How to convert 4-20 mA to 0 - 5v or 0 - 10v and vice versa? and why do we need to convert them?

 

We all know sensors play a vital role in Automation/Process control. The output of sensors is fed to a controller so that we can consider them as interrupts and automate a process.

There are two types of sensors,

* Digital/Discrete sensors : 

Where the output will be either 0 or 24v.

Examples of digital sensors are Proximity sensors, Digital level sensors(Float), and etc.

* Analog Sensors:

Where the output of sensors will vary.

Example: Ultrasonic sensors, Flowmeters, temperature sensors like RTD, etc.

The latter helps us to understand real-world parameters. For instance, if you want to measure temperature it can't be either 0 deg or 100 deg it will be variable. Hence sensors used to capture real-world parameters are termed analog sensors.

Every PLC's Analog module differs, some of them accept 4-20 mA some of them accept only 0 to 5v or 0 - 10v. So does the sensor's output, some of the sensors have 4 - 20 mA as output. some of them have voltage outputs. 

Here is where we'll need to convert current to voltage or vice versa depending on the source available.

Now we know, what is need of converting them. let's get into how to convert them part.

We all know ohm's law which basically states Voltage = Current * Resistance

Let's say for example you want to convert  4 - 20 mA to 0 - 10VDC 

This is how the circuit looks like,

The current output is going to be from any analog sensor which gives 4 - 20mA as output.

The voltage input is the converted voltage which ranges from 0 - 10VDC

Now that we know both voltage and current we'll have to calculate the resistance. 

From ohm's law, R = V/I

The maximum voltage here is 10V and the maximum current is 20mA. 

R = 10/0.02 = 500 ohms. 

This is going to be our resistor's value. let's calculate what will be the converted output for 8 mA

V = 0.008 * 500 = 4 VDC

Likewise, if you want to convert 4 - 20mA to 0 - 5 VDC you can repeat the above calculation the resistor's value will be R = 250 ohms.

Now let's say if you want to convert 0 to 10V to 4 - 20mA. 

This is how the circuit looks like, 

 

The resistance value is going to be the same here as in the previous case. The only difference is you will connect the resistor in series to the source.