4-20mA Current Loop Tester

This design will
interest technicians who work on pneumatically operated valves and other
4-20mA current loop controlled devices. Although 4-20mA signal
injector/calibrators are available, this one is both cheap to build and
easy to operate. When first powered up, the circuit sinks 4mA of
current. If switch S1 is pressed, the current level slowly ramps up to
20mA, pauses and then ramps back to 4mA. This cycle will continue unless
the switch is pressed again, whereby the output will lock to its
current level. A further push of the switch resumes the prior cyclic
operation. Output2 from the micro (IC1) is programmed to generate a
pulse-width modulated (PWM) signal to drive the current sink transistor (Q1).

Circuit diagram:

4 20mA Current Loop Tester Circuit

4-20mA Current Loop Tester Circuit Diagram

The digital PWM signal is converted to an
analog voltage using a low-pass filter formed by the 1kω series resistor
and a 4.7μF tantalum capacitor. By varying the PWM
duty cycle and therefore the DC signal level out of the filter, the
program can indirectly vary the current flow through the transistor. A
100 resistor in series with the emitter of Q1 converts the loop current
to a small voltage, which is fed into the micro on input1. The program
uses this feedback signal to zero in on the desired current level with
the aid of the micro’s analog-to-digital converter. Details of this can
be seen in the accompanying program listing.

If the PICAXE senses an open circuit, it
shuts down the output and goes into an alarm state, to alert the
operator and protect the circuit under test. The switch can be pressed
to reset operations to the start once the open circuit has been
rectified. The LED flashes a code for various
milestones, as follows: one flash at 4m and one flash to confirm a
switch press two flashes at 12m when ramping up (for the first 5
cycles); three flashes at 20m and continued fast flash sequence for
open-circuit alarm. For portable use, the circuit can be powered from
two 9V batteries, whereas for bench testing, a 12V DC plugpack will

Author: Allan Doust – Copyright: Silicon Chip Electronics

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