Here is a 4-20 mA In/Out Analog Mux with Cascade option. This is a simple circuit i designed to make a Automation System within a budget.
Mixed Circuits Analog with Digital
This takes 4-20mA from many Transmitters and gives out just one 4-20 mA output. The Mux is done with a digital byte or word. This is a slow scanner as process is slow, that way many analog inputs can be multiplexed and sent into one analog input of a D/A. In near real time systems a faster mux could be used or mux totally avoided. This was made in some numbers, so the pcb is better than others.
4-20mA Multiplexer Circuit – pdf
Here is an ancestor of the product in the earlier post. It is a Process Scanner and Indicator. There was no control, but there were individual alarms for each channel.
This is a original design of mine, obsolete now due to size and technology. The inspiration of the product concept was from a Omega Catalog Item. Even today i use the Omega catalog for inspiration, but i do not design complete products, only circuit sections.
The Omega catalog was introduced to me by the MD of a firm i worked in as a R&D Engineer in my young engineer like days. He used to call me and show a catalog product and ask me if i could build something like that. I would then design it working both at home and office, sometimes even in the night, it was just the enthusiasm or creating something new.
I even built the first Pneumatic Tyre Inflater with the help of the CNC director for that firm, this product won a national award in the nineties. I left the firm on a trivial issue which i never disclosed. It was Exhibition time, many new products were not complete, they asked me to make Mockup Dummy Instruments for demonstration only, overnight. I ran away.
Read more about Control Instruments at my pages here
This is the Photo of a RTD 3-W Mains Powered Temperature 4-20mA Transmitter. The Circuits and PCB are here
RTD PT100 Transmitter and Multiplexer.
Now with new Technologies like Zigbee and Modbus, We can classify Transmitters as shown below. The Measured Parameter Temperature, Flow or Events Has to reach an Intelligent Data Storage and Analysis System. It may just be an Human Operator who jots the data on a Notepad and Turns a Few Dials based on his Experience or an embedded controller. It could even be a Computer Network or a Web Application used by many, like Monitoring the Weather Attributes.
- Analog Transmitters – Like 4-20mA Loop.
- Electrically Isolated Analog Transmitters.
- Transmitters that need to be Intrinsically Safe.
- Digital Transmitters and Optical Interface.
- Wireless Transmitters and TCP-IP.
The job of the transmitter is to take the weak analog measured parameter information from sensor, be close to it, amplify, clean, linearize the signal if required and send strong – error free data over a long distance to an operator or system.
This is a RTD Pt-100 Transmitter, It can Convert Thermocouple mV or RTD mV to 4-20 mA Current Transmitter. The part numbers on Circuit and silk screen may not match. But the PCB may be usable. The output is a current source and not a current sink. It goes to a 4-20mA Mux with a common ground. The Circuit RTD-Pt-100-Transmitter Circuit.
This low-cost transmitter was made to send RTD information in a 4-20mA over long distance. The 4-20mA was the input to a Analog-Mux which interfaced to a GE-Fanuc PLC system and a 486 Computer. The Analog Multiplexer Built around CD4051 was was controlled by PLC to get data from numerous channels. This reduced the cost of the System. This is ok when Real-Time Acquisition is not required. The Analog 4-20 to 4-20 Mux-Buffer circuit can be seen at Industrial Process Control Circuits.
This was done as customer budget could not afford dedicated analog input modules for every channel. There were more than 50 Inputs. The thermal inertia of system did not need a real time or fast correction.
The PNGs of PCB is 600dpi 16 color. It is to be scaled appropriately.
Related Resources –
These are the circuits and boards of a Mini Temperature Transmitter for a Platinum hundred ohms temperature Sensor.
RTD-3W-Transmitter Circuit. See my Transmitter Application Note for this.
The customer wanted a two wire system, this needs low power opamps which were ten times the price. If this transmitter works at 3mA it could have been two wire. Those parts were in short supply and the budget of customer was low. So i made it with regular opamps but three wire. The customer was satisfied as he got a cost effective solution. But now the situation of markets and products are different.
If you need to transmit the temperature from a high voltage area or toxic environment. You will need to hermetically seal the transmitters, convert the Voltage to frequency, frequency to IR LED flashes or RF. This can be then remotely monitored. Then the transmitter has to be battery-solar operated, low power too. ICL7135 is a simple solution. It has a serial output that can be used to Drive IR Leds. Remotely sense these flashes in a Micro-controller and you have a reading. You may be able to use optic-fibers too. Where volatile liquids are present so that the risks of sparks can be eliminated.
The PNGs of PCB is 600dpi 16 color. It is to be scaled appropriately.
This is the same Mini RTD Pt-100 Transmitter but in its case. Encapsulated in epoxy, hermetically sealed against harsh industrial environment. This will work well even near fumes of Ammonia with no corrosion. But not near vapors which can be ignited by sparking as terminals are still open. The side view shows zero and full scale ten turn bourns trimpots heads, for calibration. After cal it can be sealed with RTV compound.
This was a Heater which was custom built for a CRT manufacturer, It comprised of many Tubular finned heaters. And probably a big fan that worked at high temperatures. This was used in a Heating Chamber which aged or tested CRT monitors. It had a temperature controller, a remote indicator. Learn about CRT here Cathode-Ray Tube .
This could take current or voltage inputs from many remote transmitters, 4-20mA or 0-1 V Inputs. Then it would display the calibrated value in a LED display. The Zone number was a digital display. This serves as a monitor of process values. It had a analog output with a digital channel byte-word output for interface with other instruments.
This uses the CD4051-52 for the analog switching. Operating this at +/- 7.5V gives best isolation from channels and low ON resistance. Now you have many types of devices to do this.
When a large area or object has to be heated, many heaters are used. This device can monitor and display the temperature at various points. The Operator then can make adjustments in heating or cooling so that he can maintain Uniform temperature or a Temperature Gradient as specified by the Process.
There are many ways, these scanners are used. Many different parameters like pressure, temperature, level are measured with different sensors and converted to a 4-20 mA level using transmitters for each sensor. Then many parameters can be measured in one equipment, when it needs only periodic attention. As the scan rate can be set from 1 to 50 seconds, the operator will be able to keep a tab on the values or record it. This reduces the real estate required on the control panel. Then the control room can be small, as it has to be at a vantage point, with a birds view of the entire factory floor.
Industrial Process Control Circuits