Transcript No Slide Title

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 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.
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.
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
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.
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
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.
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.
Transistor problem
For the circuit below, what is the voltage drop across the load
resistor. What is the input voltage?
Transistor problem
Vi = 1.925 + 0.7V
Vi = 2.625V
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
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.
Example
MOVs are rated in Vrms and the number of Joules of energy
they can absorb. MOVs typically fail in an “open” or nonconductive mode.
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.
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.
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.
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
Types of PLC I/O
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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 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.
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
• showing isolation for the CPU
Source
Sink
Optoisolator DC input example
Connection
to ground thru
switch, sensor,etc
To CPU
DC output
DC 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, 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.
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.
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 Schematics
Voltage 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
Sensor
Actuator
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%
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