HYSTERESIS A SIMPLE EXPLANATION

EDITOR: B. SOMANATHAN NAIR

The word hysteresis has been derived from Greek and means lagging behind. The term was coined by Ewing around 1881 to describe the relation between magnetic flux density B and magnetic field intensity H. It was found that when H is zero, B is not zero. Similarly, when B is zero, H is not zero. Thus one lags behind the other. The B-H curve obtained by plotting the corresponding values of B against values of H is called as the magnetic hysteresis loop. Figure 1a shows a typical B-H curve (magnetic hysteresis loop).

       

        Even though hysteresis was described as a phenomenon seen in magnetic domain, it has been found that it exists in many areas of engineering and science. For example, we find hysteresis in Schmitt trigger circuits (Fig. 1b). A Schmitt trigger circuit gets triggered when its input voltage V_in is greater than or equal to its upper trigger (trip) point (UTP). Then the output voltage will be +V volts. The circuit will switch back to ‒V, when the input V_in is less than UTP. This means that UTP will always be greater than LTP and they can never be equal. Hence Schmitt trigger circuit will always exhibit hysteresis.

To illustrate this further, consider a semiconductor silicon diode being forward biased. The diode will start conducting when its anode-cathode voltage is typically 0.5 V (Fig. 2). We find that as long as the anode voltage is greater than or equal to 0.5 V, the diode is in its ON state. It can be seen that it can be turned off only when its anode voltage is less than 0.5 V (Fig. 3) because at 0.5 V, it is still in the ON state. It further means that the ON and OFF voltages can never be the same and are different. This gives rise to hysteresis.