Campbell Scientific CR3000 Micrologger - Manual (Page 334)

Section 8. Operation
334
8.2.4.3 Component-Built Relays
Figure Relay Driver Circuit with Relay
(p. 334)
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 Vdc
from the datalogger passes through the relay coil, closing the relay which
completes the power circuit and turns on the fan.
In other applications it may be desirable to simply switch power to a device
without going through a relay. Figure Power Switching without Relay
(p. 334)
illustrates a circuit for switching external power to a device without using 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 is required.
Figure 109: Relay driver circuit with relay
Figure 110: Power switching without relay