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Transistor base current? — Parallax Forums

Transistor base current?

dev/nulldev/null Posts: 381
edited 2009-08-20 17:09 in General Discussion
What is the current rating for the base of a BC547 transistor? I can't find it in the datasheet, which only specifies collector current.

Thanks

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Comments

  • kwinnkwinn Posts: 8,697
    edited 2009-08-18 13:33
    Got curious and looked at several data sheets, none of which listed base current rating. Based on experience and a power supply I built years ago I would guess that it is close to the same as the collector current.
  • skylightskylight Posts: 1,915
    edited 2009-08-18 13:50
    shouldnt the base current be smaller by magnitudes to the collector current hence its use in a lot of circuits such as switching larger currents?
  • Mike GreenMike Green Posts: 23,101
    edited 2009-08-18 14:16
    The actual operating base current would normally be smaller than the collector current, but the maximum current rating depends on the construction of the device, how wide and thick the metalization is and how the device leads are attached to the metalization.
  • dev/nulldev/null Posts: 381
    edited 2009-08-18 14:19
    Yes, the base current is usually tiny compared to the collector current. I am putting this in my car with the base to +12V, and I need to know approx the resistor to place in front of the base. I could of course just try a 10K resistor and see if it's enough with 1.2mA. At any rate, that shouldn't be too much.

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  • dev/nulldev/null Posts: 381
    edited 2009-08-18 14:21
    I'm trying to understand the saturation thingy in the transistors, and if you have too little current the transistor won't saturate and you could burn it. Somthing like that....

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    Don't worry. Be happy
  • UnsoundcodeUnsoundcode Posts: 1,532
    edited 2009-08-18 15:17
    Hi , I would think that if the max collector current is 100 mA and the gain is 200 then the base current would be Ic / hfe =100mA/200=0.5mA.·I guess thats a resistor > 22K

    A wild assumption but that is where I would start.

    Jeff T.
  • LeonLeon Posts: 7,620
    edited 2009-08-18 15:32
    Data sheets usually have the base-emitter saturation voltage for a given base current and collector current. There isn't any point going much above that.

    Leon

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  • Mike GreenMike Green Posts: 23,101
    edited 2009-08-18 15:45
    A bipolar junction transistor is a current amplifier. You run a certain amount of current through the base-emitter junction and that allows a certain amount of current between the emitter and collector. The ratio of the two currents is the gain (hFE). At a certain point, the base-emitter current saturates. More current through the base-emitter junction doesn't result in more current between the emitter and collector although it does add to the heat dissipated by the transistor. When the transistor is saturated, the voltage drop across the emitter-collector is known as Vce(sat) and is commonly given in the datasheet.
  • Carl HayesCarl Hayes Posts: 841
    edited 2009-08-18 20:09
    Unsoundcode said...

    then the base current would be Ic / hfe

    No.· The base current would Ic/hFE, not Ic/hfe.· Remember that hFE is not the same as hfe.· It would be the same if the transistor were perfectly linear throughout its whole operating range.· No transistor meets that description.· In this particular case that is a very significant distinction.

    Size the base resistor so that it will give about twice the base current necessary for saturation.

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    · -- Carl, nn5i@arrl.net
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2009-08-20 17:09
    In addition to what Mike said it is sometimes important to understand that the hFE can change as the IC changes.

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    Chris Savage
    Parallax Engineering
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