PWM Signal Generators

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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 vwanted 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:
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 100% on. We also want the on percentage to be proportional to the potentiometer position.
The potentiometer generally has its 'top end' connected to a positive power supply, and its 'bottom end' connected to ground. Then as it rotates the voltage at its wiper changes linearly with wiper position.
2.1.2.2. Using the RxDetector circuit
This is fully described in the RxDetector page.
2.1.3. Generating the triangle wave
There are a few ways of doing this:
2.1.3.1. Weighted resistor ladder on a counter
An example circuit for this is shown below. This uses a counter and weighted resistor ladder to generate the triangle wave (in fact it will generate a sawtooth, but you'll still get a PWM signal at the end of it). The actual resistor values which are unavailable (40k, 80k) can be made up with 20k resistors, or close approximations can be used, which may distort the sawtooth somewhat, but this shouldn't matter too much.
Click on the circuit diagram to open it in a new window.
The 74HC14 is a Schmitt input inverter, which is connected to act as a simple oscillator. The frequency of oscillation is roughly
but it doesn’t matter a great deal within a few tens of percent. This square wave generated feeds the 74HC163 binary 4-bit counter. All the preset and clear inputs of this are disabled, so the outputs, QA to QD just roll around the binary sequence 0000 to 1111 and rollover to 0000 again. These outputs, which swing from 0v to +5v are fed into a binary weighted summer amplifier, the leftmost LM324 op amp section with the 80k, 40k, 20k and 10k resistors. The output voltage of this amplifier depends on the counter count value and is shown in the table below as Amp1 output. The op amp following this just multiplies the voltage by -½, to make the voltage positive, and bring it back within logic voltage levels, see the Amp2 output column in the table.
Counter value Binary value Amp1 output (Volts) Amp2 output (Volts)
0 0000 0 0
1 0001 -0.625 0.3125
2 0010 -1.25 0.625
3 0011 -1.875 0.9375
4 0100 -2.5 1.25
5 0101 -3.125 1.5625
6 0110 -3.75 1.875
7 0111 -4.375 2.1875
8 1000 -5 2.5
9 1001 -5.625 2.8125
10 1010 -6.25 3.125
11 1011 -6.875 3.4375
12 1100 -7.5 3.75
13 1101 -8.125 4.0625
14 1110 -8.75 4.375
15 1111 -9.375 4.6875
The results of two SPICE simulations are shown below. The first is with the DC threshold level set at +1V, and the second with it set at +3V. The blue line is the threshold level, the green line is at the +ve input of the rightmost comparator, and the red waveform is the output. The difference in PWM ratio can be clearly seen.