Abstract: A modulator for injecting signals into a polyphase power transmission system includes a bridge of four thyristors for each phase. The thyristors of each bridge are connected so that when the thyristors of two opposite arms are ignited simultaneously they produce a signal at the output from the modulator of opposite sense to the corresponding signal when the thyristors of the two other opposite arms are ignited. The thyristors of the adjacent arms are controlled in opposition and coupled by an induction coil having two windings which, through the back EMF generated by mutual inductance, causes the extinction of one of them when the other is ignited. The energy stored in the induction coil at the time of the extinction phase is dissipated as heat in a resistor provided for this purpose in a loop which does not contain the supply.