Digital signals have two positions: on or off, interpreted in shorthand as 1 or 0. Analog signals, on the other hand, can be on, off, half-way, two-thirds the way to on, and an infinite number of positions between 0 and 1 either approaching 1 or descending down to zero. The two are handled very differently in electronics, but very often must work together (that’s when we call it “mixed signal electronics.”) Sometimes we have to take an analog (real world) input signal (e.g., temperature) into a microcontroller (which only understands digital). Often engineers will translate that analog input into digital input for the microcontroller (MCU) by using an analog-to-digital converter. But what about outputs?
PWM is a way to control analog devices with a digital output. Another way to put it is that you can output a modulating signal from a digital device such as an MCU to drive an analog device. It’s one of the primary means by which MCUs drive analog devices like variable-speed motors, dimmable lights, actuators, and speakers. PWM controller is not true analog output, however. PWM “fakes” an analog-like result by applying power in pulses, or short bursts of regulated voltage.
An example would be to apply full voltage to a motor or lamp for fractions of a second or pulse the voltage to the motor at intervals that made the motor or lamp do what you wanted it to do. In reality, the voltage is being applied and then removed many times in an interval, but what you experience is an analog-like response. If you have ever jogged a box fan by applying power intermittently, you will experience a PWM response. The fan and its motor do not stop instantly due to inertia, and so by the time you re-apply power it has only slowed a bit.
Therefore, you do not experience an abrupt stop in power if a motor is driven by PWM. The length of time that a pulse is in a given state (high/low) is the “width” of a pulse wave.
A device that is driven by PWM ends up behaving like the average of the pulses. The average voltage level can be a steady voltage or a moving target (dynamic/changing over time). To simplify the example, let’s assume that your PWM-driven fan has a high-level voltage of 24 volts. If the pulse is driven high 50% of the time, we call this a 50% duty cycle. The term duty cycle is used elsewhere in electronics, but in every case duty cycle is a comparison of “on” versus “off.”
Going back to our fan motor example, if we know that the high voltage is 24, the low is 0v, and the duty cycle is 50%, then we can determine the average voltage by multiplying the duty cycle by the pulse’s high level. If you want the motor to go faster, you can drive the PWM output to a higher duty cycle. The higher the frequency of high pulses, the higher the average voltage and the faster the fan motor will spin. IF you were making your own PWM output by plugging the fan in and out of a socket at equal intervals of 1 second in the socket, 1 second out, then you are acting like a digital output that’s driving the fan at a steady average of 12V.
