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

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

1. ### mybuickskill6979New Member

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okay i;'m looking up transistors like a wild man trying to grasp exactly how they work.

So to see if i'm right(understanding ) i'm gonna tell you what i get from what i've read and you tell me if i'm wrong or not?

collector= like a door
base = area that has the voltage(electricity)
emmiter= where the electricity from the base wants to go.

okay for understanding purposes.
the base is connected to a 12v 20ma PS
the emitter is connected to say an LED(only thing i could think of) that uses 30mA current.

if you apply 5v at 10mA at the collector.
the transistor will put out(to the emitter and what ever is connected to it)
12v of power at 30mA?

my head hurts! so is this true or do i have something turned around? some how i feel like i mixed something up but ya. OK!

edited: to switch a few things(i got the base and collector switched up

Last edited: Jan 26, 2007
2. ### ljcoxWell-Known Member

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You need to study the transistor more.

You can't connect 12V directly to the base, it would blow the transistor sky high.

The base - emitter voltage must be limited to around 0.6 ~ 0.8 Volt.

The emitter emits charge carriers, these are attracted by the charge in the base-emitter region, most flow across to the collector (which "collects" them) and become collector current. The remainder recombine with charge carriers in the base region and become base current.

The relationship between base current (Ib) and collector current (Ic) is

Ic = Beta * Ib where Beta (otherwise called hFE) is the transistor's current gain.

I wrote the attachment for someone else recently. The LED will glow when 5 Volt is applied to the left hand end of R2.

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3. ### mybuickskill6979New Member

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yeah Transistors are confusing me!! i guess i'll just have to put it to bed until i get a book. cause it seems the more i read the more confused i get. here's the main thing i;d like to know and what im trying to grasp. in a switching type situation.
the main supply goes to?
A. collector
b. Base
c. emitter

the switch current(that opens and closes the circuit) goes to?
A. collector
b. Base
c. emitter

when switching current is applied the electricity travel out of?
A. collector
b. Base
c. emitter

i under stand that the two currents come together i just don't understand where everything goes? but i think i'm gonna drink a beer and forget about it till i get a book.

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5. ### chemelecWell-Known Member

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The Problem is Most Technical descriptions are Confusing.

Much Easier, Consider an NPN Transistor.
First Ground the Emitter. Or connect to Battey Negative.

Now, As you Increase the Positive Current going into the Base, it Flows to the Emitter and it Greatly Increases the Current flow from the Collector to the Emitter.

How Much depends on the "HFE" Gain of the Transistor.

(A Limiting Resistor is a good idea going to the Base, so you Don't go Extreme on input current to the base.)

Also your Load between the supply and the Collector must Not Exceed the Current capability of the Transistor.

Hope this Simplifies it for you.
TOO MUCH BEER Doesn't Help.

Better Yet, Build a Simple Circuit on a Breadboard.
A Potentiometer into the Base and a Small Light Bulb on the Collector. Than play with it.
(Slight Increase in Base Current and the Light will go Brighter.)

6. ### ljcoxWell-Known Member

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Did you look at what I posted?

Using your terminology, the Main Supply is what I maked as "Vcc"

The "switch current" I assume is the current that causes the transistor to switch the collector current on. If so then this is the base current which is due to the voltage applied to the left hand side of R2.

"when switching current is applied the electricity travel out of" the collector.

However, the current direction is from positive to negative (conventional current) so, in this case, the collector current goes into the collector.

In short, if you apply 5 Volt to R2, collector current flows through R1 and the LED then from C to E.

Last edited: Jan 26, 2007
7. ### AllVolNew Member

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Note that in Ljcox diagram, he is using an N-P-N transistor for illustrative purposes. Make sure you are also, as using a PNP will cause all kinds of grief while you are learning.

There are many analogies used in describing BPJ transistor function. I like the one where the base is compared to a gate, which in turn controls the amount of current flowing from collector to emitter. In the typical NPN transistor, a small amount of POSITIVE current applied to the base through a resistor will open the gate and turn the transistor on. No current will leave the transistor turned off.

