PURPOSE: To enable to correctly detect the temperature of preglow plugs used in a diesel engine or the like, by heating the preglow plugs continuously until they are heated to a temperature near a prescribed value, and then heating them intermittently so that they are used for detection of the temperature when they are not heated.
CONSTITUTION: When a start signal is given, a switching element control circuit 11 gives a timing pulse TP to a constant-current circuit CC and passes a constant current to preglow plugs GP. The voltage (et) dropped through temperature rise of the preglow plugs is compared with a reference value (ec) at a disconnection detecting circuit 12. If (et) is higher than (ec)., a control circuit 14 is prevented from producing a relay driving signal PS. On the other hand, if (et) is lower than (ec), the preglow plugs GP are heated by closing a relay RL. At the same time, a time-width setting circuit 13 produces a time-width signal t0 corresponding to a power source voltage V0 and opens the relay RL after passing of the time t0. Further, a transistor TR is turned ON and OFF alternately for each period by the drive signal DV produced by the circuit 11, and the voltage drop of the preglow plugs is measured by the constant current of the circuit CC when the transistor TR is OFF. The voltage drop is compared with a reference voltage Vs at a comparison circuit 10, and production of an ON-pulse is prohibited at Vt/Vs. Thus, since the temperature is measured when the preglow plugs are not heated, the measurement is determined to be correct.
OOSAWA MASAHIRO
KASATANI JITSUO
FUJI ELECTRIC CO LTD
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