Description
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:
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.
SP5168
AP2161FMG-7 PCA-121P2W 956-120-5000 19458 PIC16C558-20/SS
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:
COMPONENTS LIST
- R1 = 18k?
- R2 = 3k3
- R3 = 1k
- R4,R5 = 1k?5
- R6 = VDR S10K250 (or S07K250)
- P1 = 100 k potentiometer
- C1 = 330nF
- C2 = 1000 μF 25V
- T1,T2 = MJ3001
- IC1 = 555
- IC2 = 4013
- LA1 = neon light 230 V
- F1 = fuse, 5A
- TR1 = mains transformer, 2x9V 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.
Reprinted
Url Of This Article:
http://www.hqew.net/circuit-diagram/How-to-build-Cheap-12V-to-220V-Inverter_5560.html
