SIGNALS & CONTROL / LAB 01
4–20 mA transmitter & loop
Follow the signal. Find where the numbers stop agreeing.
VIRTUAL INSTRUMENT BENCH
Follow the live signal
Process connection → electrical current → engineering indication
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Change the process value to observe the response.
Arrows show signal progression, not electron motion. Functional diagram; not a wiring schematic.
PUT IT TO WORK
Diagnostic challenge
Load a scenario, inspect the readings, then choose where to investigate first. Fault controls are hidden during a challenge.
Model assumptions & technical references
This is a linear, direct-acting, steady-state model. URV must exceed LRV. Numeric entries are bounded to ±1,000,000; process values may exceed the configured range. Units are labels: changing a unit does not convert entered numbers.
Process mode limits output to 3.8–20.5 mA as an explicit teaching convention. Real alarm/saturation settings depend on the instrument. Below 3.8 or above 20.5 mA, this receiver marks quality BAD and suppresses engineering indication; raw arithmetic remains visible. A real DCS may clamp, hold last value or use configured alarm limits. Manual and stuck modes allow 0–24 mA.
Open loop means a broken series path (0 mA actual current). The transmitter number is a command, not proof of current flow. Short / low current specifically models a bypass across the receiver sensing resistor: series current continues but input current is zero. No supply voltage, wire resistance, compliance, HART, noise, square-root extraction or process dynamics are modeled.
Zero/span faults apply only in process mode. Wrong DCS scaling loads a receiver span at half the transmitter span; endpoints remain editable. Faults are selected individually. Challenges are learning exercises; the readings do not uniquely identify every real-world fault.
NI: 4–20 mA current loop fundamentals · Emerson: transmitter saturation and alarm behavior
READ THE EVIDENCE
What’s happening?
Initializing the workbench…
Formula
mA = 4 + ((PV − LRV) / (URV − LRV)) × 16
PV = LRV + ((mA − 4) / 16) × (URV − LRV)
Field check
Compare an independent process reference, measured series current and receiver indication. A plausible engineering value can still be wrong.
Use your site’s approved isolation and test procedures. Breaking a live loop can affect plant operation.