Cheap 12V to 220V Inverter

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Even though today’s
electrical appliances are increasingly often self-powered, especially
the portable ones you carry around when camping or holidaying in summer,
you do still sometimes need a source of 230 V AC – and while we’re
about it, why not at a frequency close to that of the mains? As long as
the power required from such a source remains relatively low – here
we’ve chosen 30 VA – it’s very easy to build an inverter with simple,
cheap components that many electronics hobbyists may even already have.

Though it is possible to build a more powerful circuit, the
complexity caused by the very heavy currents to be handled on the
low-voltage side leads to circuits that would be out of place in this
summer issue. Let’s not forget, for example, that just to get a meager 1
amp at 230 VAC, the battery primary side would have to handle more than 20 ADC!.
The circuit diagram of our project is easy to follow. A classic 555
timer chip, identified as IC1, is configured as an astable multivibrator
at a frequency close to 100 Hz, which can be adjusted accurately by
means of potentiometer P1.

Circuit Diagram

Cheap 12V to 220V Inverter Circuit

Cheap 12V to 220V Inverter Circuit Diagram

As the mark/space ratio (duty factor) of the 555 output is a long
way from being 1:1 (50%), it is used to drive a D-type flip-flop
produced using a CMOS type 4013 IC. This
produces perfect complementary square-wave signals (i.e. in antiphase)
on its Q and Q outputs suitable for driving the output power
transistors. As the output current available from the CMOS
4013 is very small, Darlington power transistors are used to arrive at
the necessary output current. We have chosen MJ3001s from the now
defunct Motorola (only as a semi-conductor manufacturer, of course!)
which are cheap and readily available, but any equivalent power
Darlington could be used.

These drive a 230 V to 2 × 9 V center-tapped transformer used ‘backwards’ to produce the 230 V output. The presence of the 230 VAC voltage is indicated by a neon light, while a VDR
(voltage dependent resistor) type S10K250 or S07K250 clips off the
spikes and surges that may appear at the transistor switching points.
The output signal this circuit produces is approximately a square wave;
only approximately, since it is somewhat distorted by passing through
the transformer. Fortunately, it is suitable for the majority of
electrical devices it is capable of supplying, whether they be light
bulbs, small motors, or power supplies for electronic devices.

PCB Layout

PCB Layout For Cheap 12V to 220V Inverter Circuit

PCB Layout For Cheap 12V to 220V Inverter Circuit Diagram

Component List

Resistors

  • R1 = 18k?
  • R2 = 3k3
  • R3 = 1k
  • R4,R5 = 1k?5
  • R6 = VDR S10K250 (or S07K250)
  • P1 = 100 k potentiometer

Capacitors

  • C1 = 330nF
  • C2 = 1000 µF 25V

Semiconductor

  • T1,T2 = MJ3001
  • IC1 = 555
  • IC2 = 4013

Miscellaneous

  • LA1 = neon light 230 V
  • F1 = fuse, 5A
  • TR1 = mains transformer, 2×9V 40VA (see text)
  • 4 solder pins

Note that, even though the circuit is intended and designed for
powering by a car battery, i.e. from 12 V, the transformer is specified
with a 9 V primary. But at full power you need to allow for a voltage
drop of around 3 V between the collector and emitter of the power
transistors. This relatively high saturation voltage is in fact a
‘shortcoming’ common to all devices in Darlington configuration, which
actually consists of two transistors in one case. We’re suggesting a PCB design to make it easy to construct this project; as the component overlay shows, the PCB only carries the low-power, low-voltage components.

The Darlington transistors should be fitted onto a finned anodized
aluminum heat-sink using the standard insulating accessories of mica
washers and shouldered washers, as their collectors are connected to the
metal cans and would otherwise be short-circuited. An output power of
30 VA implies a current consumption of the order of 3 A from the 12 V
battery at the ‘primary side’. So the wires connecting the collectors of
the MJ3001s [1] T1 and T2 to the transformer primary, the emitters of
T1 and T2 to the battery negative terminal, and the battery positive
terminal to the transformer primary will need to have a minimum
cross-sectional area of 2 mm2 so as to minimize voltage drop.

The transformer can be any 230 V to 2 × 9 V type, with an E/I iron
core or toroidal, rated at around 40 VA. Properly constructed on the
board shown here, the circuit should work at once, the only adjustment
being to set the output to a frequency of 50 Hz with P1. You should keep
in minds that the frequency stability of the 555 is fairly poor by
today’s standards, so you shouldn’t rely on it to drive your radio-alarm
correctly – but is such a device very useful or indeed desirable to
have on holiday anyway? Watch out too for the fact that the output
voltage of this inverter is just as dangerous as the mains from your
domestic power sockets.

So you need to apply just the same safety rules! Also, the project should be enclosed in a sturdy ABS
or diecast so no parts can be touched while in operation. The circuit
should not be too difficult to adapt to other mains voltages or
frequencies, for example 110 V, 115 V or 127 V, 60 Hz. The AC voltage
requires a transformer with a different primary voltage (which here
becomes the secondary), and the frequency, some adjusting of P1 and
possibly minor changes to the values of timing components R1 and C1 on
the 555.