Dc ups

Discussion in 'The Projects Forum' started by haseebhm, Feb 20, 2010.

  1. haseebhm

    Thread Starter New Member

    Feb 19, 2010
    Hello Everyone!
    I want to make my own small UPS for my home network equipment(ADSL MODEM + WIFI ROUTER). I am a student of Engineering but not Electronics. Just have basic knowledge and I know how to make a circuit etc but can't design it. I have found much help from internet in this regard and here's the image of the circuit I found the most suitable. Please check the circuit and suggest whether it will be fine.
    1. Which type of battery should I should use?
    2. would It prevent overcharging ?
    3. How can I make it automatically switch off when battery voltage drops?

    Source: http://www.beigebag.com/case_ups1.htm
  2. rs14smith

    New Member

    Feb 19, 2010
    I'm in the same boat as you man when it comes to designing. But I know external things you can use based on your question.

    As for the battery, I'd suggest going with a Deep Cycle battery so you can fully drain the battery without damaging it unlike a Car Battery.

    For the overcharging, again this is external but just something to think about, you can use a Charge Controller. These are fairly cheap as well depending on how many Amps it can handle.

    For your last question, I'm not sure what you mean when the voltage drops. Do you mean when the battery is fully charge, how to switch off the UPS so it will stop charging the battery? As if the voltage drops, I'd think you'd want to turn it off, but then again, I'm not sure exactly what you want to do with it.

    Again, most of what I listed here is external components, but as far as making one huge circuit to do all this, I have no idea. If you can figure out how to build a charge controller, you'll be one step closer. But they are so cheap I wouldn't even bother building one. :)
  3. haseebhm

    Thread Starter New Member

    Feb 19, 2010
    I mean to make it automatically turn off when battery is discharged in case longer power cut-offs.
  4. JDT

    Well-Known Member

    Feb 12, 2009
    I have done this. Was working fine until the battery suddenly died!

    I was supporting a WIFI router and two small fan-less PCs. These run a mail server and a back-up. The router needed 12V at about 1A and the PCs needed 5V at about 2.5A each.

    Despite the problems with my battery (which I think was caused by a cheap battery), I would use sealed lead-acid batteries (good ones) at a nominal 12V or 24V system.

    Keep them fully charged by "floating" them from a fixed 13.8V or 27.6V supply. This supply needs to be current limited in case the battery gets very discharged. Then, for efficiency, use DC-DC converters to change to the voltages you need. I found I needed a well filtered supply for the WiFi ADSL router or I would not connect properly.

    Yes, you need to switch everything off when the battery voltage falls too low. This will be about 11.5V on a 12V system or 23V on a 24V system. I did this with a series power P-channel MOSFET driven by a comparator measuring the battery voltage compared against a fixed reference. The reason I used a MOSFET instead of a relay is that the MOSFET needs no drive current once its on and has a very low on resistance.
  5. kubeek


    Sep 20, 2005
    What about building a switched regulator for the stabilization of output voltages? You could either build your own probably, similar to an ATX power supply, or use three switched regulators like the LTC series.
  6. haseebhm

    Thread Starter New Member

    Feb 19, 2010
    and what about prevention from damaging the battery due to overcharging ?
  7. haseebhm

    Thread Starter New Member

    Feb 19, 2010
    As I already said, I cannot modify the circuit a/c to the recommendations. Please suggest me the type of battery to use according to the circuit above. Sorry for the circuit is not easily readable since it doesn't contain proper symbols.

    please suggest me the type of battery
    and I hope this circuit will prevent overcharging. Tell me if I am wrong.

    Thanks in advance
  8. retched

    AAC Fanatic!

    Dec 5, 2009
    A decent lead-acid battery, once it has reached a set voltage say 14.4, can revert to a floating charge that stays at 13.8 (all numbers for a 12v battery)

    You can keep it there all day. You will DEFINITELY HAVE to read the manufactures data sheet for your specific battery. They will give you charging details that can be specific for your battery.

    But for overcharge protection, you can use the comparator set at around 14.4 to switch the charger into float mode.