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

  • Thread starter Rick Karlquist N6RK
  • Start date

Rick Karlquist N6RK

Jan 1, 1970
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

Eric Sears

Jan 1, 1970
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

Rick Karlquist N6RK

Jan 1, 1970
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

Eric Sears

Jan 1, 1970
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.