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transistor beta and saturation current for switching

Discussion in 'General Electronics Discussion' started by max_torch, Oct 10, 2014.

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  1. max_torch

    max_torch

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    Feb 9, 2014
    I know that to solve for the value of the current limiting resistor between input and the base of a transistor you have to solve first for how much current you need to feed the base by dividing the collector current by beta (the current gain i think?) And then divide the (input voltage minus the base-emitter voltage) by the current needed at the base.

    But when I looked at the data sheet of a switching transistor (http://www.futurlec.com/Transistors/C9013.shtml) in the section containing hfe it said:
    hfe @ IC =50mA and then a min value of 64 and max value of 246.
    I dont get it... am i just supposed to choose a number between 64 and 246? and then it will be 50mA divided by that number and then the 'input voltage minus 0.7' divided by the derived base current?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    for safety, take the lowest value of hfe from the datasheet our measure the device
     
  3. BobK

    BobK

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    Also, Hfe only really applies in the linear region. In saturation, a rule of thumb is to us 1/10 of collector current.

    Bob
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Yeah, I was thinking about that just after I posted it.

    The multiplier need not be 10, depending on your needs it could be 2, 3, or anything up to 10.

    I'd go for 3, especially if the load has a high startup current (like a filament bulb) because if you design it for the peak current, even if it's not completely saturated then, it will be well saturated for the average current.

    Of course there are many considerations and a multiplier of 10 may be what is needed. I think we had a thread about this previously and KrisBlueNZ posted a graph of Vce with increasing Ib that nicely illustrated this.
     
  5. max_torch

    max_torch

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    Feb 9, 2014
    You are referring to the 1/10 mentioned by Bob? Collector current is what it says on the datasheet? So 50mA / 10 = 5mA...
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Yeah, it's a bit confusing.

    The datasheet will give either (or both) ranges of hfe or hfe at certain collector currents. If there is a range, you pick the lowest, otherwise you pick the hfe at the collector current closest to your expected load.

    So, if the transistor is specified as having an hfe from 100 (minimum) to 400 (maximum), you assume 100. If it says that the hfe is 100 at Ic of 500mA, 150 at Ic = 100mA, 200 at Ic = 10ma and 160 at 1mA, and your load is 200mA then the hfe will be between 100 and 150, so pick 100.

    This tells you that at a certain (again, typically specified) voltage, if the base current is b mA, the collector current can be up to 100 x b mA (the 100 was the hfe).

    The problem with this calculation is that whilst the transistor can pass this current, it has a large voltage drop. We then come to a rule of thumb which says "multiply the base current by 3 to saturate the transistor". Saturation means that the voltage across the transistor falls to a much smaller value. This is required when you want to use the transistor as a switch. Now, as mentioned above, that "3" might be a larger number.

    So in effect, once you have found the beta, you divide that by a number (like 3) so you can determine the base current required to not only pass a particular collector current, but to do so while saturating the transistor.

    Once you have that current you can calculate the base resistor if you know the voltage that you have available (and also the Vbe which is probably around 0.7 to 0.8V for a saturated transistor)
     
  7. max_torch

    max_torch

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    Feb 9, 2014
    The collector current is determined bythe load current? so for example if i have a piezospeaker hooked up to 12v i would have to put an ammeter in series to check how much current it consumes and then use that as my collector current for the calculation?
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    KrisBlueNZ likes this.
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