![]() Therefore, this optocoupler can resolve 7,692 steps from a PWM running at 2kHz. Let’s look at a higher cost optocoupler like the ON Semi FOD8012A, which has a t R and t F of 13ns combined, with the same 2kHz PWM. But if you want to operate at a higher frequency or resolution, a high-speed optocoupler would be better. However, a 4-bit PWM yields 16 steps (2 4 = 16) and since the 4N25 can attain up to 83 steps, these parameters might work together. Unless you are willing to trade down to a much lower frequency from your PWM, the number of steps the 4N25 optocoupler would be able to resolve would be poor. Solving for n, you find that at PWM frequency of 2kHz, the 4N25 optocoupler can see a maximum of 83 steps. The 4N25 lists only turn on and turn off times at 2µs and 10 µs (max). Solve for n:Īnd you have the number of steps that the optocoupler should be able to accommodate, based on given rise and fall times in the optocoupler datasheet. It might be best to just run through some calculations to experiment with the possibilities on paper first, which is more efficient than guessing and buying optocouplers to see if they work.Ī quick calculation can be made if you know the frequency of the PWM (F PWM) and the rise time (t R) and fall time (t F) of the optocoupler: F PWM= 2/n(t R+t F), where n is the number of discrete steps that the optocoupler can accommodate. Put simply, the frequency of the PWM (F PWM) (Hz) is related to the maximum number of steps that the optocoupler must attain. But how do you tease this information from the optocoupler data sheet? However, the speed of the optocoupler is key the minimum pulse width of the PWM must be longer than the switching speed of the optocoupler. It seems as if optocouplers would be best used in a digital environment, however, it is possible to use optocouplers to isolate pulse width modulated (PWM) signals. Although optocouplers are limited by the frequency at which they can operate (which mainly depends on the type of photoreceptor inside), optocouplers provide protection from overvoltage, high voltage transients, and can be used to eliminate noise outside the optocoupler’s operating range. The optocoupler’s output mirrors the input and connecting an optocoupler is like operating an LED, which may require using a current limiting resistor (check the optocoupler datasheet). Optocouplers provide complete electrical isolation between circuits at the input and output terminals of the optocoupler. Nevertheless, many variations on the original part number are available. ![]() The ON Semi 4N25 is a good basic example but is officially obsolete. A commonly used optocoupler is the ON Semiconductor 4N25 (formerly Fairchild), as shown in Figure 1. The optocoupler has an air gap or insulating glass inside for the beam to cross, so no electrical connection exists between the input side and output side of the optocoupler. The infrared beam crosses a gap inside the optocoupler package to a light-sensitive device (e.g., photodiode, phototransistor, etc.), which changes the light back into a signal again and sends it out of the optocoupler as output. The optocoupler translates the signal on its input into an infrared light beam using an infrared light emitting diode (LED). An optocoupler achieves this isolation by taking signals that it receives at its input and transferring the signals using light to its output. ![]() Optocouplers are “fail safe” in that if subjected to voltages higher than the maximum rating, they are known to break as an open circuit. An optocoupler (or optoisolator) is a device that galvanically separates circuits and is not only great at isolation but allows you to interface to circuits with different ground planes or that operate at different voltage levels. ![]()
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