This post unifies together the following posts:
Industrial, 4-20 mA current loop, measuring basics I.
Industrial, 4-20 mA current loop, measuring basics II.
Industrial, 4-20 mA current loop, measuring basics III.
Industrial, 4-20 mA current loop, measuring basics IV.
Internal workings of process controllers I
. Internal workings of process controllers II.
There are a lot of types of transmitters nowadays. Usually a separate one (a specific type) is needed for thermocouple measurement, an other one (an other type) is needed for level measurement…
The basic concept of this post is to present a way for a universally usable transmitter, which can accept almost any type of input (Pt100, pH probe, rotation, level, light…) and produce a configurable, standard 4-20 mA output.
Continue reading Universal, 4-20 mA, two wire industrial transmitter
Continued from the previous post
Although the title says “basics”, here are some advanced circuits about the 4-20 mA current loop transmitters.
In the first example a USB connection is shown. Nowadays almost every transmitter incorporates a microcontroller. The transmitters can be programmed through their display, or through HART protocol, or via DIP switches.
Transmitters with HART communication are expensive, using displays also increases the cost, and using DIP switches provides a not so flexible way of configuration.
Some manufacturers use USB as a programming / configuration interface, but most of them are not loop powered, so they need a separate power supply, which increases the cabling costs.
The following solution shows a way of how to include the USB communication in the transmitter design, while the transmitter is powered from the loop.
Continue reading Industrial, 4-20 mA current loop, measuring basics IV.
When a switch or a pushbutton has to be connected to a digital circuit it is often (almost always) a nasty thing, because its unwanted behaviour.
It is a common practice to connect a button or a switch to a digital system (to a microcontroller, to a PLC, or to a PC). The simplest way to do it is shown in the picture below:
Connecting a pushbutton to a microcontroller
If we measure the voltage at the input of the microcontroller with an oscilloscope, it would show the following:
Contact bouncing phenomena
The problem is obvious. There isn’t a simple, clean low to high transition. If the microcontroller is fast enough, it senses this switch on event as if there were several pushbutton pressings.
Continue reading Switch debouncing in PLC software