Introduction to Industrial Automation

(b) relay set-latched and power circuit closed.

(a)

(b)

Electromagnet

Rotation

steps

NO contact

Dual disk

Surface A

Surface B

Figure 2.12  Typical internal structure of a pulse bistable relay in its two possible states: (a) a 

movable contact in front of type-A disk surface, and (b) after one pulse, the movable contact 

comes in front of the type-B disk surface and the electric contact closes.

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  Introduction to Industrial Automation

plate disk, one tooth plate for step-by-step rotation of the disk and a second plate for setting the posi-

tion of the movable contact. The electromagnet causes the step-by-step rotation of the disk at each 

positive rising edge of the supplied voltage pulse. The contact status depends on the position of the 

second plate, particularly if the surface A or surface B is in front of the movable contact. When the 

coil is pulsed, the relay armature moves a lever that, in turn, rotates the disk to position B stepwise in 

the sequence, closing the NO contact. This position will remain independent of the pulse duration 

and as long as another pulse is not supplied to the coil. Figure 2.13 shows the behavior of the electric 

contact due to a series of successive pulses. The first pulse closes the contact, the next opens it, and so 

on, back and forth. It is obvious that this relay has two discrete states, which can be retained without 

the existence of a coil voltage, justifying the term “bistable”. A typical use of the pulse bistable relay 

is remotely starting and stopping a machine from a single momentary push button.