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Little Ups for my 12W Router

Discussion in 'General Electronics Discussion' started by iamsuperuser, Jan 22, 2013.

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  1. iamsuperuser


    Jan 22, 2013
    Hello Everyone

    I'm very new to this forum, so apologies if posted in wrong section.

    I wanted a backup for my wifi router (12V * 1A = 12W) during power cuts , which is for two hours in my place Hyderabad, India.

    So I searched the net and i found THIS

    I implemented the circuit in the first post with some modifications due to lack of availability of some components.

    The first time i tested the circuit i removed the battery in the UPS of my desktop and used it. This worked perfectly.

    Anyway i needed the battery for UPS, so i bought a new battery of the same brand (exide 12V 7AH) and tested for the second time my router got damaged, it is dead now.

    While testing first time i used step down t/f for input but the second time i kept the step down t/f aside to finish the connections neatly.

    While testing second time the last connection i put was router terminals to 1N5408 and instantly i saw sparks in the breadboard.

    I also posted my breadboard connection screenshot , please find my mistakes on it.

    Anyway I spent a lot of time on it and i wanted to know what did i do wrong.

    Below are the orginal and modified circuits . Please tell me what to change and what to keep.

    Original Circuit


    Modified circuit


    I made the circuit on a bread board even the bread had burns on it.



    Please help me this time , May be i'll be careful next time. :)
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    There is so much that is wrong...

    Firstly, you cant replace an LM338 with a 7812.

    Secondly, there is no current limiting for the charging of the battery.

    The 7812 adds to this by removing any practical voltage limit as well.

    Depending on your router, it may require a very exact voltage, or it may operate from a very wide supply. What is it? Google may know what the internal power supply is.

    I'm not sure why you burnt stuff, but it is possible that you wired something incorrectly.
  3. iamsuperuser


    Jan 22, 2013
    Well i have a Belkin Router which needs 12V x 1A = 12W of power.

    I need a 2hr battery backup only for this router because i have power cuts in my area.

    I thought i could make something simple without spending much money.

    Household voltages are 220V to 240V in my country.

    So Can suggest any similar circuits that can provide 12W for 2hr ?
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    OK, I'm unsure about belkin routers. Some linksys routers can run on voltages much lower (or higher) than their internal power supply uses due to a DC-DC converter inside them -- the WRT54GL actually runs from 3.3V!). If you told me *exactly* what the router was, I might be able to investigate. For now we still don't know how close to 12V the power supply needs to be.

    Harald has given you the general idea. Get a 12V battery charger and connect that to the battery permanently (make sure it's a type that can be left connected permanently). Then connect your load to the battery.

    If you have to build this yourself, find a circuit for a battery charger that float charges the battery at a safe voltage (the battery datasheet will tell you what that is -- beware that it changes with temperature). Then get a proper LM338 and build a regulator for this voltage.

    This datasheet shows how to create a temperature compensated charger.

    A little trick can then be used to provide a current limit lower than the internal limit of the 338. A small resistor in series with the load placed between the output and the sense resistor will limit the short circuit current to 1.25/R.

    However, you're not so interested in the short circuit current, but the current at normal charging voltages. If you want the current limited to 3A when the battery is at 10V, then the resistor needs to be about 0.1 ohms (and that's based on your float voltage being about 13.75V).

    The formula is something like R = (1.25 - (1.25 * V / Vr))/Im

    Where V is the battery voltage, Vr is the regulated voltage, and Im is the maximum current you want at this voltage. (if you substitute the values above, you'll find you get 0.1136 ohms)

    Of course, your DC power supply needs to be rated for at least 3A and you will need a heatsink on the LM338.

    And NO you can't use a fixed voltage regulator.
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