Campbell Scientific CR5000 CR5000 Measurement and Control Module - Page 35
Use of Digital I/O Ports for Switching Relays, Installation and Maintenance
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Section 1. Installation and Maintenance 1.9.1 Use of Digital I/O Ports for Switching Relays Each of the eight digital I/O ports can be configured as an output port and set low or high (0 V low, 5 V high) using the PortSet or WriteIO instructions. A digital output port is normally used to operate an external relay driver circuit because the port itself has a limited drive capability (2.0 mA minimum at 3.5 V). Figure 1.9-1 shows a typical relay driver circuit in conjunction with a coil driven relay which may be used to switch external power to some device. In this example, when the control port is set high, 12 V from the datalogger passes through the relay coil, closing the relay which completes the power circuit to a fan, turning the fan on. In other applications it may be desirable to simply switch power to a device without going through a relay. Figure 1.9-2 illustrates a circuit for switching external power to a device without going through a relay. If the peripheral to be powered draws in excess of 75 mA at room temperature (limit of the 2N2907A medium power transistor), the use of a relay (Figure 1.9-1) would be required. Other control port activated circuits are possible for applications with greater current/voltage demands than shown in Figures 1.9-1 and 2. For more information contact a Campbell Scientific applications engineer. FIGURE 1.9-1. Relay Driver Circuit with Relay 1-11