Test-Measurement (Page 3)

This was a Student Project of mine, it is a very basic instruments that could do most of the tasks on a test bench.

The cabinet used, was available as a part of a Hi-Fi DIY System, The Range and Mode switches were Modular Stackable Interlocked MultiPole Switches. The rest of the Interface were wirewound pots, jacks, sockets and plugs. The front panel was Engraved Acrylic, the panel was the toughest issue we tackled.

I wanted it to look well engineered, at least on the outside. It was a total analog instrument except for the digital indication of ICL7107. Thanks to ‘Intersil Hot Ideas’ Applications Book.

The above belong to an instrument Measureall shown below.

MeasureAll Benchtop Instrument

This is a Milli-Volt and Milli-Amp Source for Calibration of Temperature Measurement and Control Instruments using Thermocouples. There is also a Passive Variable Resistance Source for Simulating a PT-100 RTD Sensor.

Current Source mA for calibration

Temperature Calibrator

This calibrator was a simple solution i built. This instrument itself was calibrated by Instruments like the Calibrators from Omega which are the best you can get.

Instruments used in Calibration

Here are some Test and measurement notes. These ideas come from practical experience. But the jots are not in any order but are quite important.

Instrumentation and Automation

Scope Notes

  • 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.
  • 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”.
  • 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.

DMM Notes

  • 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.
  • 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.
  • 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 indicators tell you to swap the leads. Use a comparator or opamp to build it.

The LM3914 is a monolithic integrated circuit that senses analog voltage levels and drives 10 LEDs, providing a linear analog display. A single pin changes the display from a moving dot to a bar graph. Current drive to the LEDs is regulated and programmable, eliminating the need for resistors. This feature is one that allows operation of the whole system from less than 3V.

LM3914 – Versatile and Easy use Display

  • Drives LEDs, LCDs or vacuum fluorescents
  • Bar or dot display mode externally selectable by user
  • Expandable to displays of 100 steps
  • Internal voltage reference from 1.2V to 12V
  • Operates with single supply of less than 3V
  • LED driver outputs are current regulated, open-collectors

The circuit contains its own adjustable reference and accurate 10-step voltage divider. The low-bias-current input buffer accepts signals down to ground, or V, yet needs no protection against inputs of 35V above or below ground. The buffer drives 10 individual comparators referenced to the precision divider. Indication non-linearity can thus be held typically to ½%, even over a wide temperature range.

My LM3914 Circuits

Millivolt Meter using a LM3914 LED Dot Display
Millivolt Meter using a LM3914 LED Dot Display

Rechargeable AA cells are very popular for a large variety of portable applications. The number of brands is also high, and the cell quality can differ considerably. Cell capacity is sometimes different from that specified, and the capacity often deteriorates dramatically after a limited number of discharge cycles. ….

See Maxim Application Note – Simple Characterization Circuit for Rechargeable AA Cells

This characterization circuit, plus a PC and some software, accurately measures the full discharge cycle for a rechargeable AA cell. Cell capacity and output resistance can easily be determined from the curve resulting from these measurements.

MAX8881 and DS2490 Characterization for Cells

In Figure 1, the simple circuit is connected to the USB port of a PC. No external power source is required because power is extracted from the USB bus voltage. U1 (DS9490R) is a USB dongle that converts the USB protocol to the 1-Wire® protocol, thereby allowing the PC to communicate with the key circuit component (U2).