| 摘要 |
PURPOSE:To protect the detecting circuit of a high input impedance from an abnormal voltage by providing a diode in the input end of the detecting circuit and operating it in the backward bias state normally and operating it in the forward bias state at the abnormality time. CONSTITUTION:A resistance Rx is provided between two points P1 and P2 in a conductive liquid; and when the AC output of an OSC is 1V, about 0.95V is impressed to the input of an FET in case of presence of ink because R1 is a high resistance; and in case of absence of ink, Rx is infinity and V0 is 0. However, the impedance at a point A is high, and consequently, the noise is easily flowed into this point to break the FET frequently, and the reliability is lowered. For the purpose of eliminating this blemish, diodes D1 and D2 are aded to the input of the FET, and power is flowed through the diode D1 when a + abnormally high voltage is impressed to the point A, and the diodes D2 is turned on to earth the input of the FET when a - abnormal voltage is impressed to the point A. Thus, the breakdown of the FET is prevented. |
