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let's see if i'm getting this right.

Discussion in 'General Electronics Chat' started by mybuickskill6979, Jan 26, 2007.

  1. ljcox

    ljcox Well-Known Member

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    As I've said before, we need a readable copy of your circuit.

    Do you have Microsoft Word or Excel?

    These both have drawing capability.

    Have you built the circuits that I or one of the others posted?
     
  2. AllVol

    AllVol New Member

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    it's enough to make a man change his avatar.
     
  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I use Microsoft Paint program for schematics. The one I posted uses parts copied and pasted from another schematic and then I moved the parts around.
     
  4. dave

    Dave New Member

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  5. AllVol

    AllVol New Member

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    Yeah, me too. It's very easy. The copy and paste feature keeps all component parts identical in a schematic.

    (Sample attached)
     

    Attached Files:

  6. mybuickskill6979

    mybuickskill6979 New Member

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    nope no drawing programs other then a cad type deal called sketchup but its more 3d type drawing!!

    i'm looking at the sheet and the drawing(the first one from len. to see how the math works !
     
  7. ljcox

    ljcox Well-Known Member

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    There is free drawing software available.

    try www.labcenter.co.uk and www.downloads-zdnet.com

    You could also search this forum. I think there has been free s/w mentioned in the past.

    I just did a search (on "drawing software") and there are plenty of hits. Some suggested eagle, and one said:-

    I have recently discovered a free schematic drawing program and a free p.c.b. layout program, both from the same source.​
    I downloaded and installed them only today so I haven't had a chance to assess them properly yet​
    This link is to the original page http://b.urbani.free.fr/pagetci/tci.htm
    and this one to the Google translated version http://translate.google.com/translat...ci/tci.htm
     
    Last edited: Feb 6, 2007
  8. mybuickskill6979

    mybuickskill6979 New Member

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    okay i have a question about the mathematics.

    the VCE Grows or decays in relation to the current supplied to it right? and Ib is very much decided by the VCE and ICE correct? i'm looking at the first drawing by len and the sheet for bc547b!

    whats the mathematics to from .250v @100mA With a Ib of 5mA, and a Vce of 90mV@10mA with a IB= .5mA

    to a Ib of 4A and a VCE of 80mV @40mA i know where the 40mA comes from but how about the 80mV?
     
  9. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Just look at the graph of its current gain in the datasheet.
    A collector current of 100mA is the max allowed for a BC547 transistor. At collector currents more than about 30mA the current gain is less. At 100mA the current gain is 60% of what it is at 10mA.
    So the transistor doesn't turn on as hard at 100mA.

    You cannot feed 4A to the base without blowing it up. You probably mean 4ma which is 1000 times less.

    To turn on an LED it doesn't matter if the collector voltage is nice and low at 80mV or not so low at 1000mV. The LED will still light brightly because the difference is less than 1V and the power supply voltage is much higher.
     
  10. mybuickskill6979

    mybuickskill6979 New Member

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    ok i get it. i thought there was a mathematical way to go about it. i was dividing and multiplying and everything else trying to figure it out lol!! thank you!!
     
  11. mybuickskill6979

    mybuickskill6979 New Member

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    okay how does the current gain work? is it max current through collect plus max current through the base? thats been confusing me for a minute!!
     
  12. ljcox

    ljcox Well-Known Member

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    Vce in the active region is the sum of voltage across the resistor (which = Ic * R1) and the voltage across the LED (assumed to be 3.6 V @ 40 mA) subtracted from the supply voltage, ie. Vcc - Vled - Ic * R1.

    In other words, you add the resistor and LED voltages together and subtract them from Vcc.
    This is Vce sat at a collector current of 10 mA and a base current of 0.5 mA (not 5 mA). A typical transistor will have a Vce sat of 90 mV under these conditions. But the worst case will have a Vce sat of 250 mV.

    What this means is if you were to buy say 1000 of these transistors and measured every one at an Ic = 10 mA and Ib = 0.5 mA, the Vce sat would be spread around 90 mV; the maximum could be as much as 250 mV.

    So Vce sat for one transistor may be 85 mV, another one may be 134 mV, another one may be 79 mV, etc. The average will be 90 mV.
    As Audio said, you mean 4 mA, not 4A.

