power supply) or does it use a lower DC voltage and have an external power supply (wall wart)?

If it is an external power supply, get a second one, combine the - outputs directly and the + outputs by two diodes (use Schottky diodes for a lower voltage drop) That will give you the automatic switching between the two supplies. You may add a big capacitor (gold cap) behind the diodes to keep the modem alive while no power at all is available.

If the modem needs 220V - tricky. Probably nothing you should build yourself - if you have to ask what to do. You need a uninterruptable power supply with two inputs. Those beasts exist but are used for professional equipment that needs ultra high availabilty and are a tad expensive therefore.

Srujan Deshpandeverence

Reply 3 years ago

Thanks a lot Verence!! I think this is exactly what i was looking for!!

My modem's adapter converts 220V to 9V DC.

verenceSrujan Deshpande

Reply 3 years ago

How to use them? See the first schematic. The central pins of the connectors are the positive wires. This will work without the capacitor (or gold cap) too, of course without backup when both inputs are without power. This works, but whenever the circuit is switched on, the capacitor will draw a high current as it is empty. That is avoided by the second schematic, adjust R1 accordingly (1..20Ohm or so, just guessing here) The downside here is that the modem will see a double Uf voltage drop when it is powered from the capacitor. I.e. if the input voltage is 9V and the diodes have a forward voltage of 0.3V, the output will be 8.7V when any of the 220V inputs is available and 8.4V when the capacitor (that is only charged to 8.7V) will provide the power. Check if your modem is working with that a low voltage (put two diodes in the plus line between power supply and modem).

Where to get them? If you are lucky there is an electronics shop near by. May be a place that repairs electronic stuff and you can ask them. Or you can mail order / buy on line. Can't tell you exactly where, that depends on where you live. You may use digikey to find a diode. Look that is has a low forward voltage and can handle the reverse voltage (at least 9V).


Reply 3 years ago

1+ !
The idea with two power supplies is great for this problem.
All modern modems work with a wall adapter, so it should be pretty straight forward.
Can't mark it as best comment but I don't think anyone will beat it ;)

Josehf Murchison

3 years ago

I have the same troubles blackouts almost every time it rains.

Well first there is a UPS. (Uninterruptable Power Supply) The hydro goes out your computer, printer, and modem, never notices. Depending on the one you get it can run your computer, printer, and modem, for up to an hour. These are very good but you have got to replace the battery once every two years.

I have a couple of them one for the TV DVD player and satellite and one for my computer network. If the hydro goes out for more than twenty minutes then I kick up the 3500 watt generator and I keep a 3 day supply of gas on hand.

To make a simple UPS just for your modem take a rechargeable battery 6, 9, or 12 volt depending on your adapter, connect the adapter to the battery then the battery to the modem. This rig should have a charging circuit to prevent over charging the battery.