An LM317-based 0-20mA to 4-20mA 2-wire converter
My recent DI contribution (Another silly simple precision 0/20mA to 4/20mA converter) used the LM337 regulator in a novel circuit arrangement to translate an input 0-20 mA current source into a 4-20 mA two-wire transmitter current loop (a standard two-terminal industrial current source).
The circuit can also be flipped over to perform the same operation using the LM317, which is more widely used, hence easily available, and lower priced. As before, it relies on tapering off an initial 4 mA current to zero in proportion to the input 0-20 mA, and adding the input and the tapered off 4 mA signal to create a 2-wire 4-20 mA output loop. The operation is identical, only with the current directions reversed.
Refer to Figure 1.
Figure 1 An input external boost transistor is used to limit U1 dissipation.
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At 0-mA input, the series combination of Rs and Rz sets the 4 mA “zero” value in the 4-20 mA output loop, set using Pz. This is pulled from the 24-V supply, going from IN to OUT, and into the output loop via Rs and Rz.
A 20-mA input current creates a 1.25 V drop in Rs, opposing the internal reference and reducing the current through Rz to 0 mA. The 20-mA input current is pushed into point X and the – (negative terminal) of the output loop, and pulled from the 24-V supply via OUT (through U1), to create a current of 20 mA + 0 mA in the output loop. Span setting is done by Ps.
Accurate current setting requires 2 S/Z passes to set the output current to within 0.05% transfer accuracy, or (actually much) better. Pots should be multi-turn 3296 types or similar, but single-turn trimmers may also work fairly well, as both pots have a small trim range by design.
The di/dV, current stability, and ease of trimming are excellent. Input to output linearity of the basic circuit is 0.02%. The heat sink has been replaced by an external boost transistor that ‘takes over’ above an output loop current of @ 6 mA, limiting U1 dissipation to give better di/dv performance of 0.02% over a voltage range from 5 V to 32 V.
For intermediate input currents, as before, a “zero” 4 mA current is tapered off to 0 mA proportional to the input 0-20 mA to give corresponding matching input and output loop currents.
A reverse protection diode is recommended in the 4-20 mA loop. Current limiting should be applied to limit fault current to safe levels. It can easily be inserted into Q1 emitter in this case.
The 0-20 mA input sees a positive drop here, of 1 to 0 V.
The comments regarding current stability and potentiometers in “Another silly simple precision 0/20mA to 4/20mA converter” are applicable here too.
In conclusion, the operation of an inexpensive, novel, and precise circuit to convert 0-20mA current signals to 4-20mA 2-wire current signals is described, using the LM317 regulator and an external boost transistor.
It easily holds a short-term stability of 0.02% of span, and has a linearity of 0.02%.
Only two stable resistors are needed for good overall temperature stability.
Pot tempco, type, and contact resistance are less critical due to the configuration used.
A 3 V minimum operating voltage allows as much as 1000E of loop resistance with a 24-V supply.
Ashutosh Sapre lives and works in a large city in western India. Drifting uninspired through an EE degree way back in the late nineteen-eighties, he was lucky enough to stumble across and be electrified by the Art of Electronics 1 and 2. Cut to now, he is a confirmed circuit addict, running a business designing, manufacturing, and selling industrial signal processing modules. He is proud of his many dozens of design pads consisting mostly of crossed-out design ideas.
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