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PWM is useful for controlling the average power or amplitude delivered by an electrical signal. The average value of voltage (and current) fed to the load is controlled by switching the supply between 0 and 100% at a rate faster than it takes the load to change significantly. The longer the switch is on, the higher the total power supplied to ...
Fig. 2. EMI noise of output voltage for programmed switching frequency (PWM): a) Spectrum and b) Spectrogram. PWM is considered the most common deterministic technique. Considering the example of a DC-DC converter, a controlled switch is designed to “cut-off” the DC waveform into a pulse-shaped waveform.
Turning off the switch causes the voltage across the inductor to reverse and the current to flow through the freewheeling diodes Da+ and Da-, decreasing linearly. By controlling the switch on-time, the topology is able to control the current in phase with the mains voltage, presenting a resistive load behavior (Power-factor correction capability).
This circuit is called a leading-edge dimmer or forward phase dimming. Waveform of the output voltage of a thyristor dimmer set for 60 volts RMS output, with 120 V input. The red trace shows the output device switching on about 5.5 ms after the input (blue) voltage crosses zero.
An example op-amp circuit drawn with Klunky and GIMP. Klunky can be used to draw basic circuits, and then a raster editing program like GIMP to modify the screenshots. The Klunky program is public domain, and he has enhanced it with additional components, etc. (See User:Omegatron#Electronics_diagrams. Enhanced version is here.
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The switch is controlled by a PWM (Pulse Width Modulation) signal, which alternately turns the switch on and off at a rapid rate. The relative proportion of off time vs. on time determines brightness. For example, if the switch is off 10% of the time, the resulting control signal would be the equivalent of 1 V produced with a variable resistor.
A circuit diagram (or: wiring diagram, electrical diagram, elementary diagram, electronic schematic) is a graphical representation of an electrical circuit. A pictorial circuit diagram uses simple images of components, while a schematic diagram shows the components and interconnections of the circuit using standardized symbolic representations.