You might have gathered by now that PWM, duty cycle, and frequency are interrelated. We use duty cycle and frequency to describe the PWM, and we often talk about frequency in reference to speed.
For example, a variable frequency drive motor produces a response like analog device in the real world. The separate pulses that the VFD motor gets are not discernable to us; as far as we can see, the pulses are so fast usually somewhere in the milliseconds that by real world standards it just seems like a motor ramping up. The duty cycle can change to affect the average voltage that the motor experiences.
The frequency of the cycles can increase. The pulse can even be increased in length. Pulse width is directly related to duty cycle, so if you decide to increase the width of a pulse, you are just altering the duty cycle. MCUs are digital. An example of an older alternative is a simple transistor circuit that varies the current passing through it by varying its resistance.
The same efficiency rule that applies to resistors also applies to transistors — Their resistance results in wasted energy because they burn off some of it as heat. They act like heaters in that regard. Fortunately, these circuits were never mainstream. Appliances such as air conditioners and refrigerators just ran at full speed all the time, making lots of noise, and wasting lots of energy because they had to cycle on and off frequently.
PWM is digital, which means that it has two states: on and off which correspond to 1 and 0 in the binary context, which will become more relevant to you if using microcontrollers.
The longer each pulse is on, the brighter the LED will be. PWM signals are typically square waves, like the one in the illustration below.
Duty cycle refers the amount of time it is on. In such a state, an LED would not be operational. In general, this method of control has many beneficial applications. For example, PWM paired with maximum power point tracking MPPT is one of the principal methods for reducing a solar panel's output to facilitate its use by a battery. Overall, PWM is principally suited for running inertial devices like motors, which are not as quickly affected by this distinct switching.
This is also equally true for LEDs with PWM because of the linear fashion in which their input voltage affects their functionality. However, the PWM switching frequency needs to be high enough not to affect the load, yet the resulting waveform that the load perceives should also be smooth. Typically, the frequency in which the power supply must switch will vary extensively depending on the device and its application. For example, the switching has to be done several times a minute in an electric stove and well into the tens or hundreds of kHz for PC power supplies and audio amplifiers.
One of the significant advantages of using PWM is that power loss in the switching devices is substantially low. In fact, during the off phase of a switch, there is virtually no current.
Also, during the on phase of a switch, there is practically no drop in voltage across the switch while transferring power to its load. Since power loss is a consequence of both voltage and current, this translates into virtually zero loss in power for PWM.
Moreover, PWM is perfectly suited for digital controls, due to the nature of digital technology i. In general, the intrinsic nature of digital technology lends itself effortlessly to PWM's functionality, and thus, it is easy to set the necessary duty cycle. A PWM signal is a method for creating digital pulses to control analog circuits. There are two primary components that define a PWM signal's behavior:. Duty cycle : A duty cycle is the fraction of one period when a system or signal is active.
We typically express a duty cycle as a ratio or percentage. Frequency : The rate at which something repeats or occurs over a particular period.
In other words, the rate at which a vibration happens that creates a wave, e. About the duty cycle, while the signal is high, we refer to it as ON, and the duty cycle describes the amount of time a signal is in its ON-state.
We measure or quantify a duty cycle as a percentage. This percentage represents the specific time a digital signal is ON during a period interval , and this interval is the inverse of the waveform frequency. Since frequency is a primary component of the PWM technique, it is understandable that frequency affects PWM's ability to exert control within an application.
Therefore, the square wave frequency does need to be sufficiently high enough if controlling LEDs, for example, to get the proper dimming effect.
0コメント