Introduction: 4mA-20mA Current Simulator With Step Control

Introduction


This guide explains the construction of a 4mA-20mA current simulator with step control, featuring up/down buttons, step display via two 7-segment displays, and precise current adjustment using a DAC (Digital-to-Analog Converter). The steps are limited to a total of 20, ensuring a linear progression of current output from 4mA to 20mA. This project uses logical ICs only, avoiding microcontrollers.

Supplies

Materials and Tools Required


  1. Operational amplifier (e.g., LM358)
  2. Logic gates (AND, OR, NOT ICs, e.g., 7400LS series)
  3. Digital-to-Analog Converter (DAC, e.g., MCP4921)
  4. Up/Down counter ICs (e.g., 74LS193 or similar)
  5. Two 7-segment displays
  6. Precision resistors (1% tolerance)
  7. Potentiometers for adjustment
  8. Push buttons for up/down counters
  9. DC power supply (12V for DAC and 5V for other components)
  10. Breadboard and connecting wires
  11. Multimeter for testing

Step 1: Design the Step Control Mechanism

Use an up/down counter IC to allow step increments and decrements with the press of dedicated buttons.

Configure the counter to loop between 0 and 19 (20 steps total).

Connect the counter output to a Binary-Coded Decimal (BCD) driver to control two 7-segment displays, showing the current step value.

Step 2: Integrate With the DAC

The counter’s output is fed into the DAC, which converts each step value into a proportional analog current signal.

Calibrate the DAC so that step 0 corresponds to 4mA, and step 19 corresponds to 20mA, ensuring equal increments.

Step 3: Assemble the Circuit

Connect the up/down buttons to the counter IC, ensuring proper debounce circuitry.

Wire the 7-segment displays to the BCD driver for step feedback.

Link the DAC’s output to the operational amplifier, which regulates the current to the device under test.

Step 4: Add the Current Regulation Circuit

Construct the current regulation circuit using an operational amplifier and a feedback resistor network.

Use a precision potentiometer to fine-tune the output current.


Step 5: Testing and Demonstration

Initial Setup


  1. Power on the circuit, ensuring the 7-segment displays initialize to step 0.
  2. Verify that pressing the up/down buttons adjusts the step value between 0 and 19.


Verification


  1. Connect a calibrated multimeter in series with the device under test to measure the output current.
  2. Confirm the current changes linearly with each step (e.g., step 1 = 4mA, step 20 = 20mA).
  3. Check for stability and accuracy across all 20 steps.


Applications


This enhanced 4mA-20mA current simulator can be used in:

  1. Sensor Calibration - Calibrating devices like temperature transmitters and pressure sensors.
  2. Industrial Systems Testing - Testing and verifying PLC or DCS modules.
  3. Educational Projects - Demonstrating DAC operation, digital counters, and analog current control.
  4. Field Maintenance - Providing a portable current source for testing industrial equipment.
  5. Prototyping - Creating scalable current signals for analog circuit development.


Conclusion


This project demonstrates the development of a precise 4mA-20mA current simulator with simple step control. Its 20-step range and logical IC-based design make it a versatile tool for both academic and industrial use.