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Electronics Symentics


 
 

Ladder Logic

 

Ladder diagrams are used to describe the logic of electrical control systems. There are differences in the way ladder logic was implemented in computerized form as compared to hard wired so I will be talking about the old way first. The basic component of the control system is the control relay which is a solenoid that operates a number of switches or contacts. The contacts come normally open and normally closed, normal being when the relay is not energized. Relays come in various breeds like time delay and latching types. Other components of the control system are the field devices such as push buttons, limit switches, lights, and controlled devices like motor starters and solenoid operated valves. As I said, ladder diagrams show the logic of the controls but they are not used to build the system, a wiring diagram is used for that. But the wiring diagram wouldn’t be used to trouble shoot with or show functionality, that’s where the ladder is most useful.

When viewing the pictorial version of the controls as in Figure 1, one can see that the devices on a rung of the ladder are in series reading horizontally and in parallel reading vertically. Control voltage is supplied to the vertical rails, L1 being hot and L2 being common or ground. In industry it is common to see 120vac control and 480vac power circuits. Anything less than 600 volts is considered low voltage and virtually everything will have an insulation rating of 600 volts. (My robot uses 5vdc control and 28vdc power circuits.) In Figure 1 we see a normally closed (N.C.) stop button and a normally open (N.O.) start button and a motor starter. The circle with the M represents the coil of a relay, not the actual motor. The M contact is physically part of the starter and actuates with the coil. The contact labeled O.L. (over load) is also part of the starter and is a circuit breaker tripped by over current in the motor legs. The M contact is called the seal contact. Without it, the motor would run as long as someone held down the start button and would stop when released. With it, the power is allowed to flow through the start button to energize the coil, which closes the M contact, maintaining the complete circuit when the start button is released. To stop the motor, any element in series with the coil can break the circuit, in this case the stop button. True, this was a long winded explanation but you now have the critical pieces, how to turn something on, make it stay on, and how to shut it off. It’s a truth table in disguise, ANDs and ORs, ONs and OFFs.

 




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