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Transistor questions

Anonymous
Anonymous
Guru 17045 points

Hello,

I am a TI chip user and I am trying to control a stepper motor with a simple transistor-based switch. Although these transistors does not come from TI, I still with to consult with the experts here on some basic questions:

The transistor I bought from en electronic market has labeling as "S8050 D 331". I used a multimeter to measure its hfe value and it shows 186 (varying between 186 and 188).

However, I googled "S8050" and found a document from http://www.unisonic.com.tw/datasheet/S8050.pdf  which appears to me somewhat confusing:

  1. There are hfe1, hfe2, hfe3 for  this S8050 transistor and for each there are associated MIN, TYP and MAX value. for hfe1and hfe3 there are only a MIN value, but for hfe2 the values MIN, TYP and MAX are all exist. Why is the difference?
  2. What does hfe1, hfe2 and hfe3 mean? If they all exist, do I still have a unique value? Which one of them correspond to the 186 value which I got from the multimeter?
  3. Why for each of them there are a range of values (MIN, TYP, MAX)?  If the hfe has, for example like hfe2, MIN value as low as 100 and MAX as high as 400, then how can I know exactly which hfe value the transistor will operate on, and consequently, how should I design the circuit (resistance values, saturation mode, etc.)?

 

I could hardly see any clue in this data sheet. Help will be greatly appreciated.

 

 

Zheng

 

 

 

 

  • 0
    Guru 21975 points

    Zheng,

    The forward current gain of a BJT varies depending on the collector current. This manufacturer has provided information on Hfe at three different operating currents that they call 1, 2, and 3.

    Manufacturers generally only provide min and max characteristics for parameters that they actually test on each device that they ship. Other information is provided so that the user can get a sense of how the device may behave in various circuits.

    The meter you are using probably tests beta or Hfe at a relatively low collector current, perhaps around 1mA. The instructions for the meter may provide this information.

    Regards, Bruce.

     

  • Anonymous
    0 Anonymous in reply to Bruce Trump
    Guru 17045 points

    Dear Bruce,

    Bruce Trump said:

    "The meter you are using probably tests beta or Hfe at a relatively low collector current".

    I am a bit confused: Isn't beta and hfe the same thing?

    http://en.wikipedia.org/wiki/Bipolar_junction_transistor said:

    "The common-emitter current gain is represented by βF or hfe; it is approximately the ratio of the DC collector current to the DC base current in forward-active region."

    From the syntax of the above sentence it seems they are the same thing, but why do you still use an "or" between them, which might suggest beta and hfe are two different quantities?

     

    Another question is that in textbook examples, it is usually assumed that beta is known, and the steps of calculation would be:

    1. Because VBE's change in negligible with respect to IB change, VBE is therefore usually taken as 0.7V, and IB is determined by IB=(VBB-0.7)/RB.
    2. Using  β, IC = β × IB.
    3. VCE=VCC - IC × RC.

    In this treatment, β (=hfe?) is basically regarded as a constant and hence the calculation is straightforward. But if β (=hfe?) changes with IC, then how can IC be determined when IB is known? The requirement is that

    1. IC = β × IB, hence IC depends on β.
      • Bruce Trump said:
        The forward current gain of a BJT varies depending on the collector current.
      Hence  β depends on IC.

    Therefore IC and β are mutually dependent, which constitute a chicken-egg dilemma in their determination. How can one then solve this and determine the correct value for both IC and β? The most unfortunately way I could thought of is to use "iteration", which I happened to see somewhere in semiconductor textbooks, merely for this basic task. Is it true? Or did I make any mistake here?

     

     

    Zheng

     

     

     

     

     

  • 0 in reply to Anonymous
    Prodigy 30 points
    its depends on beta...Ic/Ib..Dc current gain