npn vs. pnp

Discussion in 'General Electronics Chat' started by psihodelia, Aug 9, 2006.

  1. psihodelia

    Thread Starter Member

    Aug 7, 2006
    23
    1
    There is many circuits where you have to have both sorts, pnp and npn transistors. I'v heard it's possible to change a circuit so, that it will have only npns (or pnp only).

    I have a lot of npn transistors and I don't understand how to integrate them into a circuit where pnp are planed.
     
  2. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    144
    The basic premise of using npn transistors where the pnp are planned, is to reverse all polarities due to the fact one type utilises electrons for current flow and the other utilises holes.

    Dave
     
  3. psihodelia

    Thread Starter Member

    Aug 7, 2006
    23
    1
    how to reverse polarities if both pnp and npn transistors are there in a circuit graph ? (and I have only one type)
     
  4. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    144
    Can you post up the circuit that you are currently working on? It is easier to explain if we have a circuit to hand.

    Dave
     
  5. psihodelia

    Thread Starter Member

    Aug 7, 2006
    23
    1
    please,
    this is very simple LED flasher, must flash 15 times per minute

    Code ( (Unknown Language)):
    1. 2 Transistor LED-flasher
    2. vin             0       1       9
    3. r1              0       2       6.8Meg
    4. r2              2       3       1K
    5. d1              1       2       1N914
    6. c1              4       3       6.8u
    7. r3              1       4       390
    8. q1              5       2       1       2N3904
    9. q2              4       5       6       2N3906
    10. d2              0       6
    11. .model 2N3904 NPN (Is=6.734f Xti=3 Eg=1.11 Vaf=74.03 Bf=416.4 Ne=1.259 Ise=6.734 Ikf=66.78m Xtb=1.5 Br=.7371 Nc=2 Isc=0 Ikr=0 Rc=1 Cjc=3.638p Mjc=.3085 Vjc=.75 Fc=.5 Cje=4.493p Mje=.2593 Vje=.75 Tr=239.5n Tf=301.2p Itf=.4 Vtf=4 Xtf=2 Rb=10)
    12. .model 2N3906 pnp PNP (Is=1.41f Xti=3 Eg=1.11 Vaf=18.7 Bf=180.7 Ne=1.5 Ise=0 Ikf=80m Xtb=1.5 Br=4.977 Nc=2 Isc=0 Ikr=0 Rc=2.5 Cjc=9.728p Mjc=.5776 Vjc=.75 Fc=.5 Cje=8.063p Mje=.3677 Vje=.75 Tr=33.42n Tf=179.3p Itf=.4 Vtf=4 Xtf=6 Rb=10)
    13. .model 1N914 d
    14. .tran 0.2 10
    15. .plot tran v(0,4)
    16. .end
    17.  
     
  6. n9352527

    AAC Fanatic!

    Oct 14, 2005
    1,198
    4
    Are you sure the netlist is right? It seems that Q1 and Q2 are reverse biased. Ditto with D2 which is also missing the model.

    Edit:

    Just noticed that you put the positive supply at 0 (ground) and hence you are running with negative rail. Unorthodox :)

    Now, D1 seems to be revese biased, there's no use of D2 and you better have a non-polar cap for C1, otherwise it could go bang and there's no base resistor for Q2 to limit the base current.
     
  7. psihodelia

    Thread Starter Member

    Aug 7, 2006
    23
    1
    I am not sure, that this netlist is right, because I am very new in circuits design with SPICE. Can you help me, please ? This is original graphical scheme http://www.uoguelph.ca/~antoon/circ/2qflash.htm I just tried to represent it in SPICE (and simulatuon doesn't work properly)
     
  8. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Is a complimentary darlington really needed here? Wouldn't a single NPN do just as well?
     
  9. Chris Wright

    Well-Known Member

    Jul 26, 2006
    62
    0
    Unless you just want to learn SPICE or are required to use it for some course work, try the Java applet curcuit simulator at http://falstad.com/circuit/ . Choose the "blank" circuit under the examples and right click (or control click) to display †he list of components and draw your circuit. The current is animated and you can display components in a "scope". You can download the zip file and run it off line also.

    I drew your circuit in about 5 min. and the LED blinked twice until it ran into a glitch. Didn't have time to play with it. Give it a try.
     
  10. psihodelia

    Thread Starter Member

    Aug 7, 2006
    23
    1
    Java is not Open Source software, my CPU platform cannot run Java (SUN don't have a port for that)
     
  11. Joe S.

    Member

    Aug 28, 2006
    10
    0
    Electrons and holes are sure to confuse you right now.
    If you studied tube circuits you learned that electrons flow from - to +.
    In trying to adapt that Knowlege to transistor circuits you will not feel well.

    Current flow is what you need to understand, and it's really simple. Current flows from positive to negative, it goes in the direction of the little arrows in the diodes and transistors. So in an NPN transistor the positive current flows into the collector and out through the emitter. In a PNP transistor the Positive current flows into the emitter and out through the collector. a positive voltage turns on the NPN transistor and a Negative voltage turns on the PNP transistor.

    For a good Basic Electronics tutorial get one of Forrest Mims books, he is really good at giving you the basics. I have a 1/2 dozen myself and refer to them from time to time.

    regards,
    JS
     
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