| 摘要 |
High-voltage direct or alternating currents, particularly heating currents in passenger train coaches, are to be monitored with isolated potential and errors are to be indicated. To achieve this object, it is proposed to control an optocoupler (8) directly via the voltage drop across an indicator resistor (5) through which current to be monitored flows. For this purpose, the receiver transistor (10) of the optocoupler is connected to an evaluating circuit (11) which switches off a LED display diode (18) in dependence of the flowing of the current in the indicator resistor (5). To monitor alternating currents, the evaluating circuit (11) is additionally associated with a timing element (15) which delays a lighting-up of the LED display diode (18) in the half periods of the current in which the optocoupler (8) is blocked depending on the direction of the current. In arrangements having a control contactor for additional current or temperature control, the normally-open contact of which is connected in series with the heating groups to be switched, an auxiliary contact (3b) of the control contactor (3) bypasses, when the current is operationally interrupted by the normally-open contact (3a), the receiver transistor (10) of the optocoupler (8), which is then cut off, the base current being supplied to a control transistor (13) of the evaluating circuit (11) via a parallel path. <IMAGE> |
