4-20 mA 回路供电预算
检查 4–20 mA 回路离开变送器时的电压。
Reads AS/NZS 3008.1.2:2017 (NZ) and AS/NZS 3008.1.1:2017 and 2025 (AU). Tables checked 27 Sept 2026.
尝试一个工程 6
每项工作都会预填示例数值。请替换为您自己的数值。
- Check a 4–20 mA loop's power budgetThe supply, the series resistance and the cable give the voltage left at the transmitter.
- Check a loop with a HART resistorThe voltage left with a 250 Ω HART resistor in the loop.
- Check a loop through a barrierThe voltage left with 300 Ω of barrier in series.
- Check a 2 km loopThe voltage left on 1 mm² cable over 2000 m.
- Check a loop on a 12 V supplyWhether a 12 V supply drives the loop.
- Check a loop at fault currentThe voltage left at 22 mA.
输入
使用默认输入值的示例计算。
- 环路供电电压
- 24 V
- 变送器最低电压
- 12 V
- 回路电流
- 20 mA
- 输入电阻
- 250 Ω
- HART 电阻
- 0 Ω
- 其他串联电阻
- 0 Ω
- 电缆单程长度
- 500 m
- 导体电阻
- 来自导体规格
- 导体规格
- 1 mm²
- 标准
- NZ · AS/NZS 3008.1.2
电压余量
6.48V
- 变送器处的电压
- 18.48V
- 回路电阻
- 275.8Ω
- 电缆电阻
- 25.8Ω
- 最大回路电阻
- 600Ω
- 最大电缆长度
- 6,782m
此结果仅供设计参考。请对照现行标准进行核验。
计算步骤
- 导体电阻25.8Ω/km
AS/NZS 3008.1.2:2017 Tables 34 to 35
- 两根导体的电缆电阻25.8Ω
- 串联电阻250Ω
- 环路电阻275.8Ω
- 环路电流20mA
- 回路中的电压降5.52V
- 变送器处的电压18.48V
- 电压裕度结果6.48V
- 最大环路电阻600Ω
- 最大电缆长度6,782m
关于此计算器
How the tool checks the loop
A 4–20 mA loop is one series circuit. The loop supply drives the current through the transmitter, the cable and the receiver input. The transmitter needs a minimum voltage at its terminals to work.
- Conductor resistance. The tool reads the resistance of a copper conductor for the size you pick. It can also take the value from the cable data.
- Cable resistance. The current goes out on one conductor and comes back on the other. So the tool doubles the one-way length.
- Loop resistance. It adds the cable resistance to the input resistance, the HART resistor and any other series parts.
- Voltage at the transmitter. It multiplies the loop current by the loop resistance and subtracts that drop from the supply voltage.
- Voltage margin. It subtracts the transmitter minimum voltage. A margin of 0 V or more passes.
- Maximum length. It gives the longest one-way run that keeps the margin at 0 V or more.
Worked loops
Each line opens the tool with its values. Every line uses a 24 V supply, a 12 V transmitter minimum voltage and a 250 Ω receiver input.
- Pressure transmitter, 500 m of 1 mm² at 20 mA. The loop passes with a margin of 6.48 V.
- HART loop, 400 m of 1 mm² at 22 mA. A 250 Ω HART resistor sits in the loop. The loop passes with a margin of 0.55 V.
- HART loop, 500 m of a 36 Ω/km pair at 22 mA. The resistance comes from the cable data. The loop passes with a margin of 0.21 V.
- The same HART loop at 700 m. The loop fails with a margin of −0.11 V. Shorten the run, use a larger conductor or raise the supply.
New Zealand and Australia
The Standard select picks the part of AS/NZS 3008.1 the tool reads for the conductor resistance. Part 1.2 is the New Zealand part, 2017 edition. Part 1.1 is the Australian part. For Australia, the Edition select picks 2025 or the superseded 2017 edition. All three print the same resistance values. The impedance table entry says how AmpSize checked them.
Related checks
- The comms cable selector checks the cable construction and the run length for the interface.
- The DC cable size calculator sizes the 24 V d.c. supply cable to the transmitter panel.
- The segregation checker checks the spacing between the loop cable and power cables.
问题
结果仅供设计参考。请依据现行标准核对。
什么决定 4–20 mA 回路的最长线路？
供电电压减去变送器的最低电压，即为可用预算。流经各串联电阻的回路电流会消耗这部分预算。电缆会增加两根导体的电阻，因此线路越长或导体越细，消耗越多。
为何要在超过 20 mA 时进行检查？
变送器可输出超过 20 mA 的电流，例如接近 22 mA 的超量程报警。回路压降随电流增大。请按变送器可输出的最大电流进行检查。
在哪里可以找到变送器的最低电压？
变送器数据表会给出该值，通常为起作用电压或最小端子电压。部分数据表给出的是负载线。应读取最大回路电流下的电压。
背景
本工具读取的条款和表格
- AS/NZS 3008.1.2:2017 Table 30, 34, 35Reactance at 50 Hz and a.c. resistance at 50 Hz
- AS/NZS 3008.1.1:2017 Table 30, 34, 35Reactance at 50 Hz and a.c. resistance at 50 Hz
- AS/NZS 3008.1.1:2025 Table 4.1(A), 4.1(B), 4.5(A), 4.5(B)Reactance at 50 Hz and a.c. resistance at 50 Hz
表格最后核对于 27 Sept 2026。
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