12-Bit 4-20 mA Input Adapter
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Using the Switched Input
The 12-Bit 4-20 mA Input Adapter can be used with continuously powered 4-20 mA transducers as well
as remote applications with external battery-powered 4-20 mA transducers. (See Figures 2 and 3.)
To take advantage of the switched input, the 4-20 mA transducer must meet the following criteria:
•
The sensor must be able to be powered from the 4-20 mA loop.
•
The sensor must have a warmup time of less than 300 ms.
Using the switched input can significantly reduce external battery consumption because the transducer is
powered only during warmup and sampling, as shown in the following diagram, instead of being powered
continuously.
Figure 5: Logging with switched input
Note
•
Using an external battery to drive the 4-20 mA transducer does not increase the battery life of the
logger (HOBO Weather Station or HOBO Micro Station).
Compared with continuously powering the 4-20 mA transducer, the average current drain is significantly
reduced. For an example of the power savings with and without using the switched input:
•
If the logger’s sampling interval is 60 seconds and the worst case current drain for the transducer
is 20 mA, then the average current drain will be:
transducer current × sample duration ÷ sampling interval
20 mA
×
0.327 s
÷
60 s
= 0.109 mA
•
If we assume that the transducer battery used has a useful capacity of 2000 mAh, the battery life
is:
battery capacity
÷ average current
2000 mAh
÷
0.109 mA
÷
24 hr/day
= 764 days
•
Without switched input, the battery life would be:
2000 mAh
÷
20 mA
÷
24 hr/day
= 4.1 days
Therefore, using the switched input allows the device to run approximately 186 times longer!
