Version 1.0, August 31, 2001, Copyright, Hugh Jack 1993-2001

18.1.2 Output Modules

 

· WARNING - ALWAYS CHECK RATED VOLTAGES AND CURRENTS FOR PLC's AND NEVER EXCEED!

 

· Typical output voltages are,

  1. 12-48 Vac
  2. 120 Vac
  3. 230 Vac
  4. 120 Vdc
  5. 230 Vdc
  6. 5Vdc (TTL)
  7. 24 Vdc
  8. 12-48 Vdc

 

 

· 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.

  1. - 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.
  2. - 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.

 

· Try the following example,

 

 

 

· 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