## Linearizing Circuit for Thermocouples

This circuit changes the gain of opamp U1B in four steps or segments. It can be used to get a linear output from most transducers to 1% levels.U1A is a amplifying buffer use it to boost the signal to the required level.

The resistor values i have put are for an imaginary transducer, you have to design them. The buffered input signal is compared to reference switching points by LM339.

Temperature Measurement and Control

LM339 changes the gain resistors of U1B thru the mux switch 4066. JP1 to JP4 can select either amplification or attenuation of signal. The resistor switched by 4066 can be across R1 or R2 based on JP1 to JP4.

You may have to input transducer values into a spreadsheet and draw a graph. Then divide the graph into 5 segments and deduce the switch points and gain.

Learn how to use the AD590 to measure environment temperatures for display, logging or cold junction compensation.

The voltage at the point 1 of R4 will be :Vo=( 1+ ( 10K/22K)) * Vref = 3.63V as nominal Vref is
2.5V.AD590 is a current source which gives 1 uA / kelvin, It is independent of the voltage across the device. you can treat it like a current source or sink or impedance. total voltage across AD590 is 5V as opamp pin 2 is at virtual ground.

Analog Circuits – OpAmp, Signal Condition, Mixed Signal.

This is the way you try to understand the design.

The AD590, here is a constant current sink as cathode goes to -5. The current it sucks away or drains from node pin 2 of OP07 is 1uA/ kelvin. at 0 deg C the current drained is 273 uA at 26 deg C it is 300uA.

You know according to theory that the amount of current entering the node, is equal to the amount of current leaving the node. do not look at voltages now, look at the currents. the AD590 drinks 273uA from Node pin 2 of OP07 at 0 deg C. Now no current can come from opamp OP07 pin 2 as resistance is in giga ohms and leakage in pico amps. now the pot R5 and resistor R4 are just in series and connected to 3.63 V as established earlier. The TL431 is a shunt regulator with reference and has a low impedence. Now the R5 + R4 combination should not load the TL431, that is not the case as 3.6 / 10K = 360uA .

By varying R5 pot you can pump 3.6 / 10K = 360uA down to 130uA when R5 is max into node pin 2 of OP07. This pot will be calibrated with AD590 in ICE to give a 0 mV output of the Op07. When calibrated R5+R4 pump 273 uA into node pin 2 of op07. this is sucked away by the AD590 which is draining 273uA at 0 deg C. This leaves the pin 2 at zero potential as currents leaving = currents entering.

Now to understand the opamp functioning.

The pin 2 of opamp is a 0 potential as calculated above and pin 3 also is at zero pulled down by R7. Now as both inputs are at same potential the output of opamp also is zero. The feedback resistors R1 and R2 will carry no current as both their ends are at 0. the Vout is now 0 mV and AD590 is on a block of ICE and opamp is stable.

If pin 2 (-) becomes more dominant or positive than pin 3 (+) the output swings negative. If pin 3 (+) becomes more dominant or positive than pin 2 (-) the output swings positive. The opamp on feedback tries to maintain both the inputs at the same potential. This thumb rule can be used to make opamp oscillate, amplify or compute.

Now what happens when the AD590 is removed from the block of ICE. It comes to room temperature say 26 deg C which means 300uA. Now the AD590 demands to draw 300uA from node pin 2 of OP07. The R4 + R5 from 3.6 V can give 273uA as it is fixed, not a uA more. The rest which is 300 – 273 = 27uA leads to a drop in potential at pin 2 and it turns negative. as demand is greater than supply. which makes pin 3 which is at zero more positive than pin 2. ( theory : 0 is positive compared to -1) as pin 3 is more dominant opamp swings positive as per thumb rule. and a current starts flowing thru R1 + R2 till the current reaches 27uA. at this point the extra current 27uA drawn by AD590 is supplied by opamp thru R1+R2. The Pin 2 now comes to 0 as currents leaving = currents entering.

Test & Measurement, Instrumentation

At this point the voltage at opamp output is given by ( R1 + R2 ) * 27uA = 270mV (assume R1+R2 is 10K after calibration) now opamp gives 10mV per deg C.as opamp now is a closed loop control the rise and fall in temperature, results in AD590 current variation which produces a proportional OP07 output.

Now the explanation above is in steps but all that happens in real time in an instant.

## Multi Zone Process Monitor

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.

## 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.

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

## RTD 3-W Mains Power 4-20 mA Transmitter

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.

 From Soldermans Basic Electronics

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.

## Temperature Controller for Cooling Chamber

This is part of an user manual i used to give for cooling controllers – app007.pdf

The Logic of cooling in temperature control is inverted, Activating the relay powers a compressor which cools the system. A small modification at the output stage, makes any heating controller into a cooling controller. It can also be done with external relay logic, but make sure on power failure (control power line) the compressor ( 3 ph power) shuts down. In all heating and cooling control, when a broken or disconnected sensor is detected, the controller should turn off all outputs and indicate a fault condition.

Before you use a Controller to Cool a Chamber to Say +5 Deg C.

First :

Do not Connect the Controller. Directly Connect the Cooling Device e. g. : Compressor. to the system and check the maximum cooling it can produce. In case temperature goes to 0 deg or -10 deg then by using a STC1000 you can control at +5 deg.

But if directly you get cooling of only upto +10 deg then it is impossible for any controller to produce extra cooling, in such a case as this use a better compressor or more insulation.

Second :

Connect Sensor properly and replace when broken keep sensor close to the source of cooling.

Third :

Deadband (DB) at minimum is 1 Deg this is the best setting, maximum setting of deadband may increase compressor life and also save power but will produce a huge variation in temperature.

Fourth :

If large Variation of temperature is present and you need accurate control Reduce deadband to min.

Unit is factory set db at 1 deg, remove seal tape before dead band adjustment.

## Temperature Control in Plastic Injection Molding

This is an Application note i used to give with my controllers – app009.pdf – Ananth

Temperature Control in Plastic Injection Molding.

 From Elex Quna – Electrical Circuits FAQ

Terms in Process Control and Explanation.

There are three Controls to be Adjusted to make a Proportional temperature Controller Perform Properly. This method has to be practiced and experience gained from it can be used to get very good and stable Control of the temperature or other process parameters.

1. Set Point. (SP)

This is the Temperature at which you require the Heated area to be. Here we have to remember it is better to control the temperature of the metallic area closest to heater to avoid thermal Cushions. In Rubber and Plastic Molding if you are measuring the plastic temperature directly it may give rise to oscillations and proper control may not be possible.

In Controlling the Temperature of Air or Water (Bad Conductors of Heat) Forced Convection with Fans for Air or Stirrers for Liquids can be used when Sensing temperature of the Liquid or air directly. But in Plastic such things cant be done as it is a semi-solid when heated hence. Control of Temperature of the Metal Discharging Heat to the Plastic is most practical. Oscillations are inevitable if the sensor is away from heater or is in contact with a non-conductor of heat.

2. Process Value. (PV)

This is the Temperature at the Tip of the sensor or the material touching the tip of the sensor. In Non-Conductors of Heat like plastic if we are monitoring plastic at a certain point the temperature of the plastic will be very different at various points depending on the Distance of the Heater from that point due to thermal gradients.

3. Proportional Band or Dead band. (PB) –

Dead band or H % or Hysteresis are terms used in on / off Controllers in proportional controller we use the term proportional band.

The Temperature zone in which the Controller turns on or off The heaters in a time proportional manner is the proportional Band. Set Point 200 deg C It is Given in % e.g. 10% PB of 200 deg SP is 20 deg. the Heaters are on till 190 deg C and off above 210 deg. C. Between 190 to 210 is the PB. A little above 190 the Heaters are on for 90% time. A little below 210 Deg C the Heaters are on for 10% of the time. When SP=PV Process Value the Heaters are on for 50% of the time i.e. 50% Duty Cycle.

4. Cycle Time –

This is the repetitive rate at which the heaters are Turned on or off Room Temperature 26 deg C For a Cycle time of 12 Seconds, when PV=SP heaters are on for 5 seconds and off for 5 seconds and this goes on as long as PV=SP.

Tuning or Adjusting a Proportional Temperature Controller.

Step # 1 –

Ensure Sensor is properly connected to the Temperature Controller TC polarity reversal will show reducing reading in the Display as heat builds up. The Heaters used and wattage selected must be able to bring the temperature more than the maximum required control temperature with TC. If Supply Voltage is down or heaters are blown or not in contact TC can not solve the problem.

So when in doubt connect heaters directly to supply (without TC) and see observe maximum temperature e. g. if max. temp. is 500 deg C the TC can control temperature upto 480 deg C.

Step # 2

Keep PB in minimum position and power on system e. g. set temperature is 300 deg C. Now Observe maximum overshoot. and adjust proportional band as in table below.

SP 300 deg C

PV (Process Value or Measured Temperature)

 PV overshoot Proportional Band 10 % 330 deg C or more Near Maximum fully clockwise till end. 5 % 315 deg C to 360 deg C Middle of the PB Control or towards max. 2% 306 deg C Little above present setting. Less than 300 deg Droop e. g. 290 PB is Critically set Do not Change.

After each change turn on system again to see response till 2 % or less variation or overshoot or oscillations are obtained.

Thumb Rule ! –

• Increment PB to Decrease Overshoot.
• Increment PB to Decrease Oscillations.
• Stop adjustment when PV droops < SP
• Adjust EC to match SP = PV after PV is stable at a point less than SP.

Step # 3

There is an additional control called Error Cal EC ( manual reset or Integral) which is factory set for SP=PV 50% duty cycle. In certain cases after stable reading is obtained after adjusting or tuning PB the temperature may stabilize say at 290 deg for a set point of 300 deg the process is stable but a ten degrees process error is present. this can be compensated in two ways.

a. Increase setpoint to 310 deg the process settles at 300 deg but this may not satisfactory even if it is practical.
b. Adjust Error Cal provided in the back panel to increase temperature to 300 deg from 290 deg.

When this is done give some time for system to respond after every 1/2 a turn 180 deg of the control. the EC control is a Ten turn potentiometer like the SP potentiometer after 10 turns the direction of turning must change. Clockwise Increase temperature Anticlockwise decrease temperature. (at min. PB setting EC pot sets the On/Off Operating Point).

Temperature Control using SSR and STC1000PK.

 From Elex Quna – Electrical Circuits FAQ

Mains Circuit –

Always Connect Phase to Live “L “, This Can Be “R” in a RYB System 3 Phase . “L” Live Can Be Verified by a “Neon Tester” . and it is the Energy Line (Tester Glows). “N” Neutral is the Energy return line and will be close to Earth Potential in a Neon tester it will not show a Glow. Earth “E” is the Local Earth at the site of the installation. ( “N” to “E” AC Voltage should be less than 5 Volts ideally)

SSR or Solid State Relay or Electronic Relay –

Generally this is a Thyristor Based Normally Open 230V Switch that can be turned on/off at a fast rate.

• No moving parts hence no wear and tear.
• Dissipates Heat when in On Condition.
• Use adequate Heat Sink or SSR will fail.
• Input to Output is optically isolated. very tight and crimped.
• The one used here is DC Control AC 230V 15A Load SSR.

Components and Points –

• Connect Power From Lighting (5 A) for Controller.
• 1 Phase 230V Supply With Earth
• Stainless Steel Braided (SSB) Sensor “K” Type ( SSB is Earthed )
• Heater 1 kW and 15 Amps SSR

Fuse Rating of a HRC Fuse

High Rupture Capacity (HRC) Fuse is Safe and Reliable. 10KW Heater at 230V is 10,000 / 230 in Amps of Fuse Rating. i.e. Watts = Volts x Amps hence use
50 Amps Fuse.

Relay Terminology –

• C Common is connected to NC when Relay is off.
• NO Normally Open is Disconnected when Relay off. (connected to C when Relay on).
• NC Normally Closed is Connected to C when Relay off. (disconnected when relay on).

Relay outputs are Potential Free or Floating or at High Impedance.

Note – The Terminations of High Current Lines going to Heater must be very tight and crimped. Loose contacts will Spark and cause Fire.