4-20 mA current loops are widely used for communication in industrial automation systems. This paper discusses the design and implementation of a low cost microcontroller based current loop calibration device to test and calibrate systems that communicate via 4-20 mA current loop standard.
The aim of this work is to decrease the cost of these devices regarding commercial value. Designed device has 0.001 mA resolution on the current sourcing. It is also both capable of measuring and providing requested current between the range of 4 and 20 mA either automatically in the format of step or ramp based function or manually by entering current value via a numerical keypad.
4-20 mA current loop is a basic method for sending sensor information in industrial applications 
. Sensor is a device used for measuring physical parameters like temperature, pressure, speed and fluid flow. Most of the process automation sensors are standardized using the 4-20 mA current loop interface. This interface is generally used for sending sensor values over current loop to remote locations - see figure 1.
Fig 1: Overview of the current loop system.
If voltage levels are used for sending process, there will be a decrease in voltage depending on the resistance of the carrier and distance increments. In order to prevent this negation, 4-20 mA current loop is standardized.
There are lots of systems in industry working with 4-20 mA current loop standard. 4-20 mA current loop calibration devices are used in testing and calibrating those systems. 4-20 mA current loop is designed to be in the value of 4 mA when the sensor receives the minimum value and it becomes 20 mA when it reaches the maximum value [2, 3]
. So, 4 mA is assumed as a starting point (0 percent reading) and 20 mA is seen as a full scale reading (100 percent). In this condition, 0 mA level is interpreted as a communication break. It means that 0 to 4 mA range is called as the zero or offset. The 4 to 20 mA range is called as the span of the transmitter.
Fig 2: Interface of the 4-20 mA current loop.
A 4-20 current loop circuit is composed of 4 elements; sensor/converter, transmitter, receiver and current source - see figure 2. Sensor or transducer measures physical magnitudes and converts to voltage. Transmitter converts the voltage information taken from sensor to the 4-20 mA current level. Receiver after taking the 4-20 mA current level converts it back to voltage and sends to the process controller or to an indicator. Current source provides the current loop as well. There is at least one receiver in each loop. It can be an indicator (a meter or a digital readout), a chart recorder, an input to an RTU or a PLC, a valve actuator, etc.
The 4-20 mA current loop has some advantages. These are; less affected by noise, capable of sending signals to distant locations, signal being limited by only current source, and so inexistence of signal lost and ability to control broken line. Since the bottom level for current loop is at 4 mA level, the breaking of signal transmission line is interpreted to be 0 mA 
We use current loop calibration devices to observe the system behavior according to the likely values that the sensor can have when designing and testing of industrial apparatus with sensors.
Calibrators produce and also read current values in 4-20 mA intervals. For the system seen in figure 2, current loop calibration devices determine how the process controller behave for different process conditions (such as the temperature level in the 10 percent, 50 percent, and 77 percent ranges) of remote systems by taking the place of transmitter and receiver.
In commercial area, there are many types of 4-20 mA current loop calibrators having different specification, and their cost is up to $2000. In this work, we aim to decrease the cost of these devices regarding commercial value.
Most of the calibrator devices produced in the commercial area have the property of being adjusted in analog or step/ramp function based. By this work, we aim to develop a calibration device that can be adjusted to the desired current level, entered by keypad, in an adequately short period. Additionally, the developed device is considered to produce current value with adequate precision and for adjusting automatically or manually according to the step/ramp function based.
In the scientific literature, there are some publications related to this work; a current adjusting system with 0-20 kA level 
, a current source with 10 mA DC for high current/power converters [6, 7]
, a general purpose CMOS current source 
, but none of them are directly intended to the 4-20 mA current loop. Implemented device in this work has a high precision within the mentioned current loop standard. This device is also capable of performing many functions such as transmitter, receiver, source and measurement related to this standard. Additionally, depending on this standard at analog level, a digitally interfaced microcontroller based system is developed. The main reasons for this are stable operation of digital systems, being less affected from environmental conditions (noise, heat, etc.), and also easier to use.