Loop Load Worked: 24 V Against 600 ohm and a Barrier

A 24 V supply leaves 12 V for a two-wire transmitter when 20 mA flows through a 600 Ω series load—before accounting for a barrier. If the barrier adds a separate 7.2 V drop, the calculated voltage available at the transmitter falls to 4.8 V. For a transmitter requiring at least 12 V, that is a 7.2 V shortfall: the proposed circuit cannot sustain 20 mA under those assumptions.

The barrier drop and transmitter minimum in this example are illustrative inputs, not verified specifications for an identified device. Without the actual barrier model, wiring arrangement, and transmitter requirements, 24 V and 600 Ω alone are not enough to determine whether the installed loop has adequate voltage.

Calculate the Voltage Available at the Transmitter

For a single series loop containing a DC supply, a two-wire transmitter, resistive loads, and a barrier represented by an additional voltage drop:

[
V_{\mathrm{available}}=V_S-I R_{\mathrm{external}}-V_B(I)
]

[
M=V_{\mathrm{available}}-V_{\mathrm{TX,min}}
]

Where:

• (V_{\mathrm{available}}) is the calculated voltage available across the transmitter at the requested current, in volts.
• (V_S) is the supply voltage at the loop feed under load, in volts.
• (I) is the requested loop current, in amperes; divide milliamperes by 1,000.
• (R_{\mathrm{external}}) is the total series resistance outside the transmitter, in ohms, excluding any resistance already included in the barrier-drop term.
• (V_B(I)) is the barrier voltage drop at that current, in volts.
• (V_{\mathrm{TX,min}}) is the transmitter’s specified minimum terminal voltage for the applicable configuration, in volts.
• (M) is the calculated voltage margin, in volts.

Include both outgoing and return cable resistance, the receiver burden, and other series loads. Emerson’s Rosemount 3051 Quick Start Guide, document 00825-0100-4001, Rev MG, Section 5.2 explicitly includes signal leads, controller loads, indicators, and intrinsically safe barriers in the loop-load assessment. Its device-specific voltage limits must not be substituted for those of an unidentified transmitter.

Keep the signed calculation. A negative result indicates that the assumed drops exceed the supply at the requested current; replacing it with zero hides the size of the deficit. Neither a negative result nor an insufficient positive result predicts the actual terminal voltage after current regulation is lost.

Worked Example: 24 V Supply and 600 Ω Load

Use the following illustrative inputs:

First calculate the resistive drop:

[
V_R=\frac{20.0}{1000}\times600=12.0\ \mathrm{V}
]

Then subtract both external drops:

[
V_{\mathrm{available}}=24.0-12.0-7.2=4.8\ \mathrm{V}
]

Compare that result with the assumed transmitter minimum:

[
M=4.8-12.0=-7.2\ \mathrm{V}
]

The voltage budget is insufficient at 20 mA. The 4.8 V result is the voltage allocation calculated at the requested current, not a guaranteed field reading: the transmitter cannot be assumed to maintain that current below its specified operating voltage.

Results are shown to the nearest 0.1 V, with no intermediate rounding. That display precision does not imply corresponding accuracy in the supply, resistance, or barrier inputs.

Why a Low-Current Check Can Miss the Problem

For comparison only, hold the illustrative barrier drop constant at 7.2 V across the signal range:

This simplified budget is sufficient at 4 mA but insufficient at 12 mA and 20 mA. A successful low-end check therefore does not establish adequate voltage throughout the signal range.

Use the actual barrier’s current-dependent characteristics rather than assuming a constant drop. For example, the Pepperl+Fuchs Z886 datasheet, issued April 6, 2023, lists a voltage-drop expression of (1.2\ \mathrm{V}+(36\ \Omega\times\text{signal current})). That expression belongs to the specified device and connection arrangement; it does not establish the 7.2 V input used above.

Avoid Counting the Barrier Twice

A barrier can be represented by a resistance, a voltage-drop expression, or a manufacturer-specified field-side supply characteristic. Use the representation appropriate to its circuit and terminals.

If a barrier’s documented drop already includes its internal resistance, do not also add that resistance to (R_{\mathrm{external}}). Conversely, if the stated 600 Ω already includes the barrier’s resistive contribution, adding a separate drop containing that same contribution overstates the voltage loss.

For a powered, galvanically isolated transmitter supply, assess the field-side circuit against its specified available voltage and load limits. Do not automatically treat its power-supply terminals, field terminals, and control-system terminals as one series loop.

The barrier’s exact model and terminal arrangement are therefore necessary inputs—not optional details.

Interpret the Margin Before Choosing a Change

The calculated margin distinguishes three conditions:

• (M<0): the requested operating point is unsupported by the stated voltage budget.
• (M=0): the budget reaches the assumed minimum exactly, with no allowance for variation.
• (M>0): the stated inputs leave voltage headroom, but adequacy still depends on worst-case conditions and device requirements.

Use minimum loaded supply voltage, maximum applicable resistance, and the barrier’s specified worst-case drop. Check the maximum current required by the actual configuration, including any relevant overrange or high-alarm condition; do not assume that 20 mA is always the design maximum.

For the illustrative 20 mA case, the supply voltage required merely to reach zero margin is:

[
V_{S,\mathrm{required}}
=V_{\mathrm{TX,min}}+I R_{\mathrm{external}}+V_B
=12.0+0.0200\times600+7.2
=31.2\ \mathrm{V}
]

Alternatively, retaining the 24.0 V supply and the same assumed barrier drop gives a maximum external resistance at zero margin of:

[
R_{\mathrm{external,max}}
=\frac{V_S-V_{\mathrm{TX,min}}-V_B}{I}
=\frac{24.0-12.0-7.2}{0.0200}
=240\ \Omega
]

These are mathematical boundaries, not recommended settings. Raising the supply may exceed equipment or installation limits; reducing resistance may conflict with receiver or communication requirements; changing a barrier requires a separate compatibility and hazardous-area assessment. There is no universal “supply first, resistance second, barrier third” correction order.

Check the Budget in the Field

Before forcing current or disturbing wiring, obtain authorization and place the process in the state required by the site procedure. Loop-test outputs can affect control actions and protective functions. Follow the installed equipment’s access restrictions; the Rosemount 3051 guide, for example, prohibits removing transmitter covers while energized in an explosion-proof/flameproof installation.

1. Identify the actual circuit. Record the transmitter model and output option, receiver input type, barrier model and terminals, supply arrangement, and cable route.
2. Confirm the inputs. Obtain the applicable transmitter voltage requirement, barrier characteristics, receiver burden, and required current range from the installed equipment documentation.
3. Measure under load. At authorized test points, measure the loop-feed voltage and the voltage directly across the transmitter while recording actual current.
4. Check low, middle, and high current where permitted. Include any additional manufacturer-required design point. If actual current fails to reach the requested value, do not calculate component drops as though the requested current were flowing.
5. Locate the discrepancy. Compare measured drops across the receiver, cable path, barrier, and other series devices with their documented or calculated values. Include any added test instrument’s burden.
6. Recheck after an approved change. Confirm terminal voltage and current over the required operating conditions, then restore the loop according to the site procedure.

A voltage-budget check can identify inadequate electrical headroom. It does not establish transmitter calibration, measurement accuracy, intrinsic-safety compliance, or successful verification of a safety function.