A 1-to-5 volt DC signal turns up constantly on voltage input cards and at analog multiplexers, and it is really a 4-to-20 milliamp current loop in disguise. The two are linked by a single precise resistor, and understanding that link explains why 1 volt, not zero, is the bottom of the range and why the scaling math is exactly the same once you map the input endpoints. This guide covers how the 1-5 V signal is created, why the live zero sits at 1 volt, how the scaling arithmetic mirrors the current-loop case, and the offset errors that creep in from resistor tolerance and card configuration.
1-5 VDC signal scaling in one line: A 1-5 VDC signal is produced by passing a 4-20 mA current loop through a precise 250-ohm resistor, so the voltage across the resistor tracks the current: 4 mA yields 1 V and 20 mA yields 5 V. The 1 V floor is the live zero, the deliberate offset that lets a broken loop reading near 0 V be told apart from a valid minimum. Scaling is identical to the current case once you map the input range, so 1 V becomes the low engineering value and 5 V the high.
The 1-to-5 volt signal is not an independent standard so much as a voltage picture of a 4-to-20 milliamp current. By Ohm's law, a current flowing through a resistor produces a voltage across it equal to the current times the resistance. Drop a 4-to-20 mA loop across a 250-ohm resistor and the endpoints fall out arithmetically: 4 mA through 250 ohms is 1 volt, and 20 mA through 250 ohms is 5 volts, with everything in between mapping proportionally. This is why 250 ohms is the classic value; it is chosen precisely so that the standard current range lands on a clean 1-to-5 volt range that a voltage input can read.
The reason the bottom of the range is 1 volt rather than zero is inherited directly from the 4 mA live zero of the current loop. A 4-to-20 mA signal starts at 4 mA rather than 0 mA so that the minimum valid reading is a positive, powered current, which lets a genuine zero, a broken wire or a dead transmitter, be distinguished from a legitimate low reading. When that current is turned into a voltage, the 4 mA floor becomes a 1 volt floor, so the 1 V at the bottom carries the same diagnostic meaning: a signal sitting near 0 V is not a valid minimum, it is a fault.
This inheritance is the whole point of preferring 1-5 V over a 0-5 V or 0-10 V voltage signal in industrial work. A voltage signal that starts at zero cannot distinguish a true low measurement from a dead or disconnected sensor, because both read zero. The 1 V live zero preserves the current loop's most useful property, its built-in fault detection, in a form a voltage input can accept, which is why so much voltage-based field signaling in fact rides on a resistor-terminated current loop underneath.
Engineering-units scaling does not care whether the raw signal is a current or a voltage; it cares only about the two endpoints of the input range and the engineering values they represent. For a 4-to-20 mA signal you anchor 4 mA to the low engineering value and 20 mA to the high, then interpolate. For a 1-to-5 V signal you anchor 1 V to the low engineering value and 5 V to the high, and interpolate exactly the same way. The slope and the arithmetic are structurally identical; only the input endpoints change from milliamps to volts.
Because the two ranges are proportional images of each other, a channel scaled for one converts trivially to the other. If a transmitter maps 4-to-20 mA onto 0-to-150 psi, the same 0-to-150 psi maps onto 1-to-5 V through the resistor, so a card reading voltage uses 1 V for zero psi and 5 V for one hundred fifty psi and produces the same pressures. This is why datasheets often quote a range against both the current and its voltage equivalent: they are the same measurement expressed through a 250-ohm resistor. The practical rule is to enter the endpoints in whatever unit the card actually reads and let the two-point line do the rest.
The one thing you must get right is which quantity the card is actually measuring. A card configured for current input expects to carry the loop and measures milliamps directly. A card configured for voltage input expects the resistor to already be dropping the current to a voltage and measures volts. Enter voltage endpoints for a current card, or current endpoints for a voltage card, and the scaling is off by the ratio between the two, even though the line equation itself is written correctly. The math is the same; matching the endpoints to the card's real input type is the discipline.
Whether a channel should be set up as voltage or current comes down to where the loop current is being converted to a voltage. Some cards read current directly and internally handle the loop; others are voltage cards that require an external or internal 250-ohm resistor to produce the 1-to-5 V they read. Getting this wrong is a common commissioning mistake: a voltage card with no terminating resistor sees essentially no signal, and a current card fed a voltage does not behave as expected. The card's configured input type, the presence of the resistor, and the wiring must all agree, or the channel reads nonsense from the start rather than a subtly wrong value.
The subtler error is offset from resistor tolerance. The clean 1-to-5 V endpoints depend on the resistor being exactly 250 ohms; a real resistor is only close, within some tolerance, and it drifts a little with temperature. If the actual resistance is slightly high or low, the voltage produced for a given current is slightly off, which shifts and tilts the whole scaled reading by a small proportional amount. This is why precision, low-tolerance, low-drift resistors are used for this job, and why a channel that reads a touch high or low across its whole range, without any wiring fault, may simply be reflecting a resistor that is not exactly on value.
In a cloud SCADA architecture such as Merobix, the platform receives whatever engineering value the edge device has already scaled, so the current-versus-voltage decision and the resistor accuracy live in the field and the RTU or card configuration, not in the cloud. What the platform can do is make verification easy: keeping each tag's configured input range and the raw signal visible alongside the engineering value lets an analyst confirm that a channel is reading the range it should. If a reading is consistently off by a small percentage with no fault, comparing it against a known physical condition, a gauged pressure or a measured level, quickly points to a scaling endpoint mismatch or a resistor that is not exactly 250 ohms, both of which are corrected at the source.
Anchor 1 V to the low engineering value and 5 V to the high engineering value, then linearly interpolate every voltage in between, exactly as you would anchor 4 mA and 20 mA for a current loop. The math is the same two-point line; only the input endpoints change from milliamps to volts. Make sure the card is configured for voltage input and, if required, has its 250-ohm terminating resistor in place.
Because the 1 V floor is the live zero inherited from the 4 mA start of a 4-20 mA loop. A signal that started at 0 V could not distinguish a true minimum reading from a dead or disconnected sensor, since both would read zero. The 1 V floor keeps that fault-detection property: a reading near 0 V is not a valid minimum, it indicates a broken loop or a failed device.
A precise 250-ohm resistor placed in the loop. By Ohm's law the voltage across it equals the current times the resistance, so 4 mA through 250 ohms is 1 V and 20 mA is 5 V, with everything in between proportional. Because the resistor's exact value sets those endpoints, a low-tolerance, low-drift resistor is used; a resistor that is not exactly 250 ohms shifts the whole reading by a small proportional offset.
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