2019

QP MAILBAG

Quality vs. Quantity

I just finished reading the article “Qualitative Indications From Quantitative Instruments” by Graeme C. Payne (September 2007, p. 76).

The first mistake I noticed in the article was in the schematic symbol. In the old days, “+” would be on the cathode, not the anode. This made sense in power supply rectification circuits but was misleading, so the convention for a diode schematic representation today does not use any polarizing symbols.

In the article, Mr. Payne also describes the forward voltage drop measurement of a diode as a qualitative measurement. I disagree with this statement. It is a quantitative measurement, as it is frequently used to measure forward voltage drops of silicon diodes (approx 0.7V) or Shottkey diodes (approx 0.2V). If the voltage compliance of the current source is high enough, the zener voltage of low voltage zeners can also be measured.

I agree that full characterization of a diode with a curve tracer (not an oscilloscope and current source) is the best method. As with many types of component test instruments, some instruments can provide basic quantitative measurements while others can provide more exhaustive quantitative measurements. Our choice in instrument is determined by the application.

BRUCE PETERSON
Accolade Engineering Solutions
Irvine, CA

Author’s Response

Mr. Peterson is correct to say the symbol for a diode does not have polarity symbols. In technical and engineering literature, it would not be appropriate because the audience is expected to know and understand the symbol.

However, the level of technical electronics knowledge among the Quality Progress audience is unknown. Thus, the placement of the polarity symbols with the negative symbol at the cathode end, where the bar of the symbol represents the n-doped material and electron flow enters the device.

Mr. Peterson is also correct that the voltage drop across a diode junction is a crucial parameter. This is particularly true in an engineering context. A study of the device’s current-voltage (I-V) characteristic curve is an excellent visual method for learning how it works.

However, that part of the column addressed the practical aspects of testing a diode in a service shop or field repair environment. There, a technician is generally using portable instruments, with an emphasis on economy and productivity. A trained technician can distinguish types of diodes from the reading on the meter, but their primary emphasis is on the quality attributes of good versus bad by using a “diode test” function on a digital multimeter (DMM).

For that indication (a voltage) to be quantitative, the output of the current source must be known and well regulated. On a typical DMM, the current source output in the diode test function rarely is included in the performance specifications, and the function rarely is calibrated. Since the source current is unknown, there is no fast and easy way to relate the DMM reading to the diode’s I-V curve beyond “conducting” or “not conducting.” There are assumptions that can be made, but assumptions should not be the sole basis of quality decisions.

GRAEME C. PAYNE


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