Pulse-width modulation of a signal or power source involves the modulation of its duty cycle to either convey information over a communications channel or control the amount of power sent to a load.
Telecommunications
In telecommunications, the width of the pulses correspond with specific data values encoded at one end and decoded at the other.
Pulses of various lengths (the information itself) will be sent at regular intervals (the carrier frequency of the modulation).
_ _ _ _ _ _ _ _
| | | | | | | | | | | | | | | |
Clock | | | | | | | | | | | | | | | |
__| |____| |____| |____| |____| |____| |____| |____| |____
_ __ ____ ____ _
Data | | | | | | | | | |
| | | | | | | | | |
_________| |____| |___| |________| |_| |___________
Data 0 1 2 4 0 4 1 0
The inclusion of a clock signal is not necessary as the leading edge of the data signal can be used as the clock if a small offset is added to the data value in order to avoid the lack of a pulse for zero values.
Power delivery
PWM is also used to vary the total amount of power delivered to a load without the losses normally incurred when a power source drops its output voltage through resistive means. In a PWM system the solid state switch being used to control the flow of current is either not conducting any current or has very low voltage drop across it (and is conducting high current); since the instantaneous power dissipated in the switch is the product of the current and the voltage at any given time, this means no power would be dissipated if the switch were ideal. With a sufficiently high modulation rate, simple RC filters are often used to smooth the pulse train into a steady analog voltage. This method is commonly used in DC motor speed control.
PWM power limits are also used in the simple light dimmers common in homes. In this case the electricity being modulated is AC. Simple adjustment to the brightness of the light can be implemented by setting at what voltage in the AC cycle the dimmer begins to conduct electricity to the light bulb (using a triac). Because the duty cycle of the modulation is the same as the AC frequency of the line (60hz in the United Pulse-width modulation States) the human eye sees only the average intensity (see flicker fusion).
Voltage regulation
(main article: switched-mode power supply)
PWM is also used in efficient voltage regulators. By switching voltage to the load with the appropriate duty cycle, the output will approximate a voltage at the desired level. The switching noise is usually filtered with an inductor and a capacitor.
One method measures the output voltage. When it is lower than the desired voltage, it turns on the switch. When the output voltage is above the desired voltage, it turns off the switch.
Audio effects
PWM is sometimes used in sound synthesis, in particular subtractive synthesis, as it gives a nice effect similar to chorus or slightly detuned oscillators played together. The ratio between the high and low level is typically modulated with a low frequency oscillator, or LFO.
A new class of audio amplifiers based on the PWM principle is becoming popular. Called "Class-D amplifiers", these amplifiers produce a PWM equivalent of the analogue input signal which is fed to the loudspeaker via a suitable filter network to recover the original audio. These amplifiers are characterised by very good efficiency figures (>= 90%) and compact size/light weight for large power outputs.
See also
- Modulation
- Pulse-code modulation
- Pulse-amplitude modulation
- Pulse-density modulation
- Pulse-position modulation
- Radio control
Electronics Topics
The field of electronics is the study and use of systems that operate by controlling the flow of electrons or other electrically charged particles in devices such as thermionic valves and semiconductors. The design and construction of electronic circuits to solve practical problems is part of the fields of electronic engineering, and the hardware design side of computer engineering. The study of new semiconductor devices and their technology is sometimes considered as a branch of physics.
# - A | B | Co - Cz | C - Cm | D
Em - F | E - El | G - H | I - K | L - Ma
