PWM Signal Generators - PWM Signal Generators V3.02...

Info iconThis preview shows pages 1–4. Sign up to view the full content.

View Full Document Right Arrow Icon
PWM Signal Generators V3.02 27-Jul-04 Testing status: Mixed. Some untested, some SPICE simulated. 1. Introduction PWM, or Pulse Width Modulation, is a method of controlling the amount of power to a load without having to dissipate any power in the load driver. Imagine a 10W light bulb load supplied from a battery. In this case the battery supplies 10W of power, and the light bulb converts this 10W into light and heat. No power is lost anywhere else in the circuit. If we wanted to dim the light bulb, so it only absorbed 5W of power, we could place a resistor in series which absorbed 5W, then the light bulb could absorb the other 5W. This would work, but the power dissipated in the resistor not only makes it get very hot, but is wasted. The battery is still supplying 10W. An alternative way is to switch the light bulb on and off very quickly so that it is only on for half of the time. Then the average power taken by the light bulb is still only 5W, and the average power supplied by the battery is only supplying 5W also. If we wanted the bulb to take 6W, we could leave the switch on for a little longer than the time it was off, then a little more average power will be delivered to the bulb. This on-off switching is called PWM. The amount of power delivered to the load is proportional to the percentage of time that the load is switched on. In the chapter on speed controllers on this site, there is an explanation why PWM signals are used to drive speed controllers. It is the same reason as for the light bulb example above. 2. The methods The PWM signals can be generated in a number of ways. There are several methods: 1. Analogue method 2. Digital method 3. Discrete IC 4. Onboard microcontroller These will all be described. 2.1. Analogue method A block diagram of an analogue PWM generator is shown below:
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
We will now go through each of these stages and work out how to implement them. 2.1.1. The comparator We are starting at the output because this is the easy bit. The diagram below shows how comparing a ramping waveform with a DC level produces the PWM waveform that we require. The higher the DC level is, the wider the PWM pulses are. The DC level is the 'demand signal'. The DC signal can range between the minimum and maximum voltages of the triangle wave. When the triangle waveform voltage is greater than the DC level, the output of the op- amp swings high, and when it is lower, the output swings low. 2.1.2. Detecting the demand signal We need to convert the signal coming from the radio control receiver into a PWM demand signal. This can be achieved using a servo, or by using a circuit which decodes the signal from the receiver. 2.1.2.1. Using a servo In this method, we want a PWM generator that will take a signal from a servo potentiometer (these signals will need to be taken out by wires from the servo body), and deliver a logic-level PWM output to the speed controller. When the servo potentiometer is at minimum, we want the PWM signal to be 100% off 0% on, and when the servo potentiometer is at maximum, we want the PWM signal to be 0% off
Background image of page 2
100% on. We also want the on percentage to be proportional to the potentiometer
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 4
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 14

PWM Signal Generators - PWM Signal Generators V3.02...

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online