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Wednesday, November 19, 2014

Thermocouple and Pt-100 RTD

The easiest way to measure temperature is with a glass diode like 1N4148 or a metal Transistor like 2N2222. These have a small temperature range, probably -5 to +120 is possible, but non-linear. A copper coil or any metal wire or even a wirewound resistor can work as temperature sensor but the thermal inertia makes it sluggish. IC semiconductor based sensors like LM335 and AD590 are easy to work with and pretty accurate. In industrial environment many types of Thermocouples, Pt-100 RTD and Optical Infra-Red Temperature measurements are used.

Input Levels for Thermocouples and RTD Pt-100. And the ways it has to be used. Thermocouples are selected on the basis on how they are mounted or fixed, the environment, the temperature range that is measured and finally the price you can afford for a particular application. Sometimes mechanical wear or chemical environments may eat away the sensors, so a cost-effective solution has to be evolved.

One customer wanted a special Pt-PtRh very high temperature sensor for molten aluminum. It was very difficult and costly to even arrive at a solution. So a non-contact infra-red sensor may save lot of time-money and even reduce risk of injury.

Thermocouples and RTD Tables

For J, K, and T Thermocouple Read "Temperature" + Room Temperature for mV shown applied to equipment, eg....

If Room Temperature is 25 Deg C and mv applied is 10.777 for J Type STC1000 then reading should be 200 + RT = 225 deg C. The mV shown are for Reference Junction at 0 Deg C.

It would be better if you refer links below, The above table i made long ago, then a Scan with OCR later formatting. It could have errors.

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Monday, November 10, 2014

Inverting Amplifier - Op-Amp Designs

This is an Op-Amp Tutorial. Here you see an Operational Amplifier in the Inverting Amp configuration. I have used OP07 as it is almost like an ideal Opamp. The ultra low offset is the best part. Then not really, CA3140 has a Tera Ohm Input resistance but offset not the best. ICL7650 approaches an Ideal Op-Amp and chopper stabilized.

Inverting Amplifier - Op-Amp Designs

These parameters are good for DC Amps. Sensors, Strain Gauge Bridges Signal Conditioning need them. Then when you want a better AC performance, you can use a LF353 with a nice slew rate. Here the DC merits are not of much use.

Inverting Amplifier - Op-Amp Circuits

Input Impedance of this module is Ri as pin 2 is at virtual ground, the opamp with feedback tries to maintain pin 2 and 3 at same potential pin 3 is at 0V hence pin 2 is at virtual ground. Clamping diodes protect OpAmp, Rf + Ri is between 5kE and 1ME as an opamp may be able to drive around say 5mA max

Current into node pin 2 = Vin/Ri if Vin is +ve it raises potential at pin 2, in order to bring it to 0V the OpAmp sucks away the current by turning its output negative the current leaving pin 2 node is also Vin/Ri. Then Vout is given by Vin/Ri * Rf as per V=IR ohms law. Most OpAmps output swings around 1v less than VCC/VDD for full swing use CA3130 this is a FET input OpAmp, and has low bias currents in pico amps.

Slide the Potentiometers just like you would operate a Sliding Control. Drag the Knob on Pot to increase or decrease the resistance. The Resistance is shown is blue letters and dynamically alters value as you slide the pot. Both Rf and Ri have Pots which are variable resistors.

The mV Source is varied by just moving mouse pointer over the two buttons, no clicking. This reduces finger strain and also you have a long lasting mouse. The mV buttons are special, the variation picks up speed if you let the mouse pointer remain on the button. This is Ramp-up and Ramp-down. This enables you to set it faster with just two buttons. This mV Source sets Vin.

The mV DPM at Output shows Vout. Vin and Vout are in mV.  The formula you know

Vout = Vin * (-1) * (Rf / Ri)

A real pot has a Minimum 0 ohms value, but in these pots i have shown 10K as min. Even in a real design, depending on input source impedance and output voltage the resistors are better kept high. Never go less than 4.7K, the OpAmp loads or the source, even another opamp stage or sensor loads. These can be resolved by using buffers or power amps respectively, if such a need arises. For all analog computing/signal conditioning use 10K min. Battery operated designs have 100K as min and FET amps only.

Values are to give an idea, keep in mind Battery Consumption, Source Impedance and what the Op-Amp need to drive. Most Op-Amps today are short circuit protected but even little loading will upset your equations. Very high resistance, even above 1M, careful PCB design is required. Leakage currents on PCB will play spoilsport. A little sunshine and moisture, there can be copper sulfate formation on terminals and tracks on exposed PCB. There is no formula for that. Theory and Practice in Tandem, this works.

Never drive any Opamp pin to a voltage higher than the supply +/- dual supply. Use clamping diodes, read specs well.

The Force does not happen to everybody. You have to master your Theory and Formulas. You then have to Experiment, Try out and Understand in detail. Then you shall modify, create & Juggle circuits !

Thursday, November 06, 2014

Buffer Unity Gain Amp - Op-Amp Designs

If output impedance of a point is a high value then connecting another circuit at that point will load it, resulting in malfunction or error. Buffers are used as interface between circuits.

Low impedance of an output means it can source/sink lot of current, when you need 2 opamps use LF353 or TL072 which are dual opamps.

Buffer Unity Gain Amp - dapj Tech Widgets

Buffer Unity Gain Amp - Op-Amp Designs

A non-inv FET input is the best buffer, for inverting buffer use high R values. Using very high R values like 2.2M or higher requires a glass epoxy PCB and guard rings around pin 2, 3 to prevent leakage currents on the PCB reaching the PINs.

Also moisture and dust has to be prevented by using RTV coating or Varnish. Use 78L05 and 79L05 for the dual supply required by this circuit.

Inverting Amp   -   Vout = -1 * Vin

Non-Inverting Amp   -   Vout = Vin

Vary the Millivolt Source by just hovering your Mouse pointer over the Two Buttons (Increment and Decrement). If you leave the mouse pointer on a button, the variation will pick up speed. Set a Value of your choice, positive or negative, you can set from -199.9mV to +199.9mV. Note the Output values on both the DVMs on the right (digital Volt Meter) in a Table. Repeat this for at-least 7 readings of both polarities. Jot the Formula below in your book and relate it to your table.

Monday, November 03, 2014

Electronic and Instrumentation Tables

Tables, charts, quick reference cards, booklets and handbooks; are the most versatile tools of learning and putting the learning into applications. These should not have a lot of language or flowing text. These are - to the point, easy to remember, graphical, colorful (memory aid).

Electronic Tables and Charts

Electronic and Instrumentation Tables

Just like a circuit diagram, flow chart or ladder logic can graphically interface with a human mind, Tables and Charts easily embed into ones memory.

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