May 8, 1997

Relay circuits use reverse hysteresis

Giovanni Romeo, Istituto Nazionale di Geofisica, Roma, Italy

The relay circuits in Figure 1 take advantage of the disengaging (off) portion of the hysteresis curve (Figure 2). This standard hysteresis curve comprises two thresholds. The highest coil-voltage threshold occurs when the relay is inactive and you power the coil. The lowest threshold occurs when the relay is active and you power off the coil. First, assume that the relay is in the disengaged position and that you power it with an increasing voltage. Because the field is just strong enough to move the armature, the new position brings the armature to a place at which the effect of the magnetic field is even stronger. The system drops in the only possible stable state: with the armature near the coil.

The disengaging threshold works similarly and is a characteristic that you can use for some interesting applications. To take advantage of this effect, you have to keep the voltage across the coil between the two thresholds and perturb this voltage to bring the relay into one of the two stable states. The only disadvantage is that the relay requires continuous power.

The multiswitch relay control in Figure 1a allows several users to control the same load. V_CC needs to be greater than the relay voltage, and R₁ must bring the voltage on the coil between the two threshold points. The circuit in Figure 1b is similar to that in Figure 1a, except that the switches connect to ground. In this case, the voltage on the coil must be between the two threshold points when all the switches are open and must exceed the on threshold when you press an even switch. R₁ should be large enough to limit the current on odd switches.

The toggle relay in Figure 1c uses relay contacts, capacitors, and diodes to turn the relay into a T flip-flop. The one-of-N switch in Figure 1d allows you to engage any relay in an array of n (n=3 in this circuit) by activating the corresponding pushbutton switch; the circuit simultaneously disengages all the remnant n­1 relays. When you do not press a pushbutton, Q₁ and Q₃ are off, and Q₂ powers all of the coils at the voltage that R₁ and R₂ define. This voltage should be between the relay thresholds. When one of the buttons is depressed, Q₁ and Q₃ turn on, Q₂ turns off, and the only coil powered is the one corresponding to the depressed button. This relay configuration holds when you release the button. (DI #2016)

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