23 Apr
2009
23 Apr
'09
2:16 p.m.
On 23 Apr 2009 at 19:21, Tony Duell wrote:
If a transistor passed this test, you need more
complex equipment to
test it. Basically you bpass a small current trhough the base-emitter
juction and see how it affects the collector current -- in other words
you use the transisorr as am amplifier. Real enthuisats have a thing
called a 'curve tracer' which does this automatically and plots one of
a range of voltage-current or current-current traces on a CRT. But I
find my Tekky 575 (such an instrument) rarely gets used when I'm
faultfinding, the defective transsitor can be found with simpler
tests.
Most currently available inexpensive DMMs that I've seen have the
capability to check transistor hFE, The last freebie I received with
a parts order has that capability, a temperature probe, capacitance
measurement and a frequency meter-and a backlit LCD display. They
just keep packing more and more into the same little hunk of silicon.
Where the two-diode transistor falls apart is in applicatons where
Vce is less than Vbe. The transistor works, but it's hard to
rationalize why if you're using the "two diode" analogy. One of my
favorite Bob Pease columns:
http://electronicdesign.com/Articles/ArticleID/15690/15690.html
--Chuck