Direct current relay capable not only of sensing whether or not a power supply exists, but also of distinguishing polarity. It is a neutral relay made sensitive to the power supply polarity; this feature can be obtained in two different ways: with a magnetic device or with a rectifier. The first case, less used, involvesthe insertion of a permanent magnet; it is arranged so that the flow produced by it can be added or subtracted from the flow produced by the relay coil; this happens in the sense of the current itself. The windings must be arranged so that the mobile crews cannot move due to the action of the permanent magnet only, but only if the two flows add up. The excitation of the relay in this way will only occur if the coil is crossed by a current in a certain established direction. The second case involves the insertion in the circuit of a rectifier diode mounted in series with the coils; the diode allows the current to flow in one direction only (from the positive pole to the negative pole). It is generally used to carry out commands, through its contacts, different according to the polarity of its supply voltage (positive or negative), while if this voltage is zero, no command is carried out. See Relay FS
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