Learning PNP transistor

Discussion in 'The Projects Forum' started by geologie, Mar 25, 2009.

  1. geologie

    Thread Starter New Member

    Mar 25, 2009
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    0
    Hi. I'm a physic teacher that have fun learning electronic in order to make some projects with kids. I bought Dave cutcher's "electronics cricuits for evil genius".

    When learning transistor, i have not problem with NPN theory and practice but with PNP, i understand the theory but the practice isn't working: if i breadboard the "lesson 10", it simply didn't work!

    Here it is :

    [​IMG]


    It doesn't work! voltage on collector is 1 volt when the switch is not push and it is suppose to lit on the led on power ON and power slowly off the led when pushing the switch. Even removing R1, R2 and C1 and the switch, the transistor is suppose to lit on the LED. The theory said that PNP transistor is an open valve until some pressure is on the base of the transistor (opposite of 2N3904 that on power on the LED is off and need voltage on the base to on the LED). I have sevral 3906 and they all do the same.

    What is my mistake - or perhaps it is the book?

    [​IMG]
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Connecting the Base to Emitter will turn either flavor off, be it NPN or PNP.

    With the switch open current flows through R1 and R2, which will turn the transistor on, unless the resistors are too large, in which case it only turns on partly.

    The 1V you see on the collector is when the transistor is on, most of the voltage is actually being dropped across the diode D1 (0.6-0.7V), the rest is across the transistor (0.2-0.3V), which is normal.

    When you push the switch the cap charges almost instantly, and the transistor turns off as fast.

    When you release the switch the capacitor slowly releases its charge (almost like a battery) and the transistor will slowly turn on.

    With no resistors R1 and R2, the transistor will be off, it needs current to turn on.
     
  3. geologie

    Thread Starter New Member

    Mar 25, 2009
    4
    0
    I don<t understand why the circuit didn't light the LED. I'm sure i breadboard the circuit ok but nothing when i plug the 9v nor when i push the switch. Is there any flaws in the schematic?

    Thanks

    Michel
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    OK, so you're say the LED doesn't light with the button is not pushed? I read 100kΩ for R1, 22KΩ for R2, and 470Ω for R3.

    With the button not pushed the collector should be about a volt under whatever the battery is. Are you using a 9V battery?

    The base should be around .7V less than the emitter, this means is on. When the button is pushed the base and emitter are the same voltage.
     
  5. davebee

    Well-Known Member

    Oct 22, 2008
    539
    46
    My suggestion would be to start with a much simpler circuit of nothing but the transistor and R3. Connect the emitter to V+ and connect R3 between the collector and ground.

    (Make sure you've correctly identified the B, E and C leads of the transistor!)

    With this configuration, there should be little or no current through Q1, so the collector should measure just about 0 volts. Connect R1 from ground to the base. This will allow a small amount of base current to flow, and will allow a much larger amount of collector current to flow (larger by just about the current gain of the transistor). You should be able to measure an increased voltage at the collector.

    Work some numbers. They're not that hard, and they're how to really learn circuitry!

    If you connect a 100K resistor to a base that's around 9 volts, then you should get around 90 microamps of current into the base. The 2N3906 is rated at about a current gain of 100, so that 90 uA should allow about 9 mA of collector current to flow. That 9 mA through a 470 Ohm resistor should produce about 4 volts across it. Your numbers will not be exactly these, but they should somewhere in this area.

    If you can't get numbers like these at all, it is possible that the transistor is bad. It does happen.

    If you are able to get the transistor to turn on and off that way, then start adding parts. Add the LED and make it turn on and off. Add the second base resistor and the pushbutton and again test the circuit. Add the capacitor and test it again.

    I think the text in "How it works" is a little confusing, but I think it is referring to the way that base current fills the depletion layer between the collector and emitter and eliminates the "voltage pressure" caused by the depletion layer, allowing current to flow from emitter to collector. This is kind of an advanced description to tell a beginner in circuitry, in my opinion. I would just say that base current allows collector current to flow and disconnecting base current stops collector current.
     
  6. geologie

    Thread Starter New Member

    Mar 25, 2009
    4
    0
    My transistor was dead AND i didn't realised some non-contact row on my breadboard assuming that there is contacts where there wasn't. So now it is working, but the theory on the book say that NPN transitor are normally open and need voltage on the base to conduct and that PNP are normally close, but like i see on the schematic, and my breadboard, PNP also need something on the base too conduct.

    I compare NPN and PNP schematics and i, with only physic knowledge, a difference : with NPN, the base need to have positive to induce conductivity and PNP need negative on the base to conduct AND the negative on PNP base is constant and when put positive on base, transistor conduct less ; am i ok?

    Also, i see that more voltage base on a NPN induct more conduct or the transistor and more positive voltage on the base of a PNP induct less conduct for the transistor; is it ok?

    But all in all, what is the purpose of the PNP? I mean, with NPN, i understand amplification; NPN tansistor receive small + signal in base and they let more electron to pass the transistor. but what is the purpose of PNP if more base + signal induct less conductivity for the transistor?

    Thanks

    Michel
     
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