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KRAIG 106

1. Pulse Width Modulation2. IC 555

TOPICS

To understand the need of PWM, consider the case when a motor’s speed needs to be varied.We have two options:1.Vary the input voltage to the motor – However, this also reduces the motor’s torque drastically2.Supply pulses of digital signals which average out to give the effect of an analog signal.

PWM MOTIVATION

• PWM or Pulse Width Modulation refers to the concept of using a rapidly pulsating digital signal to vary the average power delivered to an electrical device.

• Common application include controlling the brightness of a LED, the speed of a motor, etc.

• Power loss is very low for PWM, since when the switch is off, there is practically no current and when the switch is on, there is practically no voltage drop across the switch.

PULSE WIDTH MODULATION

1. Frequency – The number of times one complete on-off cycle takes place in a second

2. Duty Cycle – is the ratio of the width of a pulse to the width of a cycle. It is expressed as a percentage and the average power delivered is proportional to the duty cycle.

Varying the width of the pulse varies the duty cycle and hence the output

PWM PARAMETERS

• If the frequency of the PWM signal is too low, then the continuous ON-OFF will be visible as jerks.

• However for high enough frequencies, the observed effect is that of a time averaged signal

• The working frequency for a motor drive is in the range of kHz (thousands of times per second)

FREQUENCY

• Many digital systems are powered by a 5-Volt power supply, so if you filter a signal that has a 50% duty cycle you get an average voltage of 2.5 Volts.

• Other duty cycles produce any voltage (in discrete steps depending on the resolution) in the range of 0 to 100% of the ‘high’ voltage.

DUTY CYCLE

 If we closed the switch connecting the battery and lamp for 50ms, the bulb would receive 9V during that interval. If we then opened the switch for the next 50ms, the bulb would

receive 0V. If we repeat this cycle 10 times a second, the bulb will be lit as though it were connected to a 4.5V battery (50% of 9V). We say that the duty cycle is 50% and the modulating

frequency is 10Hz.

AN EXAMPLE

• Most robots use a microcontroller to enable PWM. But alternatively, a much simpler IC called the 555 Timer can be used to generate PWM

• This is a very widely used 8 pin IC, whose usage is described in the following slides.

HOW TO SUPPLY PWM?

• The schematic pin diagram of the 555 IC is as follows:

• All details on the working of the IC are given in its datasheet (available online). We will walk you through the essentials here

555 TIMER IC

The 555 timer has different modes of operation, the most useful of which is the astable mode of operation in which the output of the IC is pulsating.

ASTABLE MODE OF OPERATION

CIRCUIT FOR 555 IC IN ASTABLE

MODE

The PWM parameters (frequency and duty cycle) depend on the values of R1, R2 and C.

PWM PARAMETERS

FOR 555 IC

LOW DUTY CYCLES