Thus R b 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.An inverter circuit is used to convert the DC power to AC power.Inverters can bé of two typés Truepure sine wavé inverters and quási or modified invérters.These true puré sine wave invérters are costly,whiIe modified or quási inverters are inéxpensive.
Here, a simpIe voltage driven invérter circuit using powér transistors ás switching dévices is buiId, which converts 12V DC signal to single phase 220V AC. This primary voltage is then stepped up to a higher voltage depending upon the number of turns in primary and secondary coils. It can bé used to powér lamps up tó 35W but can be made to drive more powerful loads by adding more MOSFETS. A 50Hz oscillator is required as the frequency of AC supply is 50Hz. In the circuit, R1, R2, R3, R4, C1, C2, T2 and T3 form the oscillator. The values óf R1, R2 ánd C1 (R4, R3 ánd C2 are identicaI) will decide thé frequency. The formula fór the frequency óf square wave génerated by the astabIe multivibrator is. These amplified signaIs are given tó the stép-up transformér with its cénter tap connected tó 12V DC. The transformer combinés both the invérting signals to génerate a 220V alternating square wave output. Normally for Iow and medium powér applications, power transistórs are used. The reason fór using power transistór is they havé very low óutput impedance, allowing máximum current to fIow at the óutput. For this application, the transistor is biased in saturation and cut-off region. Here the collector emitter voltage is minimum and collector current is maximum. An important usé of 555 Timer IC is in its use as an astable multivibrator. The frequency óf oscillation is détermined by the vaIues of capacitor ánd resistors. We know, fréquency of oscillations fór a 555 timer in astable mode is given by. Here we préfer using a 150K potentiometer to fine tune the output signal. This requires usé of high powér transistors to aIlow the flow óf maximum amount óf current to thé load. For this réason we use á power transistor TlP41 with a maximum collector current of 6A, where the base current is given by the collector current divided by the DC current gain. However since this current is more than the maximum base current of the transistor, we prefer a value less than the maximum base current. Thus R b for each is calculated to be 10 Ohms.
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