Silly simple precision 0/20mA to 4/20mA converter
This Design Idea (DI) offers an alternative solution for an application borrowed from frequent DI contributor R. Jayapal, presented in: “A 0-20mA source current to 4-20mA loop current converter.”
It converts a 0/20mA current mode input, such as produced by some process control instrumentation, into a standard industrial 4/20mA current loop output.
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Figure 1 shows the circuit. It’s based on a (very) old friend—the LM337 three-legged regulator. Here’s how it works.
Figure 1 U1 plus R1 through R5 current steering networks convert 0/20mA input to 4/20mA output.
The fixed resistance of the R1 + R2 + R3 series network, working in parallel with the adjustable R4 + R5 pair, presents a combined load of 312 ohms to the 1.25v output of U1. That causes a zero-input current draw of 1.25/312 = 4 mA, trimmed by R5 (see calibration sequence detailed later).
Summed with this is a 0 to 16 mA current derived from the 0 to 20 mA input, controlled by the 4:1 ratio current split provided by the R1/R2/R3 current divider and fine trimmed by R2 (ditto).
Note that 4 mA is below the guaranteed minimum regulation current specification for the LM337. In fact, most will work happily with half that much, but you might get a greedy one. So just be aware.
The result is a precision conversion of the 0 to 20mA input to an accurate 4 to 20mA loop current. Conversion precision and stability are insensitive to R2 trimmer wiper resistance due to the somewhat unusual input topology in play.
Calibration proceeds in a four-step linear (iteration-free one-pass) sequence consisting of:
- Set input = 0.0 mA.
- Adjust R5 for 4.00 mA loop current.
- Set input = 20.00 mA.
- Adjust R2 for 20.00 mA loop current.
Done.
The input voltage burden is a negative 1.0 volt. The output loop voltage drop is 4 volts minimum to 40 volts maximum. The maximum ambient temperature (with no U1 heatsink) is 100oC. Resistors should be precision types, and the trimmer pots should be multiturn cermet or similar.
Stephen Woodward’s relationship with EDN’s DI column goes back quite a long way. Over 100 submissions have been accepted since his first contribution back in 1974.
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