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Figure 8-3.  Proper Ohmmeter Connection
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TM-9-254 Gearcase Transfer M548 M548A1 (
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Figure 8-4.  Transistor Shown as a Two-Diode Element
TM 9-254 8-2. Meter Usage - Continued (2) The two-diode concept. (a) It is common knowledge that a semiconductor diode can be checked by measuring its forward and reverse  resistances  and  comparing  these  two  readings.    It  is  also  reasonably  well  known  that  a transistor  may  be  considered  as  a  sort  of  two-diode  device  as  shown  in  figure  8-4.    To  illustrate them, let us assume we wish to check a low-power p-n-p transistor, represented by figure 8-4.  One of the effective diodes, as shown in  figure  8-4  (b)  exists  between  base  and  collector  connections; the  other  exists  between  base  and  emitter  connection.    Quite  simply,  if  either  of  these  diodes  is defective (open or shorted), the transistor cannot function properly. (b) For our first test, we connect the negative lead of the meter to the base of our p-n-p transistor and the positive lead to the collector.  Since we are trying to force a current through the base-collector diode in the forward direction, we will get a low resistance reading if this diode is in good condition. The  exact  reading  may  vary  depending  on  the  characteristics  of  the  ohmmeter  as  well  as  of  the transistor, but a low-power type should generally read quite low, something like 120 ohms. (c) In the second step, the base-emitter diode is checked by disconnecting the positive lead from the collector and connecting it to the emitter.  Note that the forward resistance of the base-emitter diode is slightly higher than that of the base-collector diode, 140 ohms.  This is normal for the low-power transistor. (d) In the third step, we connect the positive lead of the meter to the base of the p-n-p transistor and the negative lead to the collector.  Now we are trying to force a current through the base-collector diode in the reverse direction and we will get a high resistance reading if this diode is in good condition, generally  in  excess  of  50,000  ohms.    Noteworthy  here  is  the  high  ratio  of  reverse  to  forward resistance -- in the order of 500:1 or greater.  The two readings just referred to are test steps 1 and 3 of table 8-1 which is applicable to low-power p-n-p transistors. (e) In  the  fourth  step,  the  base-emitter  diode  is  checked  by  disconnecting  the  negative  lead  from  the collector and connecting it to the emitter.  The resistance reading will be quite high if this diode is in good  condition,  generally  in  excess  of  50,000  ohms.    Note  the  high  ratio  of  reverse  to  forward resistance -- in the order of 500:1 or greater.  The two readings just referred to are test steps 2 and 4 of table 8-1. (f) So far, the readings we have taken apply only to a minority of the transistors that would ordinarily be encountered.    For  one  thing,  nothing  has  been  said  of  n-p-n  types.    For  low-power  units  in  this group, the base-collector and base-emitter diodes effectively have reversed polarity as compared to their counterparts in p-n-p transistors.  We can check them by reversing the polarity of connections. When this is done (the first four lines of table 8-2 can be used as a guide to correct connections), the readings correspond to those obtained with the p-n-p transistors we are using for illustration. 8-5

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