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18.1 SWITCHED INPUTS AND OUTPUTS
A PLC is just a computer, we must get information in so that it may make decisions, and have outputs so that it can make things happen.
Motors - motors often have their own controllers, or relays because of the high current they require.
PLCs, and other industrial controls often use methods called sourcing or sinking current for outputs of devices. In this method the output of a device does not supply any power. Instead, the device only switches current on or off.
Sinking - When active the output allows current to flow to a common ground. This is best selected when different voltages are supplied.
Sourcing - When active, current flows from a supply, through the output though the output device and to ground. This method is best used when all devices use a single supply voltage.
18.1.1 Input Modules
Input modules typically accept various inputs, depending upon specified values.
An example of a PLC input card is shown below.
DC voltages are usually lower, and therefore safer (i.e., 12-24V)
DC inputs are very fast, AC inputs require a longer time (e.g., a 60Hz wave would require up to 1/60sec for reasonable recognition).
DC voltages are flexible being able to connect to greater varieties of electrical systems.
DC input cards typically have more inputs.
AC signals are more immune to noise than DC, so they are suited to long distances, and noisy (magnetic) environments.
AC signals are very common in many existing automation devices.
18.1.2 Output Modules
WARNING - ALWAYS CHECK RATED VOLTAGES AND CURRENTS FOR PLC's AND NEVER EXCEED!
Some output modules allow us to use a single common. We refer to this type of output as current sinking.
Other output modules allow us to use a single voltage supply. We refer to this type of output as current sourcing.
Typical outputs operate in one of two ways.
- Dry contacts - a separate relay is dedicated to each output. This allows mixed voltages (AC or DC and voltage levels up to the maximum), as well as isolated outputs to protect other outputs and the PLC. Response times are often greater than 10ms. This method is the least sensitive to voltage variations and spikes.
- Switched outputs - a voltage is supplied to the PLC card, and the card switches it to different outputs using solid state circuitry (transistors, triacs, etc.) Triacs are well suited to AC devices requiring less than an amp. Transistor outputs use NPN or PNP transistors up to 1A typically. Their response time is well under 1ms.
Inductive loads - Inductance is caused by a coil building up a magnetic field. When a voltage is removed from the coil the field starts to collapse, and as it does this the magnetic field is changed back to current/voltage. If this change is too sudden a large voltage spike is created. One way to overcome this is by adding a surge suppressor. One type of design was suggested by Steel McCreery of Omron Canada Ltd.
18.1.2.1 - Relays
Contactor - special relays for switching of large loads.
Motor Starter - Basically a contactor in series with an overload relay to cut off when too much current is drawn.
Rated Voltage - suggested operation voltage. Lower levels can result in failure to operate, voltages above shorten life.
Rated Current - The maximum current before contact damage occurs (welding or melting).
DC relays require special arc suppression. AC relays have a zero crossing to reduce relay arc problems.
AC relays require a shading pole to maintain contact. If a DC relay is used with AC power on the coil it clicks on-and-off at the frequency of the AC (also known as chattering).
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