Trouble shooting basic transistor test circuit

Discussion in 'The Projects Forum' started by mahela007, Sep 27, 2011.

  1. mahela007

    Thread Starter Active Member

    Jul 25, 2008
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    Hi.. I tried to build a basic circuit which uses a transistor as a switch to turn an LED on or off.
    I tried several transistors thinking the first one was broken but I always get the same result..
    The LED stays ON even when I disconnect the base. The supply voltage is +9 volts.

    The emitter was connected to ground and collector to +9 volts. The base was connected to a voltage divider which produced 4.5 V (9/2 = 4.5).

    What could the problem be?
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you post the used schematic?
    That way we can see if you made some mistake.

    Bertus
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    The emitter is connected to ground.
    The collector is connected to +9V.
    The base is connected to a voltage divider that produces +4.5V.
    And the LED is not connected to the transistor.

    Then the transistor is shorting the battery.
    Maybe the LED is also shorting the battery.
    There is no current-limiting resistor for the LED.
     
  4. mahela007

    Thread Starter Active Member

    Jul 25, 2008
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    I forgot to mention that there was a resistor in series with the LED. BTW: I'm using a 'wall-wart' .. not a battery. (I'm certain that it produces 9V DC)

    The schematic is very similar to this one
    http://www.kpsec.freeuk.com/images/trancurr.gif

    However, I do not have LED B. The 10k resistor is replaced with a 1K resistor and the LED (C) is connected to the emitter (with the proper polarity).
     
  5. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
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    Hi mahela007,

    We need to know the part number(s) of the transistor(s) you are using. The schematic shows an NPN transistor, but you may be using a PNP transistor which will act differently. Also, the part number will allow us to better determine if your resistor values need to change.

    Also, it's always a good idea to add a pull-down or pull-up resistor on the base depending on what you're doing. In this way, the transistor is forced into a known state when you aren't pushing the switch (in this example).

    Based on this circuit, I'd suggest adding a 10kΩ resistor between base and ground so the transistor is forced off whenever the switch is not pressed (this assumes you are using a NPN transistor, not a PNP).
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    Your transistor is not used as a switch. It is an emitter-follower.
     
  7. mahela007

    Thread Starter Active Member

    Jul 25, 2008
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    One of the transistors I tried was "BC550" and the other was labelled C828, which I assumed to be 2SC828.
    By the way, in the data sheets, in the diagram showing the pin orientation, are the pins facing towards or away from the reader ?
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    You need to post links to the datasheets that you are viewing, otherwise we may very well be looking at different datasheets.
     
  9. flat5

    Active Member

    Nov 13, 2008
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    I think you have the transistor pins wrong.

    A very clear picture would be nice.
    What voltage do you see on the base to emitter legs when switch is closed?
     
  10. mahela007

    Thread Starter Active Member

    Jul 25, 2008
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  11. Audioguru

    New Member

    Dec 20, 2007
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    Usually the datasheet shows the pins from a bottom view.
    Why don't you user a transistor that has a datasheet with a very clear photo of how the pins are arranged?
     
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  12. SgtWookie

    Expert

    Jul 17, 2007
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    mahela007 likes this.
  13. mahela007

    Thread Starter Active Member

    Jul 25, 2008
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    Thanks guys.. I think I must have gotten the terminals mixed up.
     
  14. mahela007

    Thread Starter Active Member

    Jul 25, 2008
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    This transistor appears to have widely varying beta values.. ( DC current gain).
    With reference to the link posted by SgtWookie, I think my transistor falls into the R category (there is a number beinning with R after the model number).
    HOwever, the R category shots beta values from 180 to 360..
    Which value should I use in biasing calculations?
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    You're not using the transistor in the linear region; you're using it as a saturated switch.

    Use Ib=required Ic/10
    For a common emitter circuit used as a saturated switch, to calculate the base current limiting resistor, use:
    Rbase = (Vin - Vbe) / (Ic/10)
    where:
    Vin is the voltage supply to the end of the base resistor that is away from the base, referenced to the emitter.
    Vbe is the voltage on the base referenced to the emitter when applying the required current. This is usually in a range of 0.63v for very low Ic to ~.8v when the collector is carrying about 1/2 of its' maximum rated current (if you need more than 1/2 the rated current, you should look for a more capable transistor)
    Ic is the required collector current.

    So, if your required collector current is 25mA, your voltage supply is 9v, and assuming 0.7v for Vbe:
    Rbase = (9-0.7) / (25mA/10) = 8.3/.0025 = 3320 Ohms. 3.3k Ohms is the nearest standard value.
     
  16. Audioguru

    New Member

    Dec 20, 2007
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    No.
    In post #4 the OP said the LED is connected to the emitter and the base is biased at half the supply voltage.
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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    Please re-read the OP's statements and links.
    In the original post, they said:
    And in post #4, nothing was mentioned about the connections, but they linked to this schematic:

    [​IMG]

    ...which I see as a common emitter configuration where the transistor is used as a switch.

    Since they have not posted a schematic of their actual circuit, I'll have to go by what they've posted already.
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    As said in post #4:
    "The schematic is very similar to this one ....
    .... the LED (C) is connected to the emitter"

    I don't know why the OP does not post the exact schematic he used since now we don't know where the 470 ohm current-limiting resistor is connected.
     
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  19. SgtWookie

    Expert

    Jul 17, 2007
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    OK, I missed that - thanks.

    I wish people would provide schematics that actually matched what they have, rather than simply saying "It's like this, but I changed...."

    Providing an accurate schematic takes care of just about all the questions.
     
  20. mahela007

    Thread Starter Active Member

    Jul 25, 2008
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    OK.. sorry about the confusion. The reason I didn't post a exact schematic was because I wasn't sure how to draw one.
    Anyway, my main problem is solved. Thanks for the help.
     
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