This mechanism also needed to be able to drive motors for fans, pumps and robotic servos, and needed to be compact enough to interface with lamp dimmers. While the rheostat was one of several methods of controlling power (see autotransformers and Variac for more info), a low cost and efficient power switching/adjustment method was yet to be found. It was an inefficient scheme, as this also wasted power as heat in the resistor element of the rheostat, but tolerable because the total power was low.
PWM TO ANALOG CHIP SERIES
In the past, control (such as in a sewing machine's foot pedal) was implemented by use of a rheostat connected in series with the motor to adjust the amount of current flowing through the motor. Some machines (such as a sewing machine motor) require partial or variable power. Here is a pictorial that illustrates these three scenarios: When a digital signal spends more time in the off state than the on state, it has a duty cycle of <50%. When a digital signal spends more time in the on state than the off state, it has a duty cycle of >50%. When a digital signal is on half of the time and off the other half of the time, the digital signal has a duty cycle of 50% and resembles a "square" wave.
Duty cycle is expressed in percent, 100% being fully on. The term duty cycle describes the proportion of 'on' time to the regular interval or 'period' of time a low duty cycle corresponds to low power, because the power is off for most of the time. These are commonly used for direct current (DC) motor control in robotics and other applications. In electronics, many modern microcontrollers (MCUs) integrate PWM controllers exposed to external pins as peripheral devices under firmware control by means of internal programming interfaces. PWM has also been used in certain communication systems where its duty cycle has been used to convey information over a communications channel. PWM also works well with digital controls, which, because of their on/off nature, can easily set the needed duty cycle. Power loss, being the product of voltage and current, is thus in both cases close to zero. When a switch is off there is practically no current, and when it is on and power is being transferred to the load, there is almost no voltage drop across the switch. The main advantage of PWM is that power loss in the switching devices is very low. For example, switching has to be done several times a minute in an electric stove 100 or 120 Hz (double of the utility frequency) in a lamp dimmer between a few kilohertz (kHz) and tens of kHz for a motor drive and well into the tens or hundreds of kHz in audio amplifiers and computer power supplies. The rate (or frequency) at which the power supply must switch can vary greatly depending on load and application.
The PWM switching frequency has to be high enough not to affect the load, which is to say that the resultant waveform perceived by the load must be as smooth as possible.
PWM is particularly suited for running inertial loads such as motors, which are not as easily affected by this discrete switching, because their inertia causes them to react slowly. Along with maximum power point tracking (MPPT), it is one of the primary methods of reducing the output of solar panels to that which can be utilized by a battery. The longer the switch is on compared to the off periods, the higher the total power supplied to the load. The average value of voltage (and current) fed to the load is controlled by turning the switch between supply and load on and off at a fast rate. Pulse-width modulation ( PWM), or pulse-duration modulation ( PDM), is a method of reducing the average power delivered by an electrical signal, by effectively chopping it up into discrete parts.
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