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Page Revision: 2020/07/23 20:42
This article describes the design of a electronics debugging tool for electronic circuits.
It's main purpose is to give a graphical representation of the impedance at a node of the circuit, similar to a commercial product, the Huntron Tracker.
The tool allows for very fast fault finding in electronic circuits, especially when a known good circuit is available, so graphs can be compared.
If there is no known good circuit is available, still comparison can be made to similar nodes in the same circuit, and differences point to the culprit.
In example microprocessor pins are all similar, if one pin shows a different impedance, there is reason to doubt the microprocessor as being defect.
Creating an impedance graph is based on a very simple principle: apply a voltage to that pin, and measure the current that flows in the pin.
By applying different voltage, the impedance at each voltage can be shown in a graph.
A good escription of the circuit can be found here:
OctopusTypical test voltages are +/- 15V at currents of a few mA, to avoid damage to the circuit under test.
As explained in the article, for resistors, Ohms law U=IxR will show a linear relation between voltage and current. A straight line.
For a diode, at positive voltages the impedance will be low, and negative voltage will be high.
This kind of characteristics are visible on the screen of the Backpack Tracker.
The name "Backpack" is from the core building block of this tool: the Micromite Backpack.
The Micromite Backpack is a design of Geoff Graham, and is based around a PIC32 processor that can be programmed in Basic. Information can be found here:
BackpackThe Micromite Backpack is a 2 circuit board sandwich existing of a PIC32 board, and a commercial ILI9341 LCD touch color display 320x240 pixels.
In this design the LCD will display the impedance curves, and will act as a very simple control panel with only 2 (touch) buttons.
The PIC32 microcontroller will generate the voltage applied to the circuit under test, measure the current (*) and convert the measurements to an impedance graph.
The software is written in MMBasic, the native language of the Micromite Backpack.
Since the PIC32 processor can only output voltages in the range of 0V..3.3V, an additional amplifier is needed to achive the +/- 15V.
This amplifier, and it's power supply, are located on a third printed circuit board.
The amplifier is shown here:
Amplifier boardA short explanation of the circuit:
The PIC32 processor does not have a DAC output that can be used to generate different voltages, therefore a PWM output is used, running at a rather high frequency of 120kHz. This frequency is an optimum between resolution (the higher the frequency - the lower the resolution, the lower the frequency - the slower the voltages can be varied, the lower the screen update rate).
The PWM signal passes through a low pass filter, to convert it into an analog voltage, that can be amplified by a classical opamp circuit around an LM741.
The LM741 is powered from +/- 16.5V that is generated from 5V in an MC34063 switchmode convertor.
The circuitry fits on a small PCB, the size of the micromite backpack (could form a sandwich of 3).
The circuit board layout is shown here:
Board layoutJ1 is the connector that plugs into the micromite backpack.
To understand how the circuit works it is necessary to know that the current measurement, required to determine the impedance of the circuit under test, is done through a trick: the ADC inside the PIC32 measures the output voltage of the LM741 after it passed through a resistor, and subtracts the measured value from the theoretical output voltage of the LM741. result is the voltage across that resistor, and thus the current through the resistor.
Since this principle (subtract 2 large values to get a small value) is in essence inaccurate, the software performs a calibration at powerup.
A picture of the displayed waveform is shown here:
Display waveformThere are 2 buttons:
MEM: this button stores the current waveform, and displays it in red, as a reference. This would be the waveform of the "known good" circuit.
ZOOM: this button zooms in at the center, to allow more detail if low voltage circuits are probed.
In the screen there is a waveform, characteristic for the impedance of the pin under test (yellow), and 2 markers (cyan).
The markers show the voltage at the waveform extremes, and can be used to detect smaller changes between very similar impedance's. Very handy to check zener diodes.