page 218

• AC signals are very common in many existing automation devices.

ASIDE: PLC inputs must convert a variety of logic levels to the 5Vdc logic levels used on the data bus. This can be done with circuits similar to those shown below. Basically the circuits condition the input to drive an optocoupler. This electrically isolates the external electrical circuitry from the internal circuitry. Other circuit components are used to guard against excess or reversed voltage polarity.

+5V

optocoupler

DC

TTL

input

	+5V
AC	optocoupler
input
	TTL

18.1.2 Output Modules

•WARNING - ALWAYS CHECK RATED VOLTAGES AND CURRENTS FOR PLC’s AND NEVER EXCEED!

•Typical output voltages are,

12-48 Vac

120 Vac

230 Vac

120 Vdc

page 219

230 Vdc 5Vdc (TTL) 24 Vdc 12-48 Vdc

ASIDE: PLC outputs must convert the 5Vdc logic levels on the PLC data bus to external voltage levels. This can be done with circuits similar to those shown below. Basically the circuits use an optocoupler to switch external circuitry. This electrically isolates the external electrical circuitry from the internal circuitry. Other circuit components are used to guard against excess or reversed voltage polarity.

optocoupler

TTL

optocoupler

TTL

+V

TTL

+V

Sourcing DC output

AC output

	Note: Some AC outputs will
relay	also use a zero voltage
output	detect. This allows the out-
AC/DC	put to be switched on when
	the voltage and current are
	effectively ‘off’, thus pre-
	venting surges.

page 220

• Some output modules allow us to use a single common. We refer to this type of output as current sinking.

24 V DC
Output Card
00
01	Relay
02
03
04
05	24 V lamp
06
07
COM
in rack 01
I/O group 2

120 V AC

Power

Supply

COM.

Motor

+24 V DC

Power

Supply

GND

O:012

Motor

03

O:012

Lamp

07

• Other output modules allow us to use a single voltage supply.We refer to this type of output as current sourcing.

page 221

24 V DC
Output Card	Power
	Supply
V+	+24 V DC	GND
00
01	Relay
02			120 V AC
03			Power
			Supply
04		Motor	COM.
05	24 V lamp
06
07

in rack 01
I/O group 2	O:012

Motor

03

O:012

Lamp

07

page 222

e.g. output example with dry (relay) contacts
120 V AC/DC	24 V DC
Output Card
	Power
	Supply	120 V AC
00		Power
		Supply
01
02
03
04	Relay
05
06		Motor
07
in rack 01	24 V lamp
I/O group 2

O:012

Motor

03

O:012

Lamp

07

•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.

page 223

• Try the following example,

Layout the electrical wiring for the control of a hydraulic press including,

-a double actuated 24VDC solenoid controlled valve for the advance/retract

-a 220VAC, 20A single phase motor for the hydraulic pump

-bottom and top limit switches

-an emergency stop button

-a manual retract button

-an automatic cycle button

page 224

Solution

24VDC output

24VDC input

	solenoid
	advance	I/0
O/0
		I/1
	retract	I/2
O/1
		I/3
O/2		I/4
com	-	+
		com
		24VDC

• 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.