my transistor is sending current without base

Discussion in 'General Electronics Chat' started by mrSilkie, Aug 23, 2013.

  1. mrSilkie

    Thread Starter New Member

    Aug 23, 2013
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    The title has been poorly constructed and my circuit probably has been aswell.

    I got an arduino earlier this week and it has been great, it is the ultimate platform for learning digital electronics, it doesn't just drop me off in the deep end and i already have some coding experience so the learning curve isn't too high.
    Anyways, tonight i learnt about relays and transistors, turns out i have a transistor in my little arduino starter kit so i made a circuit with 5v connected to the collector, and a resistor and LED on the emitter end, i'm just using the 3v wire on my arduino to apply a current to the base. My problem is that even when the base isn't getting a current, the led is still dimly lit. Is this because the transistor has a maximum voltage? other then that, i have no clue what could be causing it.

    http://www.technologystudent.com/elec1/transis1.htm
    my transistor is similar to the bottom transistor in the animated gifs seen at the bottom of the screen.

    Also, because I've only been learning about electronics for a week or so, are there any good books that will show me the absolute basics?
     
  2. pwdixon

    Member

    Oct 11, 2012
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    You need to draw a schematic because your words do not match the quoted schematic so you might have actually done anything. You need to indicate the transistor type number as well and show how you think you have actually connected to on the Arduino.
     
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
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    How do you know the base isn't getting a current?

    Is it still connected to something?
     
  4. mrSilkie

    Thread Starter New Member

    Aug 23, 2013
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    the transistor is on a breadboard, but when i push the button on the switch it lights up to it's full brightness, untill then it stays pretty dim.

    this is the schematic, it's my first schematic ever so go easy.
    Also, the switch is there because i like to push the button. :) even without the button/resistor and the transistor's base pin in the breadboard by itself it's still making the LED shine dimly
    [​IMG]
     
  5. Bigbird

    New Member

    Aug 22, 2013
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    Without a doubt as pwdixon said we need the type of transistor to help. I assume it's a NPN BJT transistor but verification would help. If it is a PNP then a lit LED would be expected with no base current.
     
  6. mrSilkie

    Thread Starter New Member

    Aug 23, 2013
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  7. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Leakage current plus large beta plus modern sensitive LEDs is the most likely recipe. Just put some largish resistor from base to ground and see if it stops.
     
    wayneh and mrSilkie like this.
  8. mrSilkie

    Thread Starter New Member

    Aug 23, 2013
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    Thanks, a resistor going to ground on the base solved the problem but even when the base receives a charge the LED doesn't light up as bright as when i connect the 3v (not using my 5v anymore) to the resistor + LED. I heard that transistors resist some of the current but it shouldn't make a noticeable difference... should it?
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    Try changing the transistor configuration to common emitter circuit, i.e. the LED and current limiting resistor goes on the collector to +V and the emitter goes to GND.
     
  10. pwdixon

    Member

    Oct 11, 2012
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    You look like you are using an emitter follower configuration which depending on LED colour will get varying results. Why not wire it ala the example you originally had with the LED in the collector it'll work better I promise.
     
  11. BobTPH

    Active Member

    Jun 5, 2013
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    Yes, why did you link to a correct circuit for a BJT used as a switch, then wire it as an incorrect circuit?

    The circuit you have will feed about 2.4V to the LED with the base connected to 3V.

    Bob
     
  12. wayneh

    Expert

    Sep 9, 2010
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    Or rearrange the transistor to the bottom and put the LED and resistor on top.
     
  13. mrSilkie

    Thread Starter New Member

    Aug 23, 2013
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    I thought that a transistor had to be directional, or atleast, i assumed so, especially with the words collector and emitter.
    So the base just allows current to pass through, which way the current flows doesn't make a difference?
     
  14. MrChips

    Moderator

    Oct 2, 2009
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    A bipolar junction transistor (BJT) is directional. You cannot interchange the collector and emitter.

    What three people are saying is you have chosen one of three ways to configure a BJT circuit.

    The circuit you should use is called a common-emitter amplifier.
    Connect the emitter to GND.
    Connect +5V to resistor, to LED to collector.
    You will need to reverse the logic in your code.
     
  15. studiot

    AAC Fanatic!

    Nov 9, 2007
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    There are two types of junction transistor. Both allow flow in only one direction.

    NPN which allows current to flow from positive towards more negative.

    PNP which allows current to flow from negative towards more positive.

    In both types this current is controlled by a signal at the base so can be varied at will.

    The above characteristics are used to create a varying output that is not one direction or the other but greater and smaller.
     
  16. mrSilkie

    Thread Starter New Member

    Aug 23, 2013
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    [​IMG]
    Just using the arduino because i've never had access to a 5v line before and the breadboard is amazing.

    very simple circuit, don't even need an arduino but there it is.
    Red arrows indicate the direction of the current, purple arrow highlights base which isn't connected to anything but the current is still allowed to pass through the transistor
     
  17. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Useful picture, you obviously want to learn more about electronics.

    You need to be aware that the usual direction for current you will meet in the world is

    From positive to negative. - This is called conventional current.

    This website in the Ebook uses the opposite convention

    From negative to positive. - This is called electron flow or electron current.

    The direction of positive current is only a convention. All the equations are the same, except for a minus sign, whichever you choose. But you should be consistent.
     
  18. BobTPH

    Active Member

    Jun 5, 2013
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    Are you sure you have the correct pinout of your transistor? The typical pinout of a TO92 tranistor is, from left to right, E B C with the flat side facing toward you and the pins facing down. As in this datasheet for one of the most commonly used NPNs:

    http://www.fairchildsemi.com/ds/2N/2N3904.pdf

    It looks to me, from your picture, that you have the emitter and collector reversed compared to what you show in the schematic.

    Bob
     
  19. MrChips

    Moderator

    Oct 2, 2009
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    Maybe your transistor is toast.
     
  20. pwdixon

    Member

    Oct 11, 2012
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    Really stupid suggestion but couild it be that the transistor has all three pins along the line of a continuous circuit on the breadboard. That would make the whole transistor look shorted and make the led light.
     
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