Optocoupler Input Circuits for PLCs
by Lewis Loflin
Here I'll introduce programmable logic controller (PLC) input circuits using opto-couplers. We use these devices to interface high voltage sensors to low voltage microcontroller logic and to isolate sensitive circuits from noise.
I'll also discuss the concepts of SOURCE and SINK.
Fig. 1 illustrates a typical 6-pin opto-coupler. These can also be surface mount as well. All contain an LED emitter and many a photo transistor output.
Fig. 2 show just a few of the many types of optocouplers. The bottom two are often used as output while the top two can be either way.
Fig 3 illustrates a typical bipolar transistor opto-coupler to a microcontroller.
Fig 4 shows how to build you own opto-coupler with a light emitting diode and a photo transistor.
The H11AA1 is a very useful device because the dual back-to-back LEDs simplify circuits design because we don't have to worry about input polarity.
Fig. 6 shows what is called a SINK configuration. This refers to the current path in relation to the PLC input. (This is really simplified.)
U1 is an open drain or collector in this case a Hall effect sensor. When a magnet turns on the Hall sensor a path to ground is enabled through the output pin. When U1 is turned on one of the LEDs is turned on switching on the transistor. The 5-volt micro-controller reads this as a switch closure.
The switching is in the ground side of the PLC input.
Fig. 7 illustrates a SOURCE configuration. When U3 is turned on it switching on a PNP transistor completing the current path though the H11AA1 to ground. The switching is on the positive supply side of the PLC input.
Fig 8 shows both SINK and SOURCE inputs on the same PLC. Because the opto-coupler is say a 4N26 with a single LED we must observe LED polarity.
In Fig. 6 the NPN transistor was internal to the Hall sensor.
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