Randy Pangilinan Esmenda
A journey of a thousand miles begins with a single step
In places where electricity is not available, a portable electric generator and a battery are alternative power sources.
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Project Description
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Fig 1B. Functional Diagram of the Power Inverter project.
The astable oscillator is configured around a 555 timer IC. Its oscillating frequency is set to 120Hz by R1, R2, R3 and C1. the resulting output is a train square waves having a peak to peak amplitude nearly equal to the12 – Volts supply voltage. The way this oscillator works can be easily understood through Fig. 2.
Figure 2. Internal block diagram of the 555 timer IC.
Immediately upon application of the 12-volt supply voltage, the
voltage across capacitor C1 is nearly at the ground level. This voltage
sensed by the “trigger” comparator (pin2) of the timer, setting the
internal flip flop and hence, causing the output voltage at (pin3) to go
high or approximate the supply voltage. Consequently , it turns off the
“discharge” transistor and allowing capacitor C1 to charge through
resistors R1, R2 and R3. The capacitor charges towards the supply
voltage but upon reaching 2/3 of the supply voltage, the threshold
comparator (pin6) is activated. This causes the ineternal flip flop to
reset and the output voltage to go back to ground level. The discharge
transistor then conducts, which in turn, causes capacitor C1 to
discharge through R2 and R3 until the voltage across it drops to 1/3 of
the supply voltage, a level at which the “trigger” comparator switches
the timer output to a high state (or near the supply voltage level)
again, then the cycle is repeated.
The output of the oscillator is fed to the clock input of the 74C74
D-type flip flop. This IC has a dual function. First it divides equally
into two the timer frequency to obtain the nominal 60Hz output, and
secondly, it simultaneously provides complementary outputs (at its pins5
& 6) required by the succeeding switching circuit.
The part of the switching circuit that receives the flip flop outputs
are composed of buffer transistors Q1 and Q2. Resistors R4 and R5 limit
the current of the base of these transistors to a safe value. Resistors
R6 and R7 serve as pull up resistors that set the collector bias of
these transistors.
The output of the buffer transistors are in turn
connected to the push pull transistors consisting of the Q3-Q4 and Q5-Q6
pairs. The PNP transistors Q3 and Q5 act as driver transistors to Q4 and
Q6, respectively. They supply the current requirements of the main power
transistors so that the full saturation can be attained. The currents
supplied by these drivers can go up to five amperes depending on the
load connected the output of the inverter.
Transistors Q4 and Q6, wired in a push pull mode,
alternately supply current through each half of the primary winding of
the transformer. Alternate conduction here means when one transistor is
fully saturated, the other one is at cut off, and vice versa. This
alternate switching of the two transistors creates a magnetic field on
the transformer core resulting to a transfer of energy from the
secondary winding of the primary winding, a process termed as magnetic
induction. The power available at the secondary of the transformer has
been translated already to 220 volts alternating current in the form of
square waves. The primary to secondary winding turns ratio causes the
12-volt primary voltage to be stepped up to 220 Volts in the secondary
winding.
The power inverter although composed of only a few components handles large amounts of current thus necessitating extra precautionary measures:
Use only AWG#10 insulated wire in wiring connections indicated by heavy lines in Figure 1A. Use similar wire size for connections to the battery.
Transistors Q4 and Q6 must be mounted on a heatsink sufficient enough to dissipate the expected heat that may be generate. A fin type heatsink is specifically recommended for this purpose.
Transistors Q4 and Q6 must be both electrically isolated from the heatsink
The rest of procedures are mainly a matter of following good construction techniques as in doing other electronic projects:
1. Secure the ready made PCB (See Figure 3 and 4). Carefully check the foil path.
2. Insert and solder the components intended for the PCB. Take note of the lead polarity or lead orientations of the electrolytic capacitors and the transistors.
Capacitors C4 is mounted at the foil side of the PCB at the location indicated in figure 4. Resistors R10 to R13 are not mounted in the PCB, refer to Figure 6.
3. Mount the power transistors on the heatsink. If the heatsink is to be grounded be sure to put mica insulators between the heatsink and the power transistors.
4. Plan the over all work to be done to complete the project with emphasis on the layout in the enclosure of the PCB, transistors on the heatsink, and the transformer.
Note: do the wiring connections as illustrated in the wiring guide after the TESTING procedures.
Figure 3. Foil pattern layout of
Figure 4. parts placement guide of
the Power Inverter.
the Power
Inverter.
*C4 is soldered at the foil side of the PCB at the location indicated.
Power Inverter
Parts List
Semiconductor :
IC1 - 555, Timer IC
IC2 - 74C74, CMOS DUAL D Flip Flop
Q1,Q2 - 9418, NPN Transistors
Q3,Q5 - NA52W, PNP 40W 45V 4A Transistors
Q4,Q6 - MJ15015, NPN 180W 120V 15A Transistors
Resistors : (All are 1/4 watt +-5% tolerance Carbon composition type, unless otherwise noted:)
R1 - 10 Kilohms
R2,R3 - 27 Kilohms
R4,R5 - 3.3 Kilohms
R6,R7 - 1 Kilohm
R8,R9 - 47 Ohms, 1 watt
R10,R11
- 330 Ohms
R12,R13 - 15 Ohms, 1/2 watt
Capacitors:
C1,C2
- 0.1uF/50V mylar
C3
- 220uF/25V electrolytic
C4
- 4.7uF/16V electrolytic
Miscellaneous:
SI-SPST 15 Amp Switch
Transformer - 24V CT Primary/220VSecondary, 150-watt
IC Socket, ICU - 8 - 1pc.
IC Socket, ICU - 14 - 1pc
Heatsink(fin type) - 9inches in length
Mica insulators - 2 pcs
Binding Posts - 1 pair
Car Battery Alligator Clips(red & black) - 1 pair.
Banana Plugs (red & black) - 1 pair.
AC Outlet, (chassis-mounted) - 1 pair.
PCB, Eyelets, Nuts and Bolts
AWG#10 and AWG#22 Insulated Hook-Up Wires, etc.
The main function of the power inverter is to make available AC power in instances when and where electricity is not available. This inverter can supply power to any non critical appliance provided that its power rating is within the 120-Watt level. However, since the output of this inverter is a square wave and not sinusoidal as the supplied house current, the inverter could not be used for TV and similar electronic appliances. (Note: We have actually tested this inverter using a TV load and encountered no problem at all. But this is not a guarantee, however, that the same results will be obtained with other TV brands. Some may performed as well or better but others may be damaged when powered by a square wave AC source.)
Some possible applications of the power inverter are the following:
1. Lamps (fluorescent or incandescent)
2. Electric fan
3. Low wattage heating applianced (120 watts maximum)
Item Quantity Price(P)
AWG#22
4m
10
P. Inverter Kit
1 kit
1282.50
Battery Clip
1pair
25
Footings
4 pcs.
8
Screws 1/8 x 1/2 20 pcs.
15
Heatsink 12"
12"
155
TW#10
2 m 73
Housing
450
Paint
25
Transportation
2 times
250
TOTAL COST 2293.50
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