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Page 263

| Figure 13-18 Basic photoelectric electronic relay circuit. |

| Transducers |

| Transducers are devices that convert nonelectric quantities into electric ones, and vice versa. Among them are the thermostats and thermocouples that actuate relays to control heating or protect equipment from heat. Microphones, moreover, convert sound to electric current, which is then employed to make recordings as well as to actuate signals or to sound alarms. Photocells convert light to electricity to control lighting circuits (Fig. 13-18). Other devices convert pressure or pressure differences to actuate relays that open or close valves and give indications or alarms. |

| Review |

| 1. Protective relays are low-powered electric devices used to activate high-powered devices (Fig. 13-1). Placed between the circuit or equipment and associated circuit breakers, they reflect a small but definite proportion of the power in the circuit or equipment; they sense and analyze abnormal conditions, causing circuit breakers to operate to relieve or protect the circuit or equipment. |

| 2. The relays may sense abnormal values of current, voltage, frequency, temperature, pressure, speed, as well as be sensitive to sound, light, and other indicators. Actions taken may be positive (starting), negative (stopping), balancing (a combination of the two), or informative (reporting the status of a specified condition). |

| 3. Three fundamental requirements include accuracy, sensitivity and selectivity. In general, they do this by measuring the magnitude, duration and direction of the electric currents flowing through them. These currents are usually received from current transformers, potential transformers, or both. |

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