07-08-2012, 04:34 PM
Pulse width modulation
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Pulse width modulation
Triggering the 555 timer in monostable mode with a continuous sequence of pulses allows
the output pulse width to be modulated by changing the amplitude of a signal applied to the
control input pin 5 (CON). An example pulse width modulator circuit is given in Fig. 13.
In this circuit components C2, R6 and D1 convert the 555 trigger signal into a falling edge
triggering signal. This can be seen in Fig. 14 which illustrates the trigger, discharge and
resulting output waveform. The 555 timer control pin is driven from a voltage pulse source.
The specification of the control waveform has been chosen to generate a triangular shaped
signal so that the modulation of the pulse width can be clearly seen as the control signal
Pulse-width modulation (PWM), or pulse-duration modulation (PDM),
is a commonly used technique for controlling power to inertial electrical devices, made practical by modern electronic power switches.
The average value of voltage (and current) fed to the load is controlled by turning the switch between supply and load on and off at a fast pace. The longer the switch is on compared to the off periods, the higher the power supplied to the load is.
The PWM switching frequency has to be much faster than what would affect the load, which is to say the device that uses the power. Typically switchings have to be done several times a minute in an electric stove, 120 Hz in a lamp dimmer, from few kilohertz (kHz) to tens of kHz for a motor drive and well into the tens or hundreds of kHz in audio amplifiers and computer power supplies.
The term duty cycle describes the proportion of 'on' time to the regular interval or 'period' of time; a low duty cycle corresponds to low power, because the power is off for most of the time. Duty cycle is expressed in percent, 100% being fully on.
The main advantage of PWM is that power loss in the switching devices is very low. When a switch is off there is practically no current, and when it is on, there is almost no voltage drop across the switch. Power loss, being the product of voltage and current, is thus in both cases close to zero. PWM also works well with digital controls, which, because of their on/off nature, can easily set the needed duty cycle.
DEMODULATION:-
Demodulation is the process of extracting the original information from a modulated carrier wave. A demodulator is an electronic circuit used to recover the information content from the modulated carrier wave. The process of integration is employed in to demodulate a P.W. signal for recover the modulating signal.
In this circuit we are using an op-amp integrator built around IC 741. In this current is summed over period of time and the resultant voltage generated is the integral of that current as a function of time. In this process we are measuring the area under each pulse, and for this we are using integration method. And to integrate the wave we use I.C. 741 as an integrator. The 100k resistor across 0.1uf capacitor is added to provide DC stability.
However , the output waveform has some error as compared to the original modulating waveform.