Multi Zone Process Controller

This was one unique custom design i made for a Mining Company for Ore Processing. It was Multi Zone Scanner controller.

Multi Zone Temperature Controller

It might have been for a rotary kiln of a large size. Four temperature sensors were connected to the system with brushes just like in a motor. Even the supply to the trasmitters was on the brushes.

The Instrument had a Analog Multiplexer with a Synchronous Sample and hold, as the analog value was coming via the brushes. It had four relays that would turn off or on every scan cycle, depending on the input values. The kiln system was slow it had inertia, so it did not require a fast acquisition for ‘Real time’ as you may find necessary in small Thermal Systems.

This was built with analog and digital chips, no uC in this design. This was because it was a front end controller, there was a risk of a uC system getting stuck. These days, that problem may not be so worrisome, but i am not so sure.

What if the watchdog goes for a short nap?

You can find part of the designs here – Industrial Process Control Circuits

80C39 and MCS48 based Process Controller

The Early Microcontroller : 80C39 is a CMOS Version of 8048 the one that preceded 8031-8051. MCS48 is the set for that, MCS51 is the current set.

Here is an example project for 80C39-8748-8749 microcontroller. This code was written by me, and it works. But the documentation is not complete or may have errors.

80C39 and MCS48 based Process Controller

Process Controller code here 80C39 Code for MCS48 (editable spreadsheet online)

The circuit for these are on this page along with other circuits. This may be difficult to put together now. But the code and hardware is near compatible to 8051 uC. Now there are may types of ADC and uC.

This was supposed to have Analog in and Analog out. Some parts of the circuit may be in the 80C51 page. This has ramp-up and ramp-down settings for the Increment and Decrement buttons. This helps setting setpoint quickly. There is also debouncing for the pushbutton.

Two Set Point – Display Card – Process Controller

This is the pcb board details of a Two Set Point Controllers for any process, shown here for temperature. For new types of transducers or input types, module card has to be designed or modified. The other cards remain the same.

The cabinet of these process controllers were made of steel for shielding, but the display card would still pickup EMI in some cases. These were more in instances where the Instrument supply was derived from the motor 3-phase supply. Instrumentation Supplies 230V AC must come from a Lighting Circuit of another supply arm, this has to come after conditioning with EMI-RFI filters and Servo Stabilizers or UPS if possible. This way the load spikes-glitches due to turn-on and turn-off of Motors and Heaters. dont act as a feedback to instruments. If line-load regulation is bad and mains voltage unstable, more problems can be expected.

This front panel shielding was done with a semi farady cage, by having a ground plane on the front of PCB, facing operator. This is just the negative of solder mask, but is the copper layer in front, no pth processing, even though it is two layer pcb. The solution worked well.

Display Card - Shield

Proportional Temperature Controller

Proportional Temperature Controller

This is a Proportional Controller where the setpoint is derived from a Thumbwheel switch.

Proportional Temperature Controller

The conversion of Thumb-wheel Digital Data to Analog mV is similar to R-2R Weighted Resistor Network. In this case it is a 1-2-4-8 Binary Weighted Resistor Network. It has no Digital Components.

You can see an example circuit below for digit weights you just use like 10K-100K-1M etc. There is a problem of procuring 8M Resistors, so use series parallel combinations, avoid open presets. Trimpots can be used but then it raises the BOM cost.

1-2-4-8 Kilo Ohms may load opamp for high output levels. 1-2-4-8 Mega Ohms may be ok in the lowest digit. Greater than 10M designs are possible only in lab, not in commercial or industrial domains.

Binary Weighted Resistor Network

Make such R networks, solder array on thumbwheel, in some thumbwheels remove diodes or other connections. Club all of them, thumbwheels. One opamp will do. Use -2.5 V for positive mV output. The resistors should be close to 0.3% at least.

This Binary Resistor Network can also work with Digital CMOS Chips like CD4029. Use these chips on a separate supply, which is just a LM336 – 5V device. A digital thumb-wheel also can be used.

In this controller you can see a sensor open indication. When the sensor breaks, the temperature controller may continue to turn on the actuators or heaters, It may even Oscillate. So when a high impedance is detected in the sensor input terminals, the output relay is shut off and a LED is turned on to simplify operator’s diagnosis.

Mount the controller a distance away from heaters, ac-drives and vibrating parts. Avoid direct sunlight on controller, fix controller in a sealed control panel. Earth the point where the thermocouple senses heat. Some heaters leak. The machine has to be earthed.

