Electrical Circuits: Instrumentation 1

As promised in the last post, we will spend a bit of time on the Instrumentation side and maybe it is a good idea to start with the basics of what you are bound to encounter.

First we will look at the different signals you will be getting from these units as this will determine what you need in order to make the circuitry work

  • Digital

This signal is merely a set of contacts that will open or close when a pre-set condition is met. This may be a pressure switch that will monitor the pressure on a line and once it reaches the pressure to which it has been set, it activates the contacts forcing them to change state.


  • Analogue

This is where it changes a bit. The signal being returned from the instrument comes back as an analogue signal which could be 0 – 10V, 0 – 20mA or 4 – 20 mA. This value is then scaled to the engineering value and displayed as such


To use a simple example, let’s look at a pressure sensor once again. The output from the sensor is 4 – 20 mA. The pressure range you are working with is 0 – 10 Bar. That means at 4mA you have 0 Bar and at 20mA you have 10 Bar. This is your engineering value. The signal is sent to the PLC and you enter the engineering value in the program. Once you have this set up, you can control different components pending how the program has been written and you can also have the actual pressure displayed on the HMI etc.


So, which is better you may ask? Well, there is room for both and it is dictated by the process you need to run. Be very careful when selecting instruments as it is nice to have a unit with all the bells and whistles, but you need to ensure it delivers what you need to drive the process and that it is suited for the application in which you intend to install it. As with everything, talk to your supplier and make sure you get all the facts before selecting a unit


Now that we have had a look at the basic make-up of the signals, the next post will be focussed on a more practical example of how we put it into practice. Till then, stay safe and remember, it is not that hard to spend a little more time to make sure the job not only works, but looks good as well