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12v DC to 220v AC Converter Circuit Using Astable Multivibrator


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 cut-off 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.

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

  1. This circuit can be used in cars and other vehicles to charge small batteries.

  2. This circuit can be used to drive low power AC motors

  3. It can be used in solar power system.

Limitations

  1. 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.

  2. Use of transistors reduces the efficiency of the circuit.

  3. 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.

Note

Instead of 555 timer one can use any astable multivibrator. For example this circuits can also be build using 4047 astable multivibrator,whose output current is amplified and applied to the transformer.

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