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Page 327

| Contact or disc rectifier. Both copper oxide and selenium rectifier units are arranged in stacks with a specified number of them connected in series, depending on voltage requirements. They are usually assembled on an insulated bolt (Fig. 16-17). The units are designed to have radiating fins and spacers between them for cooling purposes. For most applications, a natural circulation of air is sufficient. For rectifiers of about 500-watt capacity, however, fans provide forced cooling. |

| Operation |

| The operation of rectifiers has already been described in general terms. Banks of mercury-arc and contact-disc types, are used to supply capacity requirements. Standard breakers and protective equipment can be used with mercury-arc rectifiers. Breakers on both the ac and dc sides should be trip-free throughout the closing cycle; their tripping times, including arcing and relay time, should be 8 to 12 cycles. For the sake of economy, it is possible to use a multiple breaker scheme to open the associated transformer secondary circuit to each anode. For smaller units, fuses are sometimes specified. |

| Before mercury-arc rectifiers are placed in service, or after long periods of idleness, they must be heated out. Heat is applied to the container to drive out any moisture and entrapped gasses that might affect the operation of the rectifier. More specifically, this precaution is taken to prevent arc back, |

| Figure 16-16 Ignitron tube: (A) Cross section and schematic. |

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