    I took this from Figure 4 of the data sheet. This graph shows that at a collector current of 40 mA, Vce sat will be about 80 mV and Vbe sat about 0.8V.

    Note that it is a log/log scale, not a linear one.
     
  13. ljcox

    ljcox Well-Known Member

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    Current gain is called either Beta or hFE.

    The data sheets use hFE. If you look at the Electrical Characteristics table in the BC547 Fairchild data sheet, hFE at a Vce of 5V and Ic = 2 mA can be anywhere between 110 and 800. So if you buy several transistors and measure their hFE, their hFE will be somewhere between these limits.

    Beta = hFE = Ic/Ib, ie. it is the ratio of the collector current to the base current.

    It will vary from one transistor to another (of the same type), and it also varies with temperature and with collector current.

    See Figure 3 in the BC547 Fairchild data sheet. Note that this is a typical value measured at a particular temperature.

    Figure 2 shows how the collector current varies with the base emitter voltage. For example, it is 0.2 mA @ 0.8V, 4 mA @ 0.7V & 100 mA @ 0.8V.

    This shows how the transistor collector current is controlled by the base - emitter voltage.
     
  14. mybuickskill6979

    mybuickskill6979 New Member

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    okay i thought it was actually like a magical thing where more current came out of it then was going into it lol!!

    its a ratio right? so at HFe of 100 is it like 100 to 1 or whats the comparison number!!
     
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A transistor with a current gain of 100 has a base current of 1mA causing a collector current of 100mA. hFE= Ic/Ib.

    You need to read the minimum to maximum spec's about hFE and see the curve in a transistor's datasheet. The hFE drops at high currents and is a range of numbers because the transistors are all a little different.
     
  16. mybuickskill6979

    mybuickskill6979 New Member

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    oh okay so it the relationship thing so at an IC of 40mA the Bc would have to be like like .4mA or something close right?

    sorry don't mean to be so lame about it but i'm tryin =)
     
  17. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A transistor with a current gain of 100 will be a linear amplifier if its collector current is 40mA and its base current is 1/100th which is only 0.4mA.

    But if you want to use this transistor as a switch then you need it to turn on hard. The datasheet shows it "saturated" which is turned on hard when its base current is 1/10th of its collector current so it has plenty of extra base current.
     
  18. mybuickskill6979

    mybuickskill6979 New Member

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    awesome. okay i'm gonna try and figure the stuff for one of the transsitors i have here. then try it out, then post the math here. to see if i get it right!! okey dokey?
     
  19. mybuickskill6979

    mybuickskill6979 New Member

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    okay the transistor is a mpsa14(the first one i found out of my stock with charts on the sheet)

    VCEsat= 700mV @20mA
    VBEsat= 1.3v@.2 mA
    From the math the collector resistor is 420 ohm
    the base resistor is 53.5k ohm
    the closest i had is 470 and 56k for the resistors

    the led is 3.6v at 20mA

    heres the pics of the schematic and the circuit on and off
    [​IMG]

    circuit off
    [​IMG]

    circuit on
    [​IMG]

    So how'd i do if i get this one right i'll do a few more because practice makes perfect!!
     
  20. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Nice try.

    I calculate the current-limiting resistor for the LED as (12V-[3.6V+0.7V])/20mA= 385 ohms. Use 390 ohms.

    The datasheet for the MPSA14 darlington transistor shows it saturating well with a base current 1/1000th of its collector current. So the base resistor could be (12V-1.25V)/20uA= 537.5k ohms. Use 510k.

    You forgot to mark the collector and emitter so we can't see if the transistor is wired correctly or if it is backwards.
     
  21. mybuickskill6979

    mybuickskill6979 New Member

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    okay i see what i did.
    on the led limiter i put .007 instead of .7...

    the base resistor i used 12-1.3/.0002

    i got a question where did the 1000 come from?
    i was looking at the pulsed current gain(K) chart. and it said just over 100. but just wondering so i know for next time!! Thanks
    its pretty cool with this circuit its almost like i have magical powers lol i just get closed to the base wire and the led shines a little then move away and it turns back off. i'm a magician lol :D
     
    Last edited: Feb 7, 2007

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