| 摘要 |
A protective electronic system consists of several power resistor, R last. Each circuit contains two thyristors (A,B) controlled by a bridge rectifier (1), with two transformers (2,3), their secondary windings (a,a',b,b') being connected to the thyristor gates. A control DC voltage (+Ust) is applied, through a photocoupler (5) to a voltage dioider with two parallel paths (10,12) and two auxilliary thyristors (13,14), connected to the bridge terminals (+,-). The main thyristors (A,b) are fired only when the load voltage (Ul) is smaller than the voltage on the dioider. The switching state of the circuit is determined with a supplementary circuit (S), connected to the bridge terminals (+,-), via a photocoupler V18 and a LED (V17), in seives with a resistor (R19). One terminal of the circuit is connected to a DC voltage (+UB) while the other (D) is used for detecting the switching state. When the voltage UL is bigger than the voltage on the dioider, a current (iv18) passes through detecting circuit, transistor U21 is blocked and consequently there is no voltage at the terminal D. If the control voltage (+USt) is big enough to fire the thyristors, the current iv18 becomes nuel, the transistor V21 switches to conduction and therefore a voltage is detected at terminal D.
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