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Battery charger for my UPS

Discussion in 'Alternative Energy' started by umer27, Mar 7, 2009.

  1. umer27

    umer27 New Member

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    Hi all ,

    I want to be able to build a battery charger for a UPS , that is based on SMPS technology. I dont want to use bulky old transformers..

    I have looked at how Computer power supplies have been designed.. They essentially have a buck converter sort of topology..

    Please tell me where to start..

    My requirements are the circuit should take household 220V Ac , convert it to a suitable voltage for charging (13.8 V) @ 5-15 Amps..

    It is my understanding that in the computer PSW, they rectify the 220 V AC, and turn it into DC voltage (300VDC ? ) then use high frequency transformers (the yellow ones) to step the voltage down in stages .. if someone can explain the operation of these Power supplies Id be grateful

    I have good know-how of microcontrollers.. so any solution there would be a piece of cake for me..

    Thanks

    Umer


    PS , are there any safety issues with building and using such circuits.. such as lack of isolation coz there is no transformer.. ?
     
  2. mneary

    mneary New Member

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    PC power supplies don't generally use buck converter topology, they use an isolation transformer at high frequency. Look at the back of the PC board out of a power supply and see the big wide isolation gap.

    You can get typical schematics from the web, from app notes of chip vendors.
     
  3. umer27

    umer27 New Member

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    can I find an isolation transformer at the electronics store ? how pls explain how an isolation transformer works ?


    you mean to tell me, that the yellow transformer is an isolation transformer ?
     
    Last edited: Mar 7, 2009
  4. dave

    Dave New Member

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  5. mneary

    mneary New Member

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    The yellow transformer in the PC power supply is almost what you want. Maybe you can buy one, but I don't know where. Dive into the app notes for the chip sets and output transistors. Some of them will help you design your own transformer using the yellow transformer as a starting point.

    Warning: This is not a trivial exercise. When you have a working power supply you will know far more about the subject than I do.
     
  6. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    SG1842 Power Switch IC

    Here is a neat little IC that you may find is just what you are looking for! its a complete power switching system in one easy to use IC!
    I just started playing with them. The world of HF switchers just got way simpler! :D So far I have been able to get one to run a flyback transformer out of a 300 watt computer power supply at 12 volts 20 amps (dummy load only) from a 170VDC source. All the parts are recycled from the computer power supply also. But I may be pushing the IC's PWM limit! It is rated for 8 amps switching . So 8 amps 170 volts @ 50% duty cycle stepped down to 12 volts DC should give me 56.6 amps peak in theory! But I have not pushed it to see when it shuts down. At $11.20 each from Digikey I dont want to burn it up just yet!
     

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    Last edited: Mar 8, 2009
  7. umer27

    umer27 New Member

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    what should be the rating of the fly back transformer.. amps, max.power, input voltage ?
     
  8. Diver300

    Diver300 Well-Known Member

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    If you are making a SMPS there are so many more questions about the transformer than the current, voltage and power.

    The inductance is important, as are the saturation current, the number of turns for all the windings (and there are almost always 3 or more) and the leakage inductance on each winding.

    The construction is also important so that the secondary winding should always be better coupled to the primary than the auxiliary winding.

    SMPS are tricky. If you're not yet familiar with what a transformer is I suggest you try a linear supply first.
     
  9. umer27

    umer27 New Member

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    I found a chopper (transformer) ... what is the difference between a fly back and a chopper ?
     
  10. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    Fairchild 1M0880 / AN4105 5 Pin SMPS Driver

    Here is another simple ALL IN ONE switcher IC. This one even has the switching fet built in!
    1M0880 is the IC specs. AN4105 is the application circuits data. Both come from the same reference source numbers.

    I came across this IC in a 46" plasma screen power supply. Its was driving a 250 watt load all by itself!
     

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  11. blueroomelectronics

    blueroomelectronics Well-Known Member

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    Why would a UPS need a battery charger? Isn't it already built in.
     
  12. Jules_Theone

    Jules_Theone Member

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    UPS's do have a built in charger but it uses the big transformer and i think he wants to use a more effecient charger. My 1400VA UPS draws 400mA @ 240v trickle charging the battery pack at only 100mA @ 24v.
     
  13. umer27

    umer27 New Member

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    yes... PS, I have come up with an inverter that works well.. Now i need a reliable charger for it..
     
  14. shiekh

    shiekh New Member

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    Why bother, chargers based on computer power supply technology already exist.
     
  15. umer27

    umer27 New Member

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    so does most of the stuff people are discussing..

    Grid tie inverters etc etc ..
     
  16. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    As the "GTI go to guy" (sort of) I am wondering if your looking at this from a (DIY cost) VS (factory made cost) stand point or just to see if you can build one that works stand point.
    I personaly support DIY when it also beats factory cost.
    If its for self education, great!
    If its to save money, great!
    If its just to prove you can do it, great!
    If its to save $2 worth of electricity a year then I think your a nutter that needs to do a whole house energy audit, and find where the real power waisters are.
     

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