Engineering Expert Witness Blog

     Last week we looked at a basic electric relay and saw how it was used to facilitate a choice in electricity flow between two paths in a circuit.  Now let’s see what happens when we put a relay to use within a basic industrial control system making use of lit bulbs.

Figure 1

     Figure 1 shows an electric relay that’s connected to both hot and neutral wires.  At the left side is our pushbutton and the hot wire, on the right two bulbs, one lit, one not, and the neutral wire.  No one is depressing the pushbutton, so an air gap exists, preventing current from flowing through the wire coil between the hot and neutral sides.  With these conditions in place the relay is said to be in its “normal state.”

     The relaxed spring positioned on the relay armature keeps it touching the N.C. contact.  This allows current to flow between hot and neutral through the armature and the N.C. contact.  When these conditions exist the red bulb is lit, and this is accomplished without the need for anyone to throw a switch or press a button.  In this condition the other lamp will remain disengaged and unlit.

     Now let’s refer to Figure 2 to see what happens when someone presses the button.

Figure 2

     When the button is depressed the air gap is eliminated and the coil and wire become magnetized.  They will attract the steel armature closer to them, the spring to expand, and the armature to engage with the N.O. contact.  Under these conditions current will no longer flow along a path to light the red bulb because an air gap has been created between the armature and N.C. contact.  The current instead flows through the N.O. contact, lighting the green bulb.  It will stay lit so long as someone holds the button down.

     If our monkey were faced with the scenarios presented in Figures l and 2 and a banana was placed in the position of the red bulb, the cocaine in the position of the green, he might find that the regular delivery of bananas that takes place when the relay is in the N.C. contact position is enough to keep him happy.  In this state he might be so full of bananas he won’t want to expend the energy to engage the button into the N.O. contact position for the delivery of cocaine. 

     Next time we’ll revisit the subject of ladder diagrams and see how they are used to denote the paths of electric relays. 

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