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Want schematic for 12VDC to 120VAC inverter with high frequency switching

Discussion in 'General Electronics' started by Rick Karlquist N6RK, Sep 9, 2005.

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  1. 12VDC to 120VAC inverters using high frequency switching (no heavy
    iron transformers) are now ubiquitous at ridiculously low prices. I would
    like to modify one of these to make a 12VDC to 55VDC DC-DC
    converter. I suspect they all have similar designs and am seeking a
    schematic of a representative one to reverse engineer. I've tried searching
    on the web, but the only schematics I've seen are for the 60Hz switching
    types using big iron.

    Rick Karlquist N6RK
  2. Eric Sears

    Eric Sears Guest

    Hi Rick

    Just as a comment - I have built the old type of square wave inverter
    (approx 50 - 60 cycles) which use just a few components and an iron
    transformer. Now in one sense this is the basis of what you want -
    except that intead of using an iron core you use a ferrite core at say
    10 kilocycles.
    Most of the cheap modified square wave inverters first generate the
    120 volts (actually more like the peak of a sine wave at 170volts) at
    say 10kc/s, which is then rectified to DC.
    This is then chopped by a bridge circuit to AC, using a circuit that
    turns on the output for less than a full half-cycle.

    The problem of reverse engineering is that the ferrite transformers in
    the first stage are wound for approx 150 volt out - and as a sealed
    transformer it would be hard to alter them.
    (Note - I am approximating voltages - we use 230 volts here in NZ)

    You don't say how much power you want to handle, but if you could find
    a suitable core (eg an old line output ferrite core from a TV set),
    you might be able to wind a suitable transformer. This would then be
    driven at say 10kc/s, using some high power fet's - potentially these
    might be from a cheap inverter (there are many circuits to do this on
    the internet).
    Then you would rectify and smooth the output for your 50+ volts DC.
    The actual frequency of operation would be relatively unimportant,
    provided it was fast enough for the core.

    There IS a way to do what you want using a high-power light dimmer
    (yes, even with a square wave) - just straight out of the 120 volt
    inverter and then rectified - but you might have to be careful if you
    are charging a 50 volt battery (which I suspect is what you want to
    do) as the peaks of the current may be too large for the dimmer (been
    there - done that - one burnt out 1kw dimmer later!).

    Give a few more details of what you want to DO, and how you are
    sourcing the 12v (from a battery - of perhaps to step up the output of
    a solar panel?) - then I might have more suggestions.

    Eric Sears ZL2BMI
  3. I bought a 400W unit on sale yesterday for $10 and was able
    to reverse engineer it without a schematic. The control IC was
    marked TL494. I found the voltage divider resistors that go
    to the feedback pin (#1) and altered them so the IC would
    throttle back the DC output voltage from 140V to 55V. I
    found the full wave bridge rectifier on the output of the transformer
    so I could tap into the 140VDC (now 55VDC) ahead of the H-bridge that
    converts it to so-called AC. It worked perfectly.

    BTW, this cheap inverter is non-isolated, but I don't need
    isolation for my application. I could probably convert it
    to an isolated design if I added an isolated feedback circuit.
    The transformer itself is certainly isolated.

    I am going to use it to charge a 48VDC battery bank in my
    car from the car's 12V electrical system. The battery bank
    runs my 1500W solid state linear.

    Rick N6RK
  4. Eric Sears

    Eric Sears Guest

    Hi Rick

    Good stuff and thanks for telling us how its done - I have had
    occasions when I wanted to do the same for microhydo and other
    alternative energy projects
    Unfortunately 400W units are a lot dearer here in NZ - though prices
    are dropping.

    The TL494 (probably being used as a comparator) is probably now just
    giving a much smaller pulse on each half cycle. I must have a look at
    a couple of inverters here. I have adjusted the voltage on one of them
    but it didn't have much range. Clearly altering the divider resistors
    would change that.


    Eric ZL2BMI
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