What is the difference between a relay and a PLC?


Nowadays large rigid relay based control systems are being replaced with PLCs.

PLCs have a lot more functions that just use like relay contacts, though they were first made to replace relays. The ladder logic that is used to program the PLC made in form of traditional ladder logic using NO/NC contact of I/Os. PLCs provide a grater flexibility than the relays and are easy to upgrade.

But before you start replacing your relays with PLC you should consider the following:-

  1. Cost effectiveness- As the PLC have a large I/O capability therefore do insure that atleast 50% of I/O handling capability are used.
  2. Response time- PLCs have a response time of 50 - 500 msec at max. However relays have a response time less than 10 msec. Do consider that as some safety application response time is critical.
  3. Voltage level- As relays are used to isolate the various voltage levels, and relays will be used even if you use PLCs as they typical operation at 24Vdc.

Despite these PLC add a lot more like HMI integration, Soft signal transfer, better diagnostics, Historian, trends etc and definitely better in ease of use and maintenance.


Relay is a mechanical switch which uses electromagnetic signals to operate, it is typically used in high voltage circuits or to connect several circuits which are being controlled by one signal.

Where as PLC ( Programmable Logic Controller ) is widely used for Industrial purpose which deals with Machines.

A PLC is an example of a “hard” real-time system since output results must be produced in response to input conditions within a limited time, otherwise unintended operation will result.

Before the PLC, control, sequencing, and safety interlock logic for manufacturing automobiles was mainly composed of relays, cam timers,drum sequencers, and dedicated closed-loop controllers. Since these could number in the hundreds or even thousands, the process for updating such facilities for the yearly model change-over was very time consuming and expensive, as electricians needed to individually rewire the relays to change their operational characteristics.

Early PLCs were designed to replace relay logic systems. These PLCs were programmed in “ladder logic”, which strongly resembles a schematic diagram of relay logic. This program notation was chosen to reduce training demands for the existing technicians. Other early PLCs used a form of instruction list programming, based on a stack-based logic solver.

Modern PLCs can be programmed in a variety of ways, from the relay-derived ladder logic to programming languages such as specially adapted dialects of BASIC and C. Another method is state logic, a very high-level programming language designed to program PLCs based on state transition diagrams