Hach Sigma 1600, Instrument Manual

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Page 18
4–20 mA Input Option
6507installation.fm
Section 2
2.5 4–20 mA Input Option
The 4–20 mA (1–5 V dc) circuit in the sampler is a basic voltage to frequency
converter with a 249 ohm load resistor across the input. Parallel loop wiring or
series loop wiring can be used when wiring multiple units (receivers) in a
control loop.
Parallel Loop Wiring
Parallel loop wiring is the most commonly used approach in interconnecting
electronic receivers (sampler, pump, etc.), where the same voltage feeds
several receivers that are electronically connected in parallel. There will be no
adverse effects on the receivers remaining in the loop. In order that the
addition or deletion of receivers from the parallel loop does not affect
accuracy, the receiver must have a high input resistance. A single common
earth ground is recommended.
Example:
Referring to Figure 6 , the resistor converts the two wire transmitter 4–20 mA
dc output to a 1–5 V dc signal to actuate receivers A, B, and C. Receivers B
and C may be removed or added without interrupting the transmitter output or
significantly affecting the voltage signal.
Figure 6 Parallel Loop Wiring
Series Loop Wiring
Series loop wiring consists of electrical connections in a series that use the
same current to feed various receivers. If the addition or deletion of certain
receivers from this series loop is not to affect calibration, then the source must
be a true current source, i.e. a two wire transmitter. This current loop with
several receivers in a series can be grounded only at one point. More than
one ground can cause a short in input receivers. If either current sensitive
receivers A or B are removed from the loop, the current path is interrupted and
the other receivers will no longer have an input signal (see
Figure 7 ).
The addition of a diode allows removal of receiver B without affecting the loop.
The normal voltage drop across receiver B does not allow the diode to
conduct (see
Figure 7 ). With removal of the receiver, the diode conducts at a
smaller voltage drop. It is recommended to use a true current source rather
than a true voltage source for installations requiring a long distance between
the transmitter and receiver.