Optocoupler speed-up also reduces power consumption
Standard optocoupler speed is limited mainly by the relatively slow response of the phototransistor. This Design Idea adds components to the LED drive side to speed things up.
Figure 1 The speed-up circuit not only increases speed of propagation of a rising input signal, but achieves this speed-up at lower power consumption, compared to simplistic driving of the LED through a resistor.
R1 is the original LED resistor, as used before the extra circuitry was added. Here however, its value can be higher, as the turn-on speed is determined mainly by the added circuit. You can thus save power, and also drive the LED with a less powerful driver.
The turn-on speed-up device is an emitter follower, NPN transistor Q1. The emitter follower has its emitter resistor split into a low-value part REL, and a higher-value part REH which is paralleled with capacitor C. At a steep rise of input voltage VIN, the initially uncharged capacitor C temporarily “shorts” REH. Thus, the emitter current flowing through the LED has an increased value of:
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