Campbell CURS100 Bedienungsanleitung PDF herunterladen (Seite 13)

User Manual

Figure 4-2. 2-wire with external power

4.1.1 Possible Ground Loop Problems

The resistor must be grounded at the datalogger to ensure that measurements are

within common mode range. The signal (or low) output on the transducer is

higher than the datalogger ground by the voltage drop across the resistor. A

ground-loop error may occur if the signal output is not electrically isolated but is

connected to the sensor's case. If such a sensor is in contact with earth ground (for

example, a pressure transducer in a well or stream), an alternative path for current

flow is established through earth ground to the datalogger earth ground. This path

is in parallel with the path from the signal output through the resistor; hence, not

all the current will pass through the resistor and the measured voltage will be too

low.

4.1.2 Minimum Supply Voltage

When the power supply is in the current loop, as is the case in a 2-wire transducer,

it is necessary to consider the effect of voltage drop across the resistor on the

voltage applied to the transducer.

For example, suppose a 4 to 20 mA transducer requires at least 9 volts to operate

correctly and the system is powered by a 12 volt battery. The voltage the

transducer sees is the battery voltage minus the voltage drop in the rest of the

current loop. At 20 mA output, the voltage drop across the 100 ohm resistor is 2

volts. When the battery is at 12 volts, this leaves 10 volts for the transducer and

everything is fine. However, if the battery voltage drops to 11 volts, a 20 mA

current will leave just 9 volts for the transducer. In this case, when the battery

drops below 11 volts, the output of the transducer may be in error.

Figure 4-3. 2-wire supply voltage

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Campbell CURS100 Bedienungsanleitung Seite 13