Inverters are often needed at places where it is not possible to get AC
supply from the Mains. An inverter circuit is used to convert the DC
power to AC power. For high power applications, thyristors are often
used. However, for low and medium power requirements, power transistors
can be used. Here, a simple voltage driven inverter circuit using power
transistors as switching devices is build, which converts 12V DC signal
to single phase 220V AC.
Principle Behind this Circuit:
The basic idea behind this circuit
involves the working of a voltage driven inverter circuit. An oscillator
is used to drive the semiconductor switches alternatively. Each of the
switches allows the DC source voltage to appear across the primary of
the transformer in alternate intervals, causing an alternating voltage
to be developed across the primary. This primary voltage is then
stepped up to a higher voltage depending upon the number of turns in
primary and secondary coils.
Also get an idea about 12V to 24V DC Converter Circuit
Theory Behind 12v DC to 220v AC Converter Circuit:
Inverter circuits can either use
thyristors as switching devices or transistors. Normally for low and
medium power applications, power transistors are used. The reason for
using power transistor is they have very low output impedance, allowing
maximum current to flow at the output.
One of the important applications of a
transistor is in switching. For this application, the transistor is
biased in saturation and cutoff region. When the transistor is biased in
saturation region, both the collector emitter and collector base
junctions are forward biased. Here the collector emitter voltage is
minimum and collector current is maximum.
Another important aspect of this circuit
is the oscillator. An important use of 555 Timer IC is in its use as an
astable multivibrator. An astable multivibrator produces an output
signal which switches between the two states and hence can be used as an
oscillator. The frequency of oscillation is determined by the values of
capacitor and resistors.
12v DC to 220v AC Converter Circuit Diagram:
Circuit Components:- V1 = 12V
- R1 = 10K
- R2 = 150K
- R3 = 10Ohms
- R4 = 10Ohms
- Q1 = TIP41
- Q2 = TIP42
- D1 = D2 = 1N4007
- C3 = 2200uF
- T1 = 12V/220V step up transformer
Circuit Design Explanation:
Oscillator Design: An astable
multivibrator can be used as an oscillator. Here an astable
multivibrator using 555 timer is designed. We know, frequency of
oscillations for a 555 timer in astable mode is given by:
f = 1.44/(R1+2*R2)*C
where R1 is the resistance between
discharge pin and Vcc, R2 is the resistance between discharge pin and
threshold pin and C is capacitance between threshold pin and ground.
Also the duty cycle of the output signal is given by:
D = (R1+R2)/(R1+2*R2)
Since our requirement is f =50Hz and D =
50% and assuming C to be 0.1uF, we can calculate the values of R1 and
R2 to be 10K and 140K Ohms respectively. Here we prefer using a 150K
potentiometer to fine tune the output signal.
Also a ceramic capacitor of 0.01uF is used between the control pin and ground.
Switching Circuit Design: Our
main aim is to develop an AC signal of 220V. This requires use of high
power transistors to allow the flow of maximum amount of current to the
load. For this reason we use a power transistor TIP41 with a maximum
collector current of 6A, where the base current is given by the
collector current divided by the DC current gain. This gives a bias
current of about 0.4A *10, i.e.4A. However since this current is more
than the maximum base current of the transistor, we prefer a value less
than the maximum base current. Let us assume the bias current to be 1A.
The bias resistor is then given by
Rb = (Vcc – VBE(ON))/Ibias
For each transistor, the VBE(ON) is about 2V. Thus Rb
for each is calculated to be 10 Ohms. Since the diodes are used for
biasing, the forward voltage drop across the diodes should be equal to
the forward voltage drops across the transistors. For this reason,
diodes 1N4007 are used.
The design considerations for both the PNP and NPN transistors are same. We are using a PNP power transistor TIP42.
Output Load Design: Since the
output from the switching circuit is a pulse width modulated output, it
might contain harmonic frequencies other than the fundamental AC
frequency. For this reason, an electrolyte capacitor needs to be used to
allow only the fundamental frequency to pass through it. Here we use an
electrolyte capacitor of 2200uF, large enough to filter out the
harmonics. Since it is required to get 220V output, it is preferred to
use a step up transformer. Here a 12V/220V step up transformer is used.
12v DC to 220v AC Converter Circuit Operation:
When this device is powered using the
12V battery, the 555 timer connected in astable mode produces square
wave signal of 50Hz frequency. When the output is at logic high level,
diode D2 will conduct and the current will pass through diode D1, R3 to
the base of transistor Q1. Thus transistor Q1 will be switched on. When
the output is at logic low level, diode D1 will conduct and current
will flow via and D1 and R4 to the base of Q2, causing it to be switched
on. This allows the DC voltage to be produced across the primary of the
transformer at alternate intervals. The capacitor ensures that the
frequency of the signal is at the required fundamental frequency. This
12V AC signal across the primary of the transformer is then stepped up
to 220V AC signal across the transformer secondary.
Applications of 12v DC to 220v AC Converter Circuit:
- This circuit can be used in cars and other vehicles to charge small batteries.
- This circuit can be used to drive low power AC motors
- It can be used in solar power system.
Limitations:
- Since 555 Timer is used, the output may slightly vary around the required duty cycle of 50%, i.e. exact 50% duty cycle signal is hard to achieve.
- Use of transistors reduces the efficiency of the circuit.
- Use of switching transistors has the possibility of causing cross over distortion in the output signal. However this limitation has been reduced to some extent by the use of biasing diodes.
No comments:
Post a Comment
some simple projects for Electrical&Electronics students