Loop-Powered (2-Wire) 4-20 mA Transmitter Voltage Budget
Whether a loop-powered 2-wire 4-20 mA transmitter has enough voltage to operate. The transmitter needs a minimum terminal (compliance / lift-off) voltage -- commonly 8-12 Vdc -- and at the 20 mA top of range the loop supply must push that current through ALL the series resistance (the sense/load resistor, the round-trip wire, plus barriers) and still leave the transmitter its minimum. Max total loop resistance = (supply - transmitter minimum) / 0.020; voltage at the transmitter = supply - 0.020 x total series resistance. A 24 Vdc loop with a 250 ohm sense resistor and 50 ohm of wire (300 ohm) leaves 18 V -- fine above a 10.5 V minimum -- and could carry up to 675 ohm; a 600 ohm run (850 ohm total) starves it at 7 V and the loop reads wrong. DC worst case at 20 mA; the transmitter datasheet's compliance voltage and the barrier burden govern.
Formula and source
max_loop_resistance = (supply_v - transmitter_min_v) / 0.020; voltage_at_transmitter = supply_v - 0.020 x (load_resistance + wire_resistance); within_spec when voltage_at_transmitter >= transmitter_min_v.
Loop-powered (2-wire) 4-20 mA transmitter voltage-budget / maximum-load-resistance relation by name (transmitter compliance-voltage practice; Ohm's law at the 20 mA worst case); the transmitter datasheet governs.
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