PURPOSE: To prevent the mechanical breakdown of an oxygen ion conductive solid electrolyte, by controlling the polarity and the density of a current supplied between the electrodes of an oxygen sensor on the basis of the potential difference between said electrodes.
CONSTITUTION: When a formula Ea>Eb, that is Vab>o is held, an analogue switch 12 is changed over to the side of the solid line in the drawing and DC voltage e1 is applied to an electrode 3 while a current I flowed to an oxygen sensor 1 becomes small as the output of the oxygen sensor 1 becomes large and the oxygen partial pressure P in the electrode is suppressed to a low level. On the other hand, when a formula Ea<Eb, that is, Vab<o is formed, the analogue switch 12 is changed over to the side of the broken line in the drawing while DC voltage e2 is applied to the oxygen sensor 1 through a resistor R3 and the current I flowed to the oxygen sensor 1 is reversed to flow a current -I in the reverse direction and the density thereof is changed by a Vab value. Therefore, the breakdown of a solid electrolyte 4 due to the excessive rising in oxygen partial pressure is not induced.
SONE KIMITAKE
KITAHARA TAKESHI