Proportional Process Controller

Proportional Process Controller SpecificationsTemperature Controller - Front

Front Panel Controls

Display – Process Value PV

This is a 3 1/2 digit display which shows the process value from 000 to 1999 as chosen by user it could be deg C, Kg/cm2 or Lts/hr.

Push Switch – Displays Setpoint

This is a push switch which when pressed, makes Display read SetPoint as required by user to control his process.

On-Output Status

LED Indicates Relay ON or OFF in Relay and Pulse Output Units. Units with 0-10V or 4-20mA, Output LED is always OFF.

Vary SetPoint SP

This is an endless 10 Turn Potentiometer this is used to adjust SetPoint (SP) in tandem with Push Switch. Turning Clockwise it Increments SP till max. value, then turn anticlockwise to bring SP to Zero. Use a Plastic Screwdriver or Trimmer.(Tester).

Basic Operation

Press Push Switch to make Display read SetPoint (SP), this is the user defined point at which process is controlled. Now holding Push Switch pressed, adjust SP Pot to change Set Point to required value, use a small screwdriver.

Back Panel Terminations

Temperature Controller - BackINPUT : 4 to 20 mA, Fused, Polarity is Important. From Sensor, DC Current.

OUTPUT : 4 to 20 mA, Polarity is Important. To Output Device or Actuator, which by using DC Current, drives 0% 4mA, 50% 12mA and 100% 20mA in Linear fashion.

SUPPLY : 230V 50Hz – L, N, E. Connect Proper Earth.

BAND : This is a Single Turn Potentiometer (300 deg) it sets the

Proportional Band of Control. Wide and Narrow are the ends of pot.

RESET : This is a Ten Turn Potentiometer (endless) it sets the

Integral or Reset or Error Cal. Inc. PV and Dec. PV are the two ends of pot.

Electrical Precautions –

  • Wrong Connections are the cause of most equipment failure.
  • Use Servo stabilizer for extra protection.
  • Use a Very good earth connection.
  • Use Fuses where appropriate.
  • Crimp and Solder all Lugs.



  • 4 to 20mA, 4mA 0% & 20mA 100%. < 4mA OFF, > 20mA ON.
  • 4-20mA Maximum Voltage @12V DC ie Shunt 10 Ohms to 500 Ohms.
  • Protection – use external fuse of 0.1A to safeguard from high energy circuits.


4 to 20mA, 4mA Displays 0% of Process Value ‘000’ & 20mA Displays 100% of Process Value. This is to be specified by user as … an example 750 Lts/hr.

Input @ 20 Ohms Shunt in STC1000PFC. Protection – Internal fuse of 2A and Semiconductor Protection Circuits to

safeguard from high energy circuits.

Environmental Limitations :

  • Delicate Precision Instrument
  • Maximum 50 Deg C, 70% RH Non-Condensing
  • Acid-Alkali-Salt Free area.
  • Equipment may fail if environment is Hot / Corrosive / Vibrating.

Cleaning and Maintenance : No Maintenance required, Clean front panel with moist sponge & Mild Detergent.

Power Supply : 230V AC 50Hz +/- 10%, Sine wave UPS & DG Sets.

Proportional Temperature Controller

A Temperature controller made with above process controller has the following changes in specs.

On-Output Status

LED Indicates SSR ON or OFF. Pulse Output goes High Max 15V or Low 0V. High Pulse turns on SSR. and this is Indicated by LED turning ON.

Display – Process Value PV

This is a 3 1/2 digit display which shows the Temperature value in Deg. Centigrade


INPUT : Thermocouple, Polarity is Important.

OUTPUT : DC Pulse Polarity is Important. To SSR. Pulse Maximum Voltage 12V DC at a Load of 250 Ohms.

Read More here – Analog Process Control Circuits

Four Channel Temperature Alarm Docs

Design Documents

Source Files Orcad Format

Block Diagram :

Four Channel Temperature Alarm

Zoom Image

Resources :

Relative Humidity Table for Hygrometer

When i made a wet bulb and dry bulb Digital Humidity Meter, a chart for evaluating RH was made. This chart is in PDF format below.

Table of Relative Humidity or RH 370 Kb PDF

The Sensors used were Two RTD PT-100 and Two 48*96 Digital Indicators. The Wet bulb had a Moist Wick Compartment where water had to be refilled periodically. A small Fan was used to get a faster Response.

When Weaving Textiles, Humidifiers or Atomizers are used to maintain the Humidity of the Work Area where the Looms are running. A Horse Hair Hygrometer is used to Manually Monitor and Regulate the Humidity. In Offices where the Air Conditioning needs a Humidity regulation the method used to Determine Humidity is by using a Wet and Dry Bulb Hygrometer or a Sling Psychrometer.

Related Resources –