1. Field of the Invention
The invention relates to a circuit arrangement for firing controlled, parallel-connected electric valves and, in particular, a circuit arrangement in which a control device for generating firing pulses is connected to the control path of at least one of two adjacent valves.
2. Description of the Prior Art
In firing parallel-connected electric valves, the problem of uniform division of the current arises. Typically, to obtain a uniform division of the current, selected valves from a suitably defined forward voltage group are employed. However, even when such valves are used, there a further problem arises. Immediately after firing the forward voltage across each valve may deviate to a larger or smaller degree from the defined static characteristic, due to the inductive behavior of the p-n junction. Thus, after firing, the total current divides over the parallel-connected valves according to their static characteristics only after a certain time has elapsed and a given current value has been reached. It is, therefore, typical to maintain the firing pulses for all valves for a time which is long enough to ensure that a uniform division of the total current is realized. For example, for currents over 300 A, such as might occur in converters for high-voltage d-c power systems (HGUe systems), firing pulses of 500 usec. duration may be employed. For currents below 300 A, firing pulses of a substantially longer duration in the area of 6 msec may instead be employed.
It is a primary object of the present invention to provide an apparatus for firing parallel-connected valves which is substantially immune to interference, employs a minimum of circuitry, and consumes little firing energy.