Hope this helps, and leave the suds in the fridge.

AllVol

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9. ### mybuickskill6979New Member

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awesome. i thinks i got it. i can be sort of dense at times. i will get a few trans and a pot tomorrow. i have a bread board right now with 4 LEDs on it running with 12vDC and a 330 ohm resistors on the neg side.

i thought the trans would go on the positive side of things which is what i think was confusing me the most.

i thank you guys for having mad patience with my"Noob" self!!

10. ### mybuickskill6979New Member

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the beer stayed in the fridge for tonight i had to drill a tank for a friend so i couldn't be drinking. but yeah i think i had an understanding of it. if the current is to much for the base i can split the current with two transistors no?

11. ### audioguruWell-Known MemberMost Helpful Member

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The value of the resistor in series with the base sets the amount of base current. A transistor has a current gain of about 230 (for a small transistor), so the base current is 1/230th of the collector current then you don't need a lot of base current.
Use a base current of 1/10th of the collector current to turn on the transistor as hard as it can.

12. ### mybuickskill6979New Member

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okay i can do that.

13. ### Hero999Banned

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What do you want to do with your transistors?

You can use them as amplifiers or switches.

When using a transistor as a switch it needs to be saturated (be turned on as much as possible (hee hee that sounds a bit dodgy ), the base current should be 1/10 of the collector current (as audioguru says).

When used as an amplifier the transistor needs to be biased so is't not fully on or off, saturation is bad because it can distort the signal. This is a bit harder to understand, try focussing more on using a transistor as a switch before you attempt to understand the amplier.

14. ### ljcoxWell-Known Member

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The transistor can be inserted on the positive side.

There are 2 options.
1. Use a PNP transistor configured as a common emitter or
2. Use an NPN configured as an emitter follower

15. ### mybuickskill6979New Member

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yeah i think i will stick with switching for now. but sooner or later amplifier situation sounds good. i wanna slap RS cause they didn't have my fuse i need for my Multi meter!!

16. ### Andy1845cActive Member

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I still shop for some stuff at radio shack cause it's on my way home and I'll pay the convience prices they charge if I want a component on my way home, but they're a REALLY bad supplier for electronics components in the end. Digi-key ,Mouser, Jameco, and any of the smaller module sites out there like Sparkfun are good sources for parts for anyone doing anything.

18. ### mybuickskill6979New Member

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will look into those sites.

i'm still grasping transistors and am wondering
is this right?

as far as the basics of how this would work?
sorry for the lack of ms paint hehe!!

the transistor i'm using in the drawing has the numbers 830mpsa14

not sure the part number. but is this pretty much the way a transistor would work?

19. ### mybuickskill6979New Member

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i just remembered i got the polarity wrong. i just learned today that electrons(because there negatives charged) move from the negative to the positive. i always thought it was the opposite but who knew lol!!

20. ### ljcoxWell-Known Member

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The MPSA14 is a NPN Darlington transistor which has 2 NPN transistors inside connected in the Darlington configuration.

So it is not a good starting point. It has a gain (hFE) of 10000 at 10 mA and 20000 at 100 mA.

So for a collector current of 750 mA, you would only need a base current of 750/20000 = 37.5 :mu:A.

There is no difference between your diagrams. A switch has a digital input where the input has 2 states:- high or low, eg. +5 Volt or 0 Volt.

An amp has an analogue input signal that has an infinite number of states, in other words it is continuous.

So if you input 2 Volt into an amp with a gain of 3, the output is 6 Volt.

If you input 2.01 Volt into the same amp, the output is 6.03 Volt.

21. ### ljcoxWell-Known Member

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When specifying the current direction, we use conventional current which assumes that the charge carriers move from positive to negative.

But as you said above, electrons carry a negative charge so they actually move from negative to positive.

But this is ignored when we do circuit analysis.

We use conventional current regardless of whether the charge carriers are electrons or particles that carry a positive charge such as protons.