Input Isolation with an Optocoupler (Optocoupler Isolation)
It is the method of separating the control circuit from the power circuit electrically by transferring the command signal with light. The LED at the input lights up and the phototransistor facing it starts conducting; there is no conductive connection between the two sides. This way the chassis difference and noise carried by a signal coming from a distant panel do not pass to the driver board.
The real gain from isolation is that it removes the chassis potential difference between two devices as a problem. In installations with long cables there can be a difference of hundreds of millivolts, even volts, between the chassis of two panels; with a direct connection this difference both spoils the measurement and creates a circulating current.
- Trigger current: The optocoupler LED is typically driven with 5–20 mA; a series resistor sets this current. About 2,2 kΩ is a suitable value to draw 10 mA from a 24 V line.
- Speed: Standard phototransistor types respond in the microsecond range; this is more than enough for button and PLC commands.
- Ageing: The light output of the LED falls over time; a current margin is left in the design.
There is one point where isolation alone is not enough: if the two sides are separated, each side must have its own supply source. An optocoupler arrangement fed from the same source does not provide real isolation, only a certain increase in noise immunity.
Context where this term is used: DC Motor Driver Selection Guide