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The two-diode concept.
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TM-9-254 Gearcase Transfer M548 M548A1 (
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Table 8-1.  Low Power P-N-P Transistor Characteristics
TM 9-254 8-2. Meter Usage - Continued (g) We  must  also  take  separate  account  of  transistors  designed  to  handle  more  power  than  those already considered.  Those used in the audio-output and power-supply applications may fall into the medium-power  and  high-power  class.    However,  circuit  application  is  not  a  reliable  guide.    The manufacturer’s  ratings  should  be  consulted  when  there  is  any  doubt.    Transistors  with  collector dissipation  rated  below  250  milliwatts  may  be  considered  low-power  types.    Where  the   rating exceeds 3 watts, we are dealing with high-power types. (h) To    understand    why    power    rating    will    affect    resistance    readings,    we    once    more    consider semiconductor diodes.  A diode designed for use as an video or audio detector passes little current, whereas one intended for use as a power-supply rectifier will handle a current many times greater. This difference is reflected in both forward and reverse resistance readings.  The detector diode will measure  much  higher  than  the  power  supply  rectifier.    Similarly  transistors  designed  to  handle greater power will pass more current and show lower resistance values. (i) Tables 8-3 and 8-4 give ohmmeter connections and typical resistances measured respectively on p- n-p and n-p-n medium-power types.  Readings for high-power transistors are given in tables 8-5 and 8-6.  Note how forward resistance of either diode sections, drops off from 120-140 to 80 and then 50 ohms  as  power  rating  increases.    The  same  pattern  is  observed  in  reverse  resistance  readings. However,   a   helpful   fact   can   be   noted.      Although   there   may   be   uncertainty   concerning   exact resistance readings obtained in any single measurement, there is a very clear relationship between forward and reverse resistance, with the latter being a few hundred times greater than the former.  If this does not show up on a test, the transistor may be considered defective. Figure 8-4.  Transistor Shown as a Two-Diode Element 8-6

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