4-20mA Current Loop Tester

Description: This design is intended for technicians working with pneumatically operated valves and other devices controlled by a 4-20mA current loop. While 4-20mA signal injector/calibrators are commercially available, this particular model is cost-effective to construct and user-friendly. Upon initial power-up, the circuit sinks 4mA of current. Pressing switch S1 causes the current to gradually increase to 20mA, pause, and then return to 4mA. This cycle continues until the switch is pressed again, which locks the output at its current level. A subsequent press of the switch resumes the previous cyclic operation. Output2 from the microcontroller (IC1) is programmed to produce a pulse-width modulated (PWM) signal that drives the current sink transistor (Q1). The digital PWM signal is converted to an analog voltage using a low-pass filter comprised of a 1kΩ series resistor and a 4.7µF tantalum capacitor. By adjusting the PWM duty cycle, the program can indirectly control the current flowing through the transistor. A 100Ω resistor in series with the emitter of Q1 converts the loop current into a small voltage, which is then fed back to the microcontroller on input1. The program utilizes this feedback signal, along with the microcontroller's analog-to-digital converter, to accurately achieve the desired current level. If the PICAXE detects an open circuit, it disables the output and enters an alarm state to notify the operator and safeguard the circuit under test. The switch can be pressed to reset the system once the open circuit issue has been resolved. The LED provides visual feedback through a series of flashes corresponding to various operational milestones: one flash at 4mA, one flash to confirm a switch press, two flashes at 12mA during the ramp-up phase (for the first five cycles), three flashes at 20mA, and a continuous fast flash sequence indicating an open-circuit alarm. For portable applications, the circuit can be powered by two 9V batteries, while a 12V DC plug pack is suitable for bench testing.

The schematic for this circuit consists of several key components designed to facilitate the operation of the 4-20mA current loop signal injector. The microcontroller (IC1) serves as the central processing unit, executing the programmed logic to control the PWM output. The current sink transistor (Q1) is pivotal in regulating the current flow; its operation is influenced by the PWM signal generated by the microcontroller.

The low-pass filter formed by the 1kΩ resistor and the 4.7µF capacitor smooths the PWM signal, converting it into a stable analog voltage that corresponds to the desired current output. This mechanism allows for fine control over the current levels, which is essential for calibrating devices that operate within the 4-20mA range.

The feedback loop is established through the 100Ω resistor connected to the emitter of Q1, which translates the current flowing through the loop into a measurable voltage. This voltage is analyzed by the microcontroller's analog-to-digital converter, enabling real-time adjustments to maintain the set current level. The alarm functionality is critical for ensuring safe operation; the circuit monitors for open circuits and responds appropriately by shutting down the output and alerting the user.

The LED indicator provides a simple yet effective means of communicating the status of the device, with specific flash patterns designated for different operational states. This feedback is essential for users to understand the current operation without needing additional diagnostic tools.

Overall, this circuit design presents a practical solution for testing and calibrating 4-20mA devices, combining user-friendly operation with essential safety features.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).

The digital PWM signal is converted to an analog voltage using a low-pass filter formed by the 1k ‰ series resistor and a 4. 7 F 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 suffice.