Instrumentation and Automation – 01

  • If the waveform on the scope droops then you may need to use a 10X attenuating probe or mode. this attenuator is at the tip of the probe hence better measurement possible, also very high frequency signals may be best transmitted over a 50 ohm impedance matched line to the Oscilloscope. The History of Oscilloscope, Learn more about Oscilloscope from the Innovator of the Scope as a Measuring Instrument here is XYZs of Oscilloscopes.
  • Karl Ferdinand Braun – In 1897 he built the first cathode-ray tube…oscilloscope. CRT technology is to this day used by most television sets and computer monitors. The CRT is still called the “Braun tube”.
  • Test a NPN transistor, DMM in 200K range. emitter negative probe, collector positive probe, it should show open. while still holding that way press wet finger on base and collector, it may show some reading, then it is ok. for PNP swap probes polarity and do the same thing. the wet finger when pressed applies a base current in uA.
  • Testing a MosfetTransistor Tester – Testing Transistors – Basic Testing of Semiconductor Devices
  • The most common reason of DMM failure is when a person tries to measure 230V AC with a DMM in current or ohms mode, with the probes in the wrong sockets. Use external attenuators and shunts.
  • A DMM in diode mode or resistance mode can be used to test IR diodes, take it under 100W lamp and measure, cover the IR diode and measure you will see a response in proper polarity.
  • If you put the DMM in 2V mode and measure the voltage across a ordinary LED near a 40W lamp you will see around 200mV. Turn off 40W lamp the mV will fall near 0. Even metal-can transistors with their top ground away will become photo sensors.
  • When you measure low voltages or high resistance like 1M with a DMM your body must not be in contact with the probe tips. Body resistance is low in comparison so error in reading or loading of mV from sensor, in sensitive and high impedance ranges.
  • Do not use an oscilloscope to measure 230V directly, use a transformer, differential probe or attenuator.
  • An oscilloscope 1M probe or a DMM 10M impedance can load circuits with nano-pico amps of bias.
  • Red socket and probe for positive and black socket and probe for negative
  • Is it oscillating ?…. We cannot say, it could also be DC, you need to see if a Clock is present. use a signal tracer or a DMM freq mode, or AC measurement. or build this solorb.com-elect-logprobe. When you do not have a scope you can build one for the PC.
  • Polarity and Zero Cross radio-flier.com-opamps.html see the last circuit it is a polarity indicator, which will tell you to swap the leads. VRS Zero Crossing detector circuit, Zero Crossing Detectors, Comparators, Discover Circuits – Zero Cross.

Testing Tools Resources

The Spice in Analog Design

Analog Designers created Spice, Analog Design Professionals are adding their experience in many commercial and free spice EDA tools. When you are creating a new demanding design, a working solution is achieved by combining workbench and simulation work.

Lab Test Bench and Breadboard testing needs to be done along with analog simulation. Power and mixed signal design too, needs to be verified on board. This is more true for a new design lab or new engineers. Experienced designers will not make many mistakes as design blocks would be in their collections or in their knowledge base. It is this knowledge that can build the simulator itself, this makes the simulator as skilled as the designer but also limited to his or that firm’s experience. In a full digital design, simulation does a more complete Virtual Test bench role.

A simulation tool can help you test the blocks and helps catch some design and concept errors. When you build a live breadboard prototype, some real issues facing your design can emerge.

Spice is a analog design aid and helps mixed design circuit simulation, See the Spice homepage.

SPICE (Simulation Program with Integrated Circuit Emphasis) is a general purpose analog electronic circuit simulator. It is a powerful program that is used in IC and board-level design to check the integrity of circuit designs and to predict circuit behavior. – Wikipedia

Spice Resources

Design Notes – Analog and Opamps – 01

LM335 and LM336 –

LM335 is a temp sensor, LM335 . use it for temp for CJC. Temperature Sensors – RTDs and Thermocouples in Temperature Measurement.

LM336-2.5 is an voltage Reference. precision 2.5V shunt regulator diodes, Applied as a precision 2.5V low voltage reference for digital voltmeters, power supplies or in opamp signal conditioning.

Current Loop –

4-20 ma will drive DC drives, motors and steam valves. It is a current loop, for long distance transmission, current loop is not prone to noise and hum, EMI RFI. As it is a small power transmission no loss of data, also many equipments can be in one data loop. A single transmitter with 4-20mA out can drive a strip-chart recorder, motor, controller and SCADA input. all in a series loop. Current loop is like 4-20 mA

Opamp Notes and Types

  • TL062, LF353, TL072, fet input. 1 tera ohm input imp.
  • OP07 higher price, 1 giga ohm. 75uV offset.
  • LF353 pin to pin replacement of TL062-72 and 82.
  • LM358 works on single supply too, low price, inp imp 1 meg.
  • LM324 Quad, works of batteries well.
  • All work best with dual +/- 5 V or more. LM7805 -LM 7905.

Points to Note –

  • Analog ground (opamps), digital ground (CMOS) and power ground (relays and LED) should be separate, (linked at root)
  • Glass epoxy PCB have high insulation resistance, above 10 tera ohms, and are not hygroscopic which means they do not drink water vapor, this makes them very suitable for precision instrumentation and sensitive circuits.
  • When you measure DC levels in 16bit accuracy or more you need resistors which have temp. coeff. of 10ppm, or you may have to put the entire circuit in a stable 45 deg oven. Thermoelectric effects, EMI, RFI, pA Leakage currents, ground loops, contact resistance all can make the readings drift and unusable.
  • High impedance points of circuit like 500 kilo ohm and above can pick up AC noise and DC leakage currents. this will affect the performance of circuit, so for DC you have to put a guard ring of the signal ground around that point in PCB. For RF you have to shield with things related to iron and mu metal, for low signals even a copper shield will do.
  • Percentage and ppm: when percentage becomes like 0.001% it is difficult to manage, so we use ppm-parts per million.5% means 5 parts in 100 parts. 23 ppm means 23 parts per million parts. 0.001% – shift the dp-decimal point four places to right it is 10ppm. 0.01% is 100ppm which is the variation of value of mfr resistors on temperature change.

Learn this at my Industrial Process Control Circuits

BD139 and BD140 Complementary transistor

BD139 and BD140 Complementary transistor

This pair is my favorite when driving small solenoids or motors. SOT-32 12 Watts..

BD139 – NPN, 80V and 1.5A, 40 beta min.
BD140 – PNP, 80V and 1.5A, 40 beta min.

This means any small solenoid, relay or DC motor can be controlled with this. I would use it upto 24V DC and max of 1A for applications of full on and full off.

In applications like a regulator or variable motor control, you have to keep in mind the current thru and voltage across the transistor which results in power dissipation. Let us say it is driving 0.5A thru a motor in speed control and the Voltage across is 30V then the dissipation could be 30 x 0.5 = 15 Watts.

That is the reason we use switched regulators or switched controllers. So if we chop the DC and do PWM you have only the Switching losses and also a bit of DC loss. Switched controllers are also green, they save power for the same work done, but generate EMI-RFI and care is required in sensitive uV circuits. The transistors for switching are also special as they have to be fast with less capacitance, hence the mosfets are preferred in those applications. – ananth (san)