1KW 12V-120V inverter

Discussion in 'The Projects Forum' started by magnet18, Mar 4, 2012.

  1. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    OK, as you can tell by the title, I need to build a 12VDC-120VAC power inverter capable of handling 1000Watts of power (~950) in normal operation, and a nice safety margin for peace of mind would be 1100 W or thereabouts.
    it's intended to be powered by multiple AGM batteries.

    So I understand that this- [​IMG]
    is the most typical way to do this, does anyone have any recommendations for transistors?
    I already have a 1300W MOT for the transformer.
    Also, has anyone done this before / does anyone know any things I should worry about/watch out for?
    Thanks guys :)
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    That circuit is not regulated, if it even works will only output square waves, the frequency would not be very stable, and it would be inefficient. It wouldn't be suiitable for much more than incandescent lamps; which themselves are inefficient.

    You would be far better off to purchase a commercially built unit.
     
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  3. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    All it has to power is incandescent lamps :)
    (overhead projectors and some Christmas lights)

    also, I'm working on a project with a budget, so this works better for me as it's substantially cheaper
     
    Last edited: Mar 4, 2012
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    That circuit is maybe good for 50W. If you want 1100W derived from a 12V source, the average current drawn from the 12V line is ballpark 120 Amps. Designing a switching converter that can crank that is no job for a novice.
     
  5. debe

    Well-Known Member

    Sep 21, 2010
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    Its far cheaper to buy one of Ebay.
     
  6. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    soo... I guess they dont make 120 amp transistors then...
    :p
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    They do make high-power transistors. However, that circuit is not suitable to drive them.

    I don't have the energy nor inclination to start on a 1kw inverter project. It would take a LOT of my time, and wind up being far more expensive than an off-the-shelf solution.

    Already having a MOT doesn't mean a lot; it would need to be re-wound for the new application, and the wire alone would be rather pricey - odds are that the first attempt at rewinding it won't produce the desired output.
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    You have to run FETs in parallel.
     
  9. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    which would require different and more complex control circuitry because of the "field effect" rather than current gain?
    making this overly complicated and expensive?
     
  10. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    They operate as saturated switches in parallel, and the driver cranks them either fully ON or OFF. Believe me, it works I have seen dozens of designs using parallel FETs for switches. No problem
     
  11. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    So what part makes it "not a job for a novice?"

    I'm not disagreeing that it's a better idea to buy one, I'm just curious what makes it so difficult to build
     
  12. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    The best answer to that question would be found by trying to do it.
     
  13. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    All of the high power inverters I have use 4-6 separate transformers to drive the output, each transformer is driven by several power transistors.

    Even the "pro" high efficiency aren't that efficient, hitting 300A running a 1kW load (Sawzall cutting up a car). I ran 000 wire to the board to jumper cable clamps on the outside of the one I loan out the most, inside they only use about 2 gauge, which gets too hot.

    They also have several overload protection options, a waterfall display for 12V current usage, output voltage, and output current, undervoltage cutoff (input or output), and the output is even a GFCI connector that works with a GFCI outlet tester, which is nifty.
     
  14. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    how much do those cost?
    I'm not the only one asking, my dads been thinking about putting in a battery system for the house (obviously not the whole house)
     
  15. harry99932

    Member

    Dec 30, 2010
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    I never like to put anyone off of a project as ive undertaken a few silly ones in my time but take it from someone who recently built an ac/dc tig welder it is 100% not worth the time or effort it takes vs the cost. my welder was done as i needed a few odd features but a square wave inverter is a square wave inverter and i dont know about were you are but you can buy cheap and cheerfull chinese that will be a lot more reliable for a hell of a lot less than you can build one for! Either way good luck:D
     
  16. Audioguru

    New Member

    Dec 20, 2007
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    The horrible circuit you found has two very long threads on its website (Aaron Cake.net) complaining about its poor performance:
    1) Its capacitors blow up because the transistors have "avalanche breakdown" of their emitter-base junctions when transformer action drives the base far below ground. The maximum allowed reverse voltage is only 7V. Then the emitter-base junction conducts like a zener diode and has a very high current in it and in the capacitor. A diode in series with each emitter to ground will prevent the avalanche breakdown but will cause a 10% voltage loss. The transistors were originally shown with their polarity backwards but they still blow up when they have correct polarity.
    2) The 180 ohm base bias resistors have an average current of 67mA. Then the saturated output current of each transistor is only 670mA. Then the circuit power is only 12V x 670mA= 8W. If the transistors have super high gain then the output power is 16W but most of it creates avalanche breakdown and blows up the capacitors.
     
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  17. Wendy

    Moderator

    Mar 24, 2008
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  18. hwy101

    Active Member

    May 23, 2009
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    Audioguru is right! that horrible circuit is based on a vintage Radio Shack inverter they sold back in the early 80's
    I have one, the transformer is huge and heavy, even at the rated 300w it would barely operate a drill but it lit up some 60w bulbs just fine.
    I haven't hooked it up in over 20 years, It just sits on my shelf as a museum piece.
     
    Last edited: Mar 9, 2012
  19. Audioguru

    New Member

    Dec 20, 2007
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    Hi Hwy101,
    I looked on Google Earth to see where you live.
    In BC, East of Prince George and close to a meandering river, EH?:D
     
  20. hwy101

    Active Member

    May 23, 2009
    91
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    pretty close, south of Prince George actually :)
     
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