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

  • Thread starter Rick Karlquist N6RK
  • Start date
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Rick Karlquist N6RK

Jan 1, 1970
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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
 
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Eric Sears

Jan 1, 1970
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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
 
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Rick Karlquist N6RK

Jan 1, 1970
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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
 
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Eric Sears

Jan 1, 1970
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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.

Cheers

Eric ZL2BMI
 
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