Campbell CR800 Dokumentacja Pobierz pdf (Strona 62)

Section 1. Installation and Maintenance

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Make certain that the primary source of power for the CR800 can sustain the

current drain for the period of time required. Contact a CSI applications

engineer for help in determining a power budget for applications that approach

the limits of a given power supply’s capabilities. Be particularly cautious

about any application using solar panels and cellular telephone or radio,

applications requiring long periods of time between site visits, or applications

at extreme temperatures.

TABLE 1.8-2. Typical Current Drain for Some CR800 Peripherals

Typical Current Drain (mA)

Peripheral Quiescent Active

AM25T .5 1

COM210 Phone Modem 0.0012 140

SDM-INT8 0.4 6.5

1.9 Controlling Power to Sensors and Peripherals

Controlling power to an external device is a common function of the CR800.

Many devices can conveniently be controlled with the SW-12 (Switched 12

Volt) terminals on the CR800. Table 1.8-1 shows the current available from

SW-12 port.

Applications requiring more control channels or greater power sourcing

capacity can usually be satisfied with the use of Campbell Scientific’s

A21REL-12 Four Channel Relay Driver, A6REL-12 Six Channel Relay

Driver, SDM-CD16AC 16 Channel AC/DC Relay Module, or by using the

control (C1-C4) ports as described in Section 1.9.1

1.9.1 Use of Digital I/O Ports for Switching Relays

Each of the eight digital I/O ports (C1 – C4) 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.

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Campbell CR800 Dokumentacja Strona 62