A photocoupler that receives lights from the phototransistor is usually called a standard type photocoupler and operates without any circuit connection to the base of the phototransistor. Since the base of the phototransistor is a floating state, unexpected noise is sometimes generated. Two typical noise phenomena are described below.
Noise Due to a Change of Potential Difference Between the Light Emitting Side and Light Receiving Side
In a photocoupler, signals are transferred by light (photons) from the light emitting side to light receiving side. However, there is slight coupling capacitance as shown at "CI-O" in the figure below.
Accordingly, by application of a step voltage signal (Vin) between the light emitting side and light receiving side, some signal may be sometimes output in the light receiving side, as shown in the following figure.
Naturally, in the photocoupler signals not transmitted by photons are defined as noise. Basically, the complete elimination of such noise is difficult. In general, as the countermeasure, the architecture is designed to avoid steep change of potential difference. Otherwise, the noise can be reduced compromising with the operation speed by transmitting output signals through the integration circuit as shown in the figure below.
However, as the following figure shows, there are photocouplers with an electrostatic shield incorporated between the light emitting side and light receiving side.
In this, because the noise is induced not to the base but to the emitter, the transistor does not amplify the current caused by the noise, resulting in less output noise level. This noise immunity is referred to as an specification of "Common mode transient immunity".
Noise due to Power Source Rise at the Light Receiving Side
The phototransistor used for receiving lights has capacitance between the collector and base as shown at "CCB" in the figure below.
Therefore, by voltage application with steep rise to the collector, a current which charges the capacitance "CCB" flows into the base for an instant. By this current the collector current also flows and noise can be output as shown in the picture below.
In general, the voltage seldom rises as steeply as shown in this example; however, an adequate evaluation should be made before operating a high output voltage photocoupler at high power supply voltage. Basically, the power supply voltage should be controlled to rise gradually and the output signal is not evaluated until the power supply voltage stabilizes.