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SKA6516: Difference between revisions
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{{MissingSpecs}} | {{MissingSpecs}} | ||
==Suspected mass failures== | |||
[[User:Zenwizard|Zenwizard Studios]] found thirteen of these in a [[475]] and showed that thirteen of those tested as "having a parasitic reverse diode junction" on a cheap component tester. | [[User:Zenwizard|Zenwizard Studios]] found thirteen of these in a [[475]] and showed that thirteen of those tested as "having a parasitic reverse diode junction" on a cheap component tester. | ||
''There is no such diode, parasitic or not'' – the cheap tester displays this symbol when the DUT has a low reverse breakdown voltage as seen in the traces below. | |||
See the thread at https://groups.io/g/TekScopes/topic/101335545#201002 – [https://engineering.stanford.edu/people/thomas-lee Prof. Tom Lee] says: | |||
<blockquote> | |||
It's relevant to mention that many high-ft transistors have vey low breakdown voltages in inverted mode (i.e., collector and emitter exchanged). | |||
Depending on how those Chinese component testers do their analysis, it's possible that the transistor is actually driven into reverse avalanche breakdown, which could very well be (mis)interpreted by the tester's algorithm as a diode being present and forward-biased when the emitter voltage is above that of the collector. | |||
For grins, I tried some 5 GHz microwave transistors (Toshiba 2SC3302) that are known good (my students use these by the bushel in the microwave circuits class I teach). | |||
They (the transistors, not the students) all behave the same on the Chinese component tester as the "bad" 151-0367-00 transistors. | |||
The tester insisted that these transistors had the infamous C-E diode. | |||
A standard DMM diode test does not show this diode, because the DMM's applied voltage is too low to provoke reverse breakdown. | |||
So, the tester is the problem, and not the transistor. | |||
Based on this set of results, an automatic "replace on sight" policy for these transistors based solely on a component tester's say-so seems unjustified, especially since the tester is most likely to make an error when evaluating possibly expensive high-ft devices. | |||
</blockquote> | |||
Since reverse breakdown can damage the DUT, small-signal RF transistors should not be tested on component testers that may trigger that breakdown. They should be checked only on a curve tracer. | |||
'''Suitable Modern Replacement''' | '''Suitable Modern Replacement''' | ||