# The LED light up-- but HOW?

Discussion in 'The Projects Forum' started by bigcape, Oct 3, 2009.

1. ### bigcape Thread Starter Member

Sep 18, 2009
158
0

I found this tutorial but it has me stumped!

• The base-emitter (BE) junction should behave like a diode and conduct one way only.
• The base-collector (BC) junction should behave like a diode and conduct one way only.
• The collector-emitter (CE) should not conduct either way.
The diagram shows how the junctions behave in an NPN transistor.

HOW DOES THE CURRENT
EVEN REACH THE LED? WHAT AM I MISSING?

IF DIODES ARE ONE WAY GATES
I SEE A LOCKED DOOR?

The current flows FROM the 0V TO the +9V right?

If the LED was turnrd around it would make sense to me!
A simple switching circuit
to test an NPN transistor

2. ### tibbles Active Member

Jun 27, 2008
249
3
until the experts reply, this is my understanding,in the example you have shown you are using the transister as a switch and probably a more accurate analogy would be think of the transister as a single pole relay.with the base as the coil.
as you rightly assume, the "diodes" would initially exhibit a high resistance,
however when a voltage is applied to the base the transistor switches on and the "diodes" change to a low resistance,hence the full description of the word transister 'transfer resistor'.
in your circuit, current would flow from pos to neg for all practical purposes,however on an atomic level i believe it does the opposite.

im a relative newcomer to electronics and if my understanding is in error i apologise and hope the experts will admonish me accordingly
regards
dougal

3. ### Rick Martin Active Member

Jun 14, 2009
31
2
I get the feeling you are confused with conventional and electron current flow.

4. ### bigcape Thread Starter Member

Sep 18, 2009
158
0

"Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol shows the direction in which the current can flow. Diodes are the electrical version of a valve and early diodes were actually called valves"
Diodes a = anode
k = cathode

If a transistor is basically
two diodes then how
does it reverse its
polarity so the LED lights?

A rectifier is four diodes which make total sence why it works. Yet a transistor has two as I understand it but one looks backwards to me?

Last edited: Oct 3, 2009
5. ### rjenkins AAC Fanatic!

Nov 6, 2005
1,015
69
The tests you mention are testing one junction of the transistor at a time.

When a transistor is in an appropriate circuit, current fed into the base injects current carriers (electrons or holes) into the body material of the transistor.

This allows the (normally non-conducting) collector to emitter path to conduct.

The ratio of base current to collector current is the gain or Hfe figure of that particular transistor.

6. ### tibbles Active Member

Jun 27, 2008
249
3
thanks robert
even i was getting confused, hence i suppose, the word semiconductor
dougal

7. ### tibbles Active Member

Jun 27, 2008
249
3
ps
something wrong there- "to test an npn transistor" bigcape is right ,you would normally test a transistor as 2 diodes, the circuit shown would not test the transistor,if the led had blown the circuit wouldnt work, dosent mean to say the transistor is faulty...

8. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
A transistor is not two diodes obviously, it is something else. The comparison to two diodes is an analogy. There is no wire connecting two diodes, it is one piece, a complete component. When a transistor conducts there is no junction voltage as there is between the base emitter, the collector emitter drops around 0.1 volts.

The AAC book and others explains this concept. It can be hard to wrap your head around, but it works.

For what it's worth, I went through the same thought process when I was your age.

9. ### bigcape Thread Starter Member

Sep 18, 2009
158
0
I don't see how it is an "analogy" because of how you test to see if a transistor is good. I believe you so:

1. Why are they using the "diode symbol" to describe it?

2. what opens the th "oneway" gate so the current can flow through the emitter to the positive side of the circuit.

10. ### rjenkins AAC Fanatic!

Nov 6, 2005
1,015
69
It's an anaogy as, when testing with a multimeter, it reads like two diodes.

In fact, the transistor is a single piece of silicon (or some other semiconductor) with three differently-doped regions.

The critical item is that the base connection is in the same piece of silicon as both the collector and emitter.

A diode does not conduct in reverse as it has a 'depletion zone' which is a region of the semiconductor with no free charge carriers.

The base current feeds charge carriers into this depletion zone, allowing it to conduct and effectively creating a direct connection between emitter and collector, at some level depending on the base current.

This is why Semiconductors are so called - they can be made to conduct under the proper conditions.

11. ### bigcape Thread Starter Member

Sep 18, 2009
158
0
So it's the "Collector" that has the real special properties and using a "Diode" symbol (analolgy) is confusing to us Noob's?

When I see a diodes "skirt" im thinking "you ain't getting up that!"

Kinda like dating!

If this is correct then I am going to rewrite the books and use a POWER MOSFET to use as the "analogy" (?)

Hey could you use a Power MOSFET instead of a transistor if you needed a substitute?

Last edited: Oct 3, 2009
12. ### bigcape Thread Starter Member

Sep 18, 2009
158
0
I swore I'd NEVER use that line!

But, I found I do.

13. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
I figure you're between 13 and 16, how close did I get?

If you look at the AAC book you'll see your quandry covered in several sections, starting with this one.

In some ways it is better to think of the Base Emitter as being only loosly coupled to the Collector Emitter, as is explained in this chapter (look at the bottom of the article for the illustration).

If you research it you'll eventually understand it, the AAC section on BJT transistors does cover it very well.

You don't have to totally understand them to use them. Base Emitter current is amplified (this number is β, beta, or Hfe) and will cause a larger current in Collector Emitter. Since