SFT Component Tester Project
![A photograph of the newest prototype](sft_tester.prototype.jpg)
The SFT Component Tester is a microcontroller-driven test signal and measurement unit that uses
various common "ohm meter" style test methods to determine whether or not a component
is good or bad, and the type of component being tested. The primary purpose of the design was
to test a number of 'Proof of Concepts' for microcontrollers in general, as well as discovering
the limitations, known design issues, and as many "unknown" or "unexpected"
issues that a single project might be able to uncover.
This device is certainly a "work in progress". It was my original hope that I could build
an in-circuit test device that would allow me to identify bad parts without actually removing them
from a circuit, by testing the device with "safe" signals. Although the hope of that feature
remains, I am still working on the basic functionality of the device.
This project remained 'on the shelf' for quite a while, in favor of other projects, because of a problem
I discovered with the A:D converter on the AVR microcontroller: Even though the DC input impedence of the
A:D converter is relatively high, the nature of the conversion causes 'inaccuracies' to appear when a series
resistance greater than about 10KΩ exists between a measured voltage and the A:D converter. This meant
that resistance measurements greater than 100KΩ would be highly inaccurate. All of my attempts to
reconcile this were pretty much futile. It meant a re-design to include op amp 'buffers' as well as a split
power supply to allow for the input voltage range being anywhere between 0V and 5V.
Features
- Simple interface - 3 colored test leads and one pushbutton with LCD display
- Reasonably fast operation
- Operates on 9V battery or external 9V (regulated) power supply
- Tests semiconductors, resistors, inductors, and capacitors.
- Detects semiconductor devices and determines whether good or bad (still need work on MOSFET, JFET)
- Designed to accurately measure resistance from 1Ω to above 10MΩ
- Design minimum measureable inductance approximately 100µH [TBD]
- Design minimum measureable capacitance approximately 10pF
- Design maximum measureable capacitance above 100,000µF
- (reserved, to be implemented) Electrolytic capacitor ESR calculation, based on charge rate and measured impedence
For a large value capacitor, the charge rate is used to determine its capacitance. However, electrolytic
capacitors typically have a large 'Effective Series Resistance' that can affect their impedence and
ability to filter power supply ripple. By calculating an estimate of the ESR, you can often determine
whether a capacitor is wearing out and needs replacement, even if its capacitance is still 'good'
- Calibration procedure via external software and serial port
- POSIX-compliant command line utility
- Also builds and runs on Microsoft Windows
- Procedure involves taking measurements on a single known resistor (10.0KΩ) and a 10% tolerance 0.1µF capacitor,
plus shorting specific test leads and taking 'open lead' measurements (when prompted).
- Calibration data is stored to NVRAM on the AVR microcontroller
Project Status
- Latest prototype, in progress 7/2013
- Placed CPU and regulator directly on the controller board, making for a more compact design. Official board design to follow.
- Added LF353 op amps to isolate A:D converters, and a simple charge pump to provide appx -4.5V to the op amps (supply is +9V/-4.5V)
- AVR's A:D converters were unequally loading high impedence devices, causing inconsistent measurements
- Research on the AVR microcontroller's A:D suggests that a high-Z buffer amplifier is necessary for any measurement above 10KΩ
- Charge pump is driven by the same 500Hz PWM output that provides an AC signal for testing reactive components, using the 9VDC power leg
- The re-designed controller board fits both a 2-line and 4-line LCD without difficulty.
- Additional software work needed to improve accuracy and accomodate latest design changes
Additional Photos
![Exploded view of SFT Component Tester prototype](sft_tester.prototype.exploded.jpg)
This is an exploded view of the assembled unit shown at the top of the page
![Original breadboard layout that tested the basic features](tester.breadboard.jpg)
This is the original breadboard layout that tested the basic features of the device
![First prototype, Nov 9, 2011 (board underneath is not visible)](tester.prototype.20110119.jpg)
This is the first prototype, Nov 9, 2011. The main board underneath is not visible.
©2013 by Stewart~Frazier Tools, Inc. - all rights reserved
Last Update: 7/24/2013
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