The constant current source (4) uses at least 2 elementary sources (5) connected in cascade, each having a transistor (1) acting as a controlled resistance, in series with a feedback resistance (6), with a voltage-dependant resistance (7) in parallel across the series resistances. The transistor is controlled via an optocoupler (10), with a voltage source (12) connected in parallel with the series resistances on its output side, together with a further parallel resistor (11), for providing a controlled voltage divider. The voltage source has an annular core transformer (3) with its secondary winding coupled via a diode (21) to a capacitor (22) and a parallel voltage-limiting element (23). The transformer primary to provided by a current loop (13) coupled to a pulse generator (16).
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