Saturday, January 23, 2016
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.
Tuesday, October 06, 2015
The Resistor R6 limits the current. The zeners should be low leakage or use clamping pull-up and pull-down diodes to +5 and -5 respectively.
The RC low-pass filter formed by R6 and C2 reduce the mains hum or 50 Hz pickup of long thermocouple cables laid close to high current heater wiring. R1 is a offset null use or add if required. R11 is gain control of OP07. The TL072 is a FET input opamp used here as a summing amp.
Adding one more inverting amp with some gain to the output of this circuit can give you a 1-5V suitable for ADC or PC analog I/O cards. C1 also serves to filter, it is an integrator here. It suppresses EMI and RFI from motors, contacters etc., R13 sets an output value for 0mV input.
Wednesday, September 23, 2015
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.
Saturday, October 14, 2006
Measurement Of Temperature - When power transistors are used, they may tend to over heat. Likewise resistors may also overheat in the event of faults or short-circuits. The knowledge of their temperatures may be advantageous. In addition, measurement of temperature constitutes a basic necessity in day-to-day life.
Measuring the temperature of a body, depends upon the establishment of thermo-dynamic equilibrium between the body and the device used to sense the temperature. In practice, this condition is rarely attained since it is difficult to establish complete instantaneous equilibrium. Hence great care must be exercised in choosing a method suited to the problem so that satisfactory conditions for temperature measurements are obtained. Temperature sensors possess thermal characteristics dependent largely on their size and shape and the materials from which they are made. These characteristics affect precise measurements. The introduction of a temperature sensor into a body tends to modify the temperature conditions at that point. In most cases the sensor is connected to a recording instrument by means of an intermediate system, along which the signal is carried. The intermediate system and the recorder may be subject to temperature and other changes. Hence compensating devices become a necessity to reduce or eliminate errors.
The measurement of temperature in our instrument depends on the fact that the forward voltage drop of a silicon diode changes by about - 2 millivolts per degree centigrade. Thus, by measuring the change in forward voltage of silicon diode kept in a temperature probe, the voltage drop can be converted into temperature.
Since this involves the measurement of millivolt level accurately a precision voltage source is needed. This can be conveniently obtained from the 3 pin + 5v voltage regulator. This voltage is tapped using a preset VR6 whose output is used for adjusting the ice bath temperature reading to zero degree. This tapped voltage is fed to the diode in the temperature probe and the other end of the diode is returned to a negative supply of -8v. The negative supply uses a (-8v regulated output from IC 7808 voltage regulator) which has the least variation with temperature. Now, the voltage at the probe point is connected to the input of DPM via function selector switch ST.
The temperature probe can be made by a length of shielded audio cable connected to any type of mini plug and fitted onto the front panel socket SSG/T. The free end of the cable is soldered to the diode. The diode is kept just at the tip of the cable. A miniature glass diode like 1N4148 is preferred. The soldering makes a good fixture at the end of the cable. The meter can thus measure temperatures from 0°C to 150°C continuously and upto 200°C momentarily since above that the cable starts melting.
Epoxy Resin and a used Metal Pen Refill can be used to make a sensor to insulate the cable. The diode must be thermally and electrically isulated from metal tube.
(above text may have ocr and concept errors)
Extra Reading -