About the P2N2222 and BC547 datasheets -- Many pages of stuff I don't understand yet.

Discussion in 'General Electronics Chat' started by bumba000, Oct 7, 2015.

  1. bumba000

    Thread Starter Member

    Oct 7, 2015
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    I don't mean to bring a dead horse back to life just to beat it, but I'm really new to this stuff. I've been over the P2N2222 and BC547 datasheets. Many pages of stuff I don't understand yet. Is power dissipation wasted power or the total amount of power that can be put through the device? I have built a joule thief using some random on hand transistor and would like to make the most efficient joule thief possible. So to power a single led from a single cell would it be better to use the BC547 or the P2N2222 and why? if you don't mind the "why" I would really appreciate the explanation.

    Thank You, John




    Mod edit:
    Welcome to AAC ... :)
    Please don't hijack other member's post, now you have your own.

    This thread was split from -- BC547 v/s 2N2222 transistor.
     
  2. ScottWang

    Moderator

    Aug 23, 2012
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    P2N2222A,Npn 40V 600mA 625mW
    BC547,.....Npn 50V 100mA 500mW

    From the rating specs you can see the P2N2222A is better than the BC547, why, because the IC of BC547 only 100mA, but the P2N2222A has 600mA.

    If the current of led is 20mA then you need to choosing an Ic has 5 times of 20mA, that is 100mA, so 100mA is enough for your need, and P2N2222A has some more current ability for using more time, thinking in another way, the rating current is your money in your pocket and the drawing current of led is the stuff you want buy, so which bjt is better for you?
     
  3. bumba000

    Thread Starter Member

    Oct 7, 2015
    72
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    I have done more research since posting this question and I'm finding the same elsewhere. Yes the 2N2222 can handle more current over a longer time and is less expensive. Yes just pennies, but less expensive none the less. Thank You.
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The power rating for a transistor is the amount of power the device can dissipate internally, not the amount of power it can control. Device dissipation is approximately equal to the voltage between the collector and emitter times the current through the collector. For a linear amplifier such as an audio application this changes continuously, while for digital or switching applications it is relatively easy to calculate. Note that the max power is not related to the max voltage or current. 40 V x 0.6 A = 24 W, significantly more than the 0.625 W rating.

    ak
     
  5. dl324

    Distinguished Member

    Mar 30, 2015
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    It's the maximum power the transistor can dissipate. Most likely specified at room temperature and derated for higher operating temperatures.
    Using a "better" transistor won't necessarily make a joule thief more efficient. The transistor is either on or off; one with a lower saturation voltage would give you a marginally higher current, but these things usually run from "dead" batteries so it's not worth worrying about.
     
  6. ScottWang

    Moderator

    Aug 23, 2012
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    There is no free lunch for that.

    P2N2222A,Npn 40V 600mA 625mW
    Imax = 625mW/40V = 15.6mA
    Vmax = 625mW/600mA = 1.0412V, how poor the voltage?
    Above just a calculation, if calculate in the real world then it should be less more.
     
  7. bumba000

    Thread Starter Member

    Oct 7, 2015
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    just trying to make a dead battery go as far as possible and to me it seems as though the 2n2222 would be the best bet.

    as far a real world calculations - what do you mean by " it should be less more ". Now I'm really confused. o_O

    Thank you, John
     
  8. ScottWang

    Moderator

    Aug 23, 2012
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    That was talking about when you using the rating value to do the calculation, in the real world you should only using it less than 1/3 to 1/5 as 100mA only using less 33mA~20mA, 600mA only using less than 200mA to 120mA, what will caused that, because the heat issue, sometimes could be less than 1/5 or more less.
     
  9. Bordodynov

    Active Member

    May 20, 2015
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    For an electronic circuit, which is powered by a single element is better to use transistors that have lower operating voltage of the base-emitter. This will allow the scheme to operate at lower voltages.
    Here is an example:

    Vbe=F(ic).png
     
    AnalogKid likes this.
  10. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    One thing to minimize power dissipation in a switching transistor is to look for a low Vce saturation voltage when ON (shown in the data sheet).
     
  11. Bordodynov

    Active Member

    May 20, 2015
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    Datasheet==>
    ZTX690B VCE(sat)<100mV IC=0.1A

    Vce(sat).png
     
  12. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Scott calculated the theoretical maximum Vce at max collector current, and the max collector current at max Vce. These are the boundaries of the transistor's capabilities. But they exist only under perfect thermal conditions and with zero margin for device-to-device variations. In the real world, conservative practice is to run devices at no more than around 50% of their ratings. So use a 100 V transistor rather than a 50 V transistor in a 48 V circuit, use a 50 V electrolytic cap in a 24 V power supply, etc.

    ak
     
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