Component Signature for Oscilloscope

Posted on Jun 7, 2012

The general (and very basic) principle of operation is shown in Figure 1. The test signal is simply derived from the mains, and is a sinewave at 50 or 60Hz. In most locations, the sinewave will be distorted, but this barely matters. With nothing connected to the DUT (Device Under Test) terminals, the oscilloscope simply displays a horizontal line. This represents voltage, and is applied to the X axis of the oscilloscope. If the DUT terminals are shorted, the display changes to a vertical line (Y axis).

Component Signature for Oscilloscope
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This behaviour is easily explained by looking at Figure 1. With the terminals open, there is no voltage across R1, so nothing is applied to the Y axis of the oscilloscope. The full voltage is applied to the X axis, and produces a horizontal line. R1 is selected to limit the current through the DUT to a safe value. Now, if we short the test leads, the signal to the X axis is shorted out, and the voltage from the transformer is now across R1, and is directed to the Y axis resulting in a vertical line. When any component that is not open or short-circuited is probed, there will be a mixture of X and Y axis signals applied to the CRO, and a distinctive pattern is produced. Diodes, inductors, capacitors, transistor junctions and resistors all provide a unique signature, and any mixture of components will produce a result that is easily recognised. The tester is easy to build, and no PCB is needed. Although construction is somewhat fiddly because of the switching, it is very straightforward. The switching is easy to get wrong though, so refer to the circuit diagram and make sure that everything is where it should be. All switch positions are shown on the diagram ... both voltage and current switches are in the 'Low' position. The transformer can be any size you have available or can find - the current drain is no more than 20mA. As shown, a 30V multi-tapped transformer is ideal. Because the first tap on those commonly...

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