Problem with transisor current

Discussion in 'General Electronics Chat' started by qbvbsite, Sep 30, 2012.

  1. qbvbsite

    Thread Starter New Member

    Jun 29, 2010
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    Hey,
    I have been learning about basic electronics for a few weeks (again) and currently i'l working with transistors. I created a very basic circuit in a simulator and got the exact expected result (photo below). I then went ahead and bread boarded the circuit and didn't get what I was expecting. I was wonder if someone could look at it for me?
    Simulated Circuit
    [​IMG]
    I get the pretty much the expected result.... 1.5mA into Base and 115mA on the LED
    Bread Board Circuit
    [​IMG]
    This I do not get what I see on the simulated circiut.... I get 1.7mA in the Base (i'm fine with this).... but I only get 3.8mA to the LED
    Can anyone explain why this happens? do I have something wrong?
    Parts
    - 2x 1.2v AA Battery
    - 1K resistor
    - PN2222A Tranisitor
    - Red LED
    Thanks,
    --James
     
  2. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    1. How is the LED spec Vf, If? (The basic spec of LED)
    2. You should series a resistor between LED and C of Q1, that is the limitation resistor protection the LED.
    R = (2.4V - V_Led - 0.2(Q1 Vce)) / I_Led
    3. Redo the labs again.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    You should not go near a transistor without looking at its datasheet.
    You connected the transistor UPSIDE DOWN!

    Then its collector and emitter are backwards.

    If the circuit was wired correctly then the LED would have burned out in a moment because its DATASHEET says so.
     
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  4. qbvbsite

    Thread Starter New Member

    Jun 29, 2010
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    Hey,
    Thanks for the tip... I just checkout the spec sheet briefly just clearly miss read the pins. Its working much better now... with no load now it goes from 16.7mA base input to 660mA on the collector. I'm guessing that I get about 39x gain. But when I reduce the current on the base to 1.5mA i get 230mA output on the collector which is like 153x gain... why is this? Also on either circuit if I put a LED on it the collection current drops to about 34mA.... Add a 22ohm resistor between the collector and the LED it drops to 16mA... I'm guessing the LED automatically limits the current to around 34mA? but its better to limit it with resistors to ease the load on the LED is this correct? sorry if this seems like beginner stuff, but I am just starting :).
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    You forgot to tell us which transistor you are using and which LED so we can see their spec's on their datasheets.
    Most little transistors work well up to a collector current of about 100mA but the hFE drops with more current. At 230mA your transistor had a current gain of 153 and at 660mA it gain dropped to only 39 which is normal. I suspect that your battery voltage is dropping which is limiting the current, measure its voltage when the transistor is conducting 660mA.

    The current drops when you connect an LED between the collector and the positive supply because the transistor does not have enough base current for it to be a switch. The datasheet for nearly every transistor shows its low "saturation voltage loss" which is when it is a switch. Then hFE is not used and the base current must be 1/10th the collector current. But some transistors will have more current gain.
    If you use the correct amount of base current then the LED will quickly burn out if it does not have a current-limiting resistor.

    Most 5mm diameter LEDs are spec'd at 20mA. 30mA or 40mA is their maximum allowed continuous current. So of course you need a resistor or circuit to limit the current.
     
  6. qbvbsite

    Thread Starter New Member

    Jun 29, 2010
    25
    0
    Hey,
    The transistor I'm using is PN2222A Transistor (Generic NPN Transistor). As for the Red LED I'm not exactly sure as its from components I orders quite some time ago. I tried reading the Transistor datasheet and I currently have no idea what all the values are :S. Lots to learn I suppose.

    --James
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    Sorry, you DID tell us that you are using a PN2222A transistor.
    Here are important spec's from its datasheet:
     
  8. qbvbsite

    Thread Starter New Member

    Jun 29, 2010
    25
    0
    I'm guess these are what the symbols mean:

    Ic = Current on Collector
    Ib - Current on Base
    Vce - Voltage on the Collector and Emitter

    Also when voltage on the Collection and Emitter is above 0.3v will this then become a saturated switch and anything below that wouldn't work? Is this basicly the "junction forward voltage"?

    In their test conditions lets say I do what a Ic of 500mA with a Vce of 10V... I would have to put a Ib of 12.5mA? Or if I wanted a Ic of 150mA with a Vce of 10V I would need a Ib of 1.5mA?

    Thanks for all the help it's quite useful.

    --James
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    Yes.

    It is not a junction forward voltage. Instead it is when the transistor is turned on hard so that its collector to emitter voltage is as low as it can go, like a turned on switch. When a transistor has plenty of collector to emitter voltage then it is a linear amplifier, not a saturated switch.

    First of all then the power in the transistor will be 5W and it will smoke and burn. The hFE at 500mA is 40 to maybe 100 so the base current will be 5mA to 12.5mA. You must design a circuit so it works with any hFE of the transistor.

    Then its power is 1.5W and again it will smoke and burn.
    The hFE at 150mA is from 100 to 300 so the base current will be from 0.5mA to 1.5mA.
     
  10. qbvbsite

    Thread Starter New Member

    Jun 29, 2010
    25
    0
    Hey,
    Thanks for all the help. I have ran a few tests to see what kinda of current I get they are as follows:

    V1: 2.55v
    LED1: Generic Red LED
    Q1: PN2222A
    R1: 1K
    R2: 1K

    Test #1 - Complete Circuit with All Resistors and LED
    - Base: 1.7mA
    - Collector: 0.6mA
    - Emitter: 2.5mA

    Test #2 - Removed R1
    - Base: 20mA
    - Collector: 0.6mA
    - Emitter: 20.7mA

    Test #3 - Removed R1 & R2
    - Base: 20mA
    - Collector: 29mA
    - Emitter: 48.6mA

    Test #4 - Removed R2 & LED1
    - Base: 1.5mA
    - Collector: 220mA
    - Emitter: 220mA

    Test #5 - Removed R1, R2, & LED1
    - Base: 20mA
    - Collector: 660mA
    - Emitter: 660mA

    I few questions I have:
    - Will the Emitter pretty much equal Base + Collector?
    - In Test #3 why when adding the LED do I only get 29mA when 20mA is on the base? Is the because the LED has a DC forward current of 30mA?
    - In Test #2 the Collector current doesn't change from Test #1 is this because of the 30mA forward current of the LED?

    I know some of the test are not how you would make a circuit and will probably burn out component is connected to long but there are just tests to better understand the working of the transistor.

    Thanks Again,

    --James
     
    Last edited: Oct 2, 2012
  11. Audioguru

    New Member

    Dec 20, 2007
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  12. qbvbsite

    Thread Starter New Member

    Jun 29, 2010
    25
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    Hey,
    Thanks for the answers.. I still don't understand when adding the LED I'm only measuring a fraction of what the current is. I'm gonna start to read more of the DC book found on this website which will hopefully clear this up. Thanks again for all your help, I have learned much from this thread.

    --James
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    When you add the LED to the collector then the collector voltage drops about 2V from 2.55V to about 0.6V. Then its current gain is a little less than with 2.55V Vce.

    But something is wrong with your measurement because your collector current dropped much too low. I think the contacts on your breadboard are intemittent or the connections of your current-measuring meter.
     
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