Out put module

Output Modules

As with input modules, output modules rarely supply any power, but instead act as

switches. External power supplies are connected to the output card and the card will

switch the power on or off for each output. Typical output voltages are listed below, and

roughly ordered by popularity.

120 Vac

24 Vdc

12-48 Vac

12-48 Vdc

5Vdc (TTL)

230 Vac

These cards typically have 8 to 16 outputs of the same type and can be purchased

with different current ratings. A common choice when purchasing output cards is relays,

transistors or triacs. Relays are the most flexible output devices. They are capable of

switching both AC and DC outputs. But, they are slower (about 10ms switching is typical),

they are bulkier, they cost more, and they will wear out after millions of cycles. Relay

outputs are often called dry contacts. Transistors are limited to DC outputs, and Triacs are

limited to AC outputs. Transistor and triac outputs are called switched outputs.

- 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 1A. Transistor outputs use NPN or

PNP transistors up to 1A typically. Their response time is well under 1ms.

WARNING - ALWAYS CHECK RATED VOLTAGES AND CURRENTS FOR PLC’s

AND NEVER EXCEED!

Caution is required when building a system with both AC and DC outputs. If AC is

accidentally connected to a DC transistor output it will only be on for the positive half of

the cycle, and appear to be working with a diminished voltage. If DC is connected to an

AC triac output it will turn on and appear to work, but you will not be able to turn it off

without turning off the entire PLC.

ASIDE: A transistor is a semiconductor based device that can act as an adjustable valve.

When switched off it will block current flow in both directions.While switched on it

will allow current flow in one direction only. There is normally a loss of a couple of

volts across the transistor. A triac is like two SCRs (or imagine transistors) connected

together so that current can flow in both directions, which is good for AC current.

One major difference for a triac is that if it has been switched on so that current flows,

and then switched off, it will not turn off until the current stops flowing. This is fine

with AC current because the current stops and reverses every 1/2 cycle, but this does

not happen with DC current, and so the triac will remain on

A major issue with outputs is mixed power sources. It is good practice to isolate all

power supplies and keep their commons separate, but this is not always feasible. Some

output modules, such as relays, allow each output to have its own common. Other output

cards require that multiple, or all, outputs on each card share the same common. Each output

card will be isolated from the rest, so each common will have to be connected. It is

common for beginners to only connect the common to one card, and forget the other cards

then only one card seems to work!

The output card shown in Figure 3.5 is an example of a 24Vdc output card that has

a shared common. This type of output card would typically use transistors for the outputs.

In this example the outputs are connected to a low current light bulb (lamp) and a

relay coil. Consider the circuit through the lamp, starting at the 24Vdc supply. When the

output 07 is on, current can flow in 07 to the COM, thus completing the circuit, and allowing

the light to turn on. If the output is off the current cannot flow, and the light will not

turn on. The output 03 for the relay is connected in a similar way. When the output 03 is

on, current will flow through the relay coil to close the contacts and supply 120Vac to the

motor. Ladder logic for the outputs is shown in the bottom right of the figure. The notation

is for an Allen Bradley PLC-5. The value at the top left of the outputs, O:012, indicates

that the card is an output card, in rack 01, in slot 2 of the rack. To the bottom right of the

outputs is the output number on the card 03 or 07. This card could have many different

A major issue with outputs is mixed power sources. It is good practice to isolate all

power supplies and keep their commons separate, but this is not always feasible. Some

output modules, such as relays, allow each output to have its own common. Other output

cards require that multiple, or all, outputs on each card share the same common. Each output

card will be isolated from the rest, so each common will have to be connected. It is

common for beginners to only connect the common to one card, and forget the other cards

- then only one card seems to work!

The output card shown in Figure 3.5 is an example of a 24Vdc output card that has

a shared common. This type of output card would typically use transistors for the outputs.

voltages applied from different sources, but all the power supplies would need a single

shared common.

The circuits in Figure 3.6 had the sequence of power supply, then device, then PLC

card, then power supply. This requires that the output card have a common. Some output

schemes reverse the device and PLC card, thereby replacing the common with a voltage

input. The example in Figure 3.5 is repeated in Figure 3.6 for a voltage supply card.

In this example the positive terminal of the 24Vdc supply is connected to the out-

put card directly.When an output is on power will be supplied to that output. For example,

if output 07 is on then the supply voltage will be output to the lamp. Current will flow

through the lamp and back to the common on the power supply. The operation is very similar

for the relay switching the motor. Notice that the ladder logic (shown in the bottom

right of the figure) is identical to that in Figure 3.5. With this type of output card only one

power supply can be used.

We can also use relay outputs to switch the outputs. The example shown in Figure

3.5 and Figure 3.6 is repeated yet again in Figure 3.7 for relay output.

In this example the 24Vdc supply is connected directly to both relays (note that

this requires 2 connections now, whereas the previous example only required one.) When

an output is activated the output switches on and power is delivered to the output devices.

This layout is more similar to Figure 3.6 with the outputs supplying voltage, but the relays

could also be used to connect outputs to grounds, as in Figure 3.5. When using relay outputs

it is possible to have each output isolated from the next. A relay output card could

have AC and DC outputs beside each other.

(resource : on internet Hugh Jack's book)