Schematics of delabs

Here 4052 is used as an analog multiplexer, U3A TL062 opamp is wired as a 1mA Constant Current Source. It pumps 1mA into U2 4052 pin 13 X . Depending on BCD code on inputs A, B of 4052 the current is routed to any one of the four RTD 100E, whose one end is connected to X0-X3. The current then flows to GND thru the Platinum 100E resistance. The Y0-Y3 monitors the mV developed on RTD in tandem with X0-X3 positions. Like a ganged rotary switch. The Output mV at Y is the mV of Active Channel as selected by the BCD of 4029.

RTD Pt-100 Four Point Alarm - del90001

The BCD is provided by 4029 counter which is clocked by a 555. U5A cancels out the 100E mV (1mA * 100E) of a cold RTD and Amplifies the differential mV. This output of U5A is in proportion with measured temperature. U5B compares the mV that was obtained with a preset mV of POT1, which is a user setting called setpoint.

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This is a Low cost controller, Analog Dial Temperature Controller. It is also called Blind Controller. This essentially means Open Loop, just control the fuel or energy input to the system to regulate heat. This is not a Blind Controller that way, it only cannot display the temperature value, that could be another reason it is called blind.

Blind Dial Proportional Temperature Controller

Dial cyclic timers were used to control heat, these were purely mechanical clockwork devices. They could regulate well, when the material flow (liquid) is constant and mains power is regulated. But when the job to be heated, varies in quantity, control temperature is close to ambient or when a precise control is required; closed loop controllers are used. Even a thermostat is like closed loop, as the bimetallic sensor is temperature dependent. But not good enough.

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The OP07 is in a non inverting amplifier so as not load the mV of thermocouple, the zeners are to protect circuit if junction contacts heaters or the earth gets broken.

Thermocouple and Pt-100 RTD

The RC is to filter out 50Hz pick up in thermocouple wires if near heater wiring and also reduces reading jumps when high current three phase contacter operates.

The Pull-up 10M is when a Thermocouple breaks the output of circuit will be max. This is open sensor protection, in case Thermocouple breaks, Required only in industrial temperature controllers for protection. This means it will be 3.5V which should make you turn off the heater in software.

J and K Thermocouple with 4-20 mA

The other opamp is for further amplification as OP07 is set to around 30 gain and offset has to be adjusted with R9. If OP07 is kept in > 100 gain it may be difficult to adjust offset of 75uV. If you need very high gain in the first stage use some instrumentation amplifier or chopper stabilized amplifier. I am not very sure. This is the very basic Thermocouple Amplifier used as a front end signal conditioning in Process Control.

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.

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