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Transistor Basic Basics

Discussion in 'General Electronics Discussion' started by kchrisc, Oct 25, 2011.

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  1. kchrisc


    Oct 25, 2011
    First Time Caller


    Just getting into electronics and having a time of it with transistors. I get the physics and all of that, but just can't get my mind around how to feed and control these things.

    I have read many Websites and watched countless videos, but most of these do the "use this, now connect this" sort of thing.

    I understand that I must "+" bias the base of an NPN (2N2222) transistor while the collector is also "+" biased. I know that the flow on the EC junction should be about 100 times the flow on the base. Thing is, I have, what I call, "splayed" (transistor being fed and then to load with several multimeters going) several transistors and measured the numbers and they just don't add up.

    Next, I cannot find anywhere a break out of the various feeds to a transistor, say 6V and then discussing the impact that each resistor choice is going to have on the action at the base and collector. I need to know what the interaction parameters are so I may chose the correct components and have a more or less accurate output/outcome.

    I have read the datasheets and understand the basics, but just do not know yet how to control things to get the required results.

    Ironically, I get the hard stuff and am having trouble with this "easy" stuff. And I can use transistors in what I am doing, but I am just guessing and using trial and error which is not acceptable to me.

    Can anyone assist me in this most basic of basic questions.


    K. Chris C.
  2. jackorocko


    Apr 4, 2010
    First thing the gain (HFE) of the transistor is hardly ever going to be exactly 100. If you want a clearer picture read the datasheet. The 2n2222 can have a gain of 35 in the worse case scenario

    With a bipolar transistor, such as the 2n2222, you control the flow of electrons through the transistors c-e junction by controlling the flow through the b-e junction. This is the reason for the resistor on the base of the transistor, to introduce a controlled current. You seem to understand that you need a +V on the base compared to the emitter to get current to flow. This is approximately .7V, the time when the transistors 'turns on'

    I think the biggest thing you need to realize maybe is that there is no perfect number. If the typical gain of a transistor is 100 then use a value smaller(bigger) then 100 when doing your calculations. In each situation you will have different criteria and you need to find either the MIN or MAX values and select your components from there.

    as an example, you need transistor to switch on a load that requires 100mA of current. Your transistor has a gain of 100. So that means you need 1mA of base current. Then you would double that and go with 2mA and, using ohms law, size the resistor appropriately.
    Last edited: Oct 25, 2011
  3. davelectronic


    Dec 13, 2010
    Electronics kit

    Hi Chris. To some extent i have trouble at times the other way round, i can get the results, understand the physics and theory, but get stuck with the math of it all, not ohms law or basic math, more complex formula. What Jackorocko said is right, the problem is tolerance of components, voltage diff's line load regulations etc. But you should be able to put the individual circuit numbers together, and your answer will be accurate for those numbers, input output etc plus values of components.
  4. davelectronic


    Dec 13, 2010
    Electronics kit

    What you get out in numbers is what you put in + or - the tolerance incuding efficiency of the circuit. your never come up with exact data sheet spec unless you can control all parameters to the letter, unless this is under lab conditions i doubt you will. you learn to expect losses, and other differances, but the principles of function are the same, ie base emmiter collector base, the functions are as per the data sheet, some components / circuits will be more accurate than others. Allow some leaway.
  5. BobK


    Jan 5, 2010
    Perhaps it would be best if you gave an example of a calaculation that you cannot manage, then we could walk you through it. Or an example of where you are seeing results that do not match your calculations, perhaps we could point out what you missed or where your expectations are wrong.

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