| 摘要 |
<p>A safety circuit for a temperature- regulated heating device permits 'power heating' during heating up but thereafter automatically switches to 'low- power heating' to prevent over-heating in the event of defects. A generator (ZG) supplies triggering signals to the triac (Tr) but a control generator (SG) blocks the generator (ZG) in every second half-wave of the a.c. supply voltage to provide 'low-power heating'. In 'power heating', blocking of signal generator (ZG) is eliminated and a desired value voltage determined by temperature regulator (TR) is raised. A comparator (V3) responsive to the temperature desired value generator (SWG) and an actual temperature sensor (F) switches storage means (SP) between the two states. A capacitor (C7) is charged during 'low-power heating' only if current flows through the heater during those half-waves in which the triggering signal is blocked and simultaneously discharged more slowly than charging so that if due to a fault a predetermined charge is reached, the circuit breaker (S) is opened to disconnect the supply. <IMAGE></p> |
