PLC I/O

1. PLC I/O Semiconductor devices I/O modules

2. Semiconductor components Semiconductor devices are made of p-type and n-type materials. A device with one p-type and one n-type layer is a Diode. Or said another way, diodes have a PN junction. Bipolar transistors are either PNP or NPN. There are three layers. Thyristors have PNPN or four layers.

3. Electronic control With a diode, once the voltage across the PN junction exceeds 0.7V, the diode conducts easily in one direction. There is no way to control how much current flows or to turn it off, unless the voltage source is reduced. Transistors “electronic switches” can provide this control and are used in DC circuits. Current flow is also only in one direction. Thyristors can work with AC circuits and some devices can conduct in both directions.

4. Diodes Diodes consist of a PN junction of semiconductor material. They only conduct in one direction and block current in the other direction. They are used as rectifiers to convert AC signals to DC. When a diode is forward biased, the voltage drop across it is 0.7V

5. Types of diodes There are two major types of diodes we are concerned with. The first is the general purpose power diode used for rectifying AC signals. The other is a LED (Light Emitting Diode). LEDs have a higher forward voltage drop than regular diodes. LEDs are used as indicator lights and also in something called an optoisolator.

6. Optoisolator Optoisolators remove the electrial connection between input and output. This way if there is a component failure or exceptionally high voltage on one side, the circuitry on the other side of the optoisolator is protected. PLCs use optoisolators on all input and output modules to protect the CPU. Optoisolators are also used in medical applications to isolate a person from medical equipment

7. Electrical isolation A familiar example of electrial isolation is the transformer. Transformers provide DC isolation between input and output, while allowing AC signals to pass through. Transformers used for this purpose are called Isolation transformers and usually have a 1:1 turns ratio.

8. Transistor circuit The transistor is an electronic switch. We will use it as either being on or off. A small control current controls a larger current. The Greek letter Beta is used to indicate the current gain of the transistor. Beta=100 is typical. Therefore a 1mA current from the CPU could control a 100mA load, such as a relay.

9. Transistor problem For the circuit below, what is the voltage drop across the load resistor. What is the input voltage?

10. Transistor problem Vi = 1.925 + 0.7V Vi = 2.625V

11. Thyristors Thyristors are a family of semiconductor devices that can control both DC and AC currents. We will look at examples of AC control. There are many devices in the thyristor family. The main devices are: 1. DIAC 2. SCR 3. TRIAC

12. Power conditioning When inductive load are turned on and off, voltage transients, or “spikes” are created. Spikes are often above several hundred volts and last a few microseconds. A car using a 12V battery, ignition coil and distributor can generate 20kV or more for the spark plugs. Each time the points open they cause the electro-magnetic field in the coil to collapse which causes a tremendous rise in voltage.

13. Example MOVs are rated in Vrms and the number of Joules of energy they can absorb. MOVs typically fail in an “open” or non- conductive mode.

14. LC filters LC filters act as low pass filters, removing noise from both signal and power lines. Sometimes car audio systems pick up the alternator noise or “hum”. The installation of a simple power line filter inline with the audio system can remove the noise.

15. UPS UPS or Uninterruptable Power Supply is a device that will detect if the AC power is lost, then use the internal batteries to power an inverter that can supply AC power to the load for a period of time. UPS devices are rated two ways. First the number of volt-amps (VA) or (kVA) they can supply. Secondly, the amount of time the device can supply the power. Critical computer, medical and control systems employ these devices.

16. Example of a combination UPS/Ferroresonant transformer • Advanced, Line Interactive Protection Against Power Problems. • The SOLA 510 is an advanced, sine wave, uninterruptible power system that operates in both 50 Hz and 60 Hz enviroments. It is designed to protect state-of-the-art microprocessor-based equipment and specifically engineered to work with the variety of power supplies found in computer, communications and industrial equipment. • At the moment of system start up, the SOLA 510 automatically tests functionality and internal battery status. When incoming power is lost, the SOLA 510 smoothly and instantaneously transfers to internal battery power. You choose whether to perform an orderly shut down or ride through a brief blackout. • When AC input (mains) is available, the SOLA 510 filters surges and sags that could potentially harm equipment. If the line voltage is low, it regulates incoming voltage to the appropriate level without depleting the battery. The SOLA 510 will automatically transfer to battery back up if the input line voltage exceeds proper levels. • Software packages are available that can perform an unattended, orderly shut down with most Unix, DEC, VMS/ULTRIX, IBM, Sun, Silicon Graphics, Novell and many other systems. • Features and Benefits. • Sine wave output. • Sine wave inverter technology is compatible with the widest variety of loads, including switch-mode power supplies. • Critical protection when power fails. • When a blackout occurs, the SOLA 510 instantly transfers to internal battery without interrupting your critical load. • Voltage regulations. • If incoming voltage is low, the SOLA 510 boosts output voltage without draining the internal battery. If line voltage is too high, the SOLA 510 automatically provides power from its battery. • System communications. • SOLA 510 communicates critical information to your computer or network via RS-232 throught the DB-9 port on the rear panel of the unit. SNMP ready.

17. Ferro-resonant Transformers Also called CVT or Constant voltage transformers will maintain a nearconstant VAC output for a varying VAC input. They are designed as resonant devices which tend to smooth out transients on the AC waveform. They also employ a specially designed transformer secondary that works in such a way as to keep the output near 120 Vac for inputs of 80 to 140Vac. These protect equipment from “brown outs” or periods where the AC line voltage drops, often for periods of 1-2 seconds

18. Types of PLC I/O DC input/output AC input/output Relay output Analog input/output DeviceNet™

19. Transient protection devices There are a number of transient protection devices that are used. - MOV (Metal Oxide Varistors) are common. They work by acting like an open circuit until the voltage across them exceeds a predesigned limit, after that they will conduct, thereby shunting the spike. When the voltage across the device drops below the limit, the device stops conducting. Inductors and capacitors are also used.

20. DC input Typically 24VDC Can be either Sinking or Source inputs Utilizes a voltage divider to reduce the input voltage to processor level

21. Sinking /Sourcing To activate a sinking input, the terminal is brought to ground potential To activate a sourcing input, the terminal is brought to 24VDC

22. Sink/Source digram showing isolation for the CPU Source Sink

23. Optoisolator DC input example Connection to ground thru switch, sensor,etc To CPU

24. DC output DC output modules use transistors to control each output terminal. Many relays and indicator lamps use 24V outputs.

25. DC output example - sinking Uses an optoisolator to protect CPU

26. AC input 120 or 240 VAC uses DIAC and voltage divider uses an optoisolator to protect the CPU

27. AC input diagram Since the CPU can only work with a 5V or 0V signal, AC inputs must be converted to DC then reduced in voltage. This circuit uses a half-wave rectifier and voltage divider to translate the high voltage AC input for the CPU. Then a optoisolator provides electrial isolation to protect the CPU from a fault on the input side.

28. AC output AC outputs can turn on small motors, relays, lights and heaters. Although it is common practice to have the output turn on a relay since the relay can control much higher currents. The CPU only generates 0V or 5VDC outputs. So additional circuitry is needed to turn on or off 120VAC.

29. Relay output Typically rated at 2A per point Can connect either AC or DC devices Each point is isolated from the others

30. Relay output wiring

31. Driving relays Relays create a voltage spike when the coil is de-energized Diodes are used across the coil of DC relays to protect the driving circuitry

32. Analog input 0-10V or 4-20mA Typically has 12 bit resolution 1 part in 4096 Can detect changes of 2.4mV used to measure pressures, speeds and temps Temperature inputs are often RTD or thermocouple

33. Analog output 0-10V or 4-20mA 12 bit resolution used to control variable speed drives, control valves, etc

34. Example of resolution calculation

35. Op-Amps is a universal amplifier/electronic building block Voltage follower has a gain of 1 is used as an impedance buffer Inverting amplifier used to increase the voltage of small signals

36. Op-Amp Schematics Voltage follower Inverting amplifier

37. 4-20mA 4mA equals 0% on 20mA equals 100% on Can detect a fault in the system if no current flows. This offers protection over 0-10V signal. Current signals have much higher immunity to noise

38. Calculating resolution Resolution = Full scale value / 2N -1 N is the number of bits on the converter Example, for a 0-10V, 12 bit system. The resolution is 10/4096-1 = 2.44mV

39. Device Net Remote I/O for Allen-Bradley PLCs Requires an interface card in the rack Uses a local area network to send/receive signals to/from devices Each device has an address

40. Device Net diagram Sensor Sensor Actuator

41. Pulse Width Modulation (PWM) Is used to control the power delivered to the load. The power delivered is directly proportional to the duty cycle of the waveform DC is 100% duty cycle for a PWM waveform The switching frequency is high, above 1 kHz

42. PWM waveform Freq = 1kHz Duty cycle = 20%

43. Resources Allen Bradley - Www.ab.com Electronics dictionary - http://www.twysted-pair.com/dictionary.htm The Institute of Electrical and Electronics Engineers- www.ieee.org www.eetimes.com