Basic Electricity II - Reading and Troubleshooting Ladder Diagrams
Reading the ladder
Ladder diagrams are drawn between power rails, with each rung read left to right (or top to bottom). Contacts in series must all be closed - a logical AND. Contacts in parallel offer an alternate path - a logical OR. A coil energizes when its rung is complete. Symbols to recognize: normally open and normally closed contacts, the coil, overload contacts, pressure and temperature switches, float switches, timers, and selector switches. Control circuits usually operate at reduced voltage from a control transformer, with the common or neutral side forming the right rail.
The circuit to master is start–stop with a seal-in: a normally closed stop button in series with a normally open start button, a holding contact wired in parallel with the start button, and an overload contact, all in series with the coil. Press start and the coil pulls in; the holding contact keeps the rung complete after the button is released; press stop and the circuit drops out.
Traps: omitting the overload contact, which must be in series and is normally closed; treating an untouched normally closed pushbutton as an open circuit; assuming a timer acts instantly; forgetting that a pressure switch labeled "open on rise" may be drawn either way unless you read the legend.
Rule of thumb for troubleshooting: with the circuit energized and the coil dropped out, measure voltage across each contact in turn. A good closed contact reads about zero volts; a good open contact reads full control voltage. Voltage across a contact that should be closed points at that device.