PLC I/O

• Semiconductor devices

• I/O modules

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 saidanother way, diodes have a PN junction.

Bipolar transistors are either PNP or NPN. There are three layers.

Thyristors have PNPN or four layers.

Electronic control

With a diode, once the voltage across the PN junction exceeds0.7V, the diode conducts easily in one direction. There is noway to control how much current flows or to turn it off, unlessthe 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 canconduct in both directions.

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 is0.7V

Types of diodes

There are two major types of diodes we are concerned with.The first is the general purpose power diode used for rectifyingAC signals.

The other is a LED (Light Emitting Diode). LEDs have a higherforward voltage drop than regular diodes. LEDs are used as indicator lights and also in something called an optoisolator.

OptoisolatorOptoisolators remove the electrial connection between input and output. This way if there is a component failure or exceptionallyhigh 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 protectthe CPU.

Optoisolators are also used in medical applications to isolate a person from medical equipment

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 transformersand usually have a 1:1 turns ratio.

Transistor circuitThe transistor is an electronic switch. We will use it as eitherbeing on or off. A small control current controls a larger current.

The Greek letter Beta is used to indicate the current gain of thetransistor. Beta=100 is typical. Therefore a 1mA current from the CPU could control a 100mA load, such as a relay.

Transistor problemFor the circuit below, what is the voltage drop across the loadresistor. What is the input voltage?

Transistor problem

Vi = 1.925 + 0.7VVi = 2.625V

ThyristorsThyristors are a family of semiconductor devices that cancontrol both DC and AC currents. We will look at examples of AC control.

There are many devices in the thyristor family. The maindevices are:1. DIAC2. SCR3. TRIAC

Power conditioningWhen 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 cangenerate 20kV or more for the spark plugs. Each time the pointsopen they cause the electro-magnetic field in the coil to collapsewhich causes a tremendous rise in voltage.

Example

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

LC filters

LC filters act as low pass filters, removing noise from bothsignal and power lines.

Sometimes car audio systems pick up the alternator noiseor “hum”. The installation of a simple power line filterinline with the audio system can remove the noise.

UPS

UPS or Uninterruptable Power Supply is a device that will detectif the AC power is lost, then use the internal batteries to power aninverter 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 timethe device can supply the power.

Critical computer, medical and control systems employ thesedevices.

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.

Example of a combination UPS/Ferroresonant transformer

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 speciallydesigned transformer secondary that works in such a way as tokeep the output near 120 Vac for inputs of 80 to 140Vac.

These protect equipment from “brown outs” or periods wherethe AC line voltage drops, often for periods of 1-2 seconds

Types of PLC I/O

• DC input/output

• AC input/output

• Relay output

• Analog input/output

• DeviceNet™

Transient protection devices

There are a number of transient protection devices that are used.- MOV (Metal Oxide Varistors) are common. They work byacting like an open circuit until the voltage across them exceedsa predesigned limit, after that they will conduct, thereby shuntingthe spike. When the voltage across the device drops below the limit, the device stops conducting.

Inductors and capacitors are also used.

DC input

• Typically 24VDC

• Can be either Sinking or Source inputs

• Utilizes a voltage divider to reduce the input voltage to processor level

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

Sink/Source digram

Source

Sink

• showing isolation for the CPU

Optoisolator DC input example

To CPUConnectionto ground thruswitch, sensor,etc

DC outputDC output modules use transistors to control each output terminal.

Many relays and indicator lamps use 24V outputs.

DC output example - sinking

• Uses an optoisolator to protect CPU

AC input

• 120 or 240 VAC

• uses DIAC and voltage divider

• uses an optoisolator to protect the CPU

AC input diagram

•

Since the CPU can only work with a 5V or 0V signal, ACinputs 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 CPUfrom a fault on the input side.

AC outputAC outputs can turn on small motors, relays, lights and heaters.Although it is common practice to have the output turn on a relaysince the relay can control much higher currents.

The CPU only generates 0V or 5VDC outputs. So additionalcircuitry is needed to turn on or off 120VAC.

Relay output

• Typically rated at 2A per point

• Can connect either AC or DC devices

• Each point is isolated from the others

Relay output wiring

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

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

Analog output

• 0-10V or 4-20mA

• 12 bit resolution

• used to control variable speed drives, control valves, etc

Example of resolution calculation

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

Op-Amp SchematicsVoltage follower

Inverting amplifier

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

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

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

Device Net diagram

Sensor

ActuatorSensor

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

PWM waveform

Freq = 1kHz Duty cycle = 20%

ResourcesAllen Bradley - Www.ab.comElectronics dictionary - http://www.twysted-pair.com/dictionary.htm

The Institute of Electrical and Electronics Engineers- www.ieee.org

www.eetimes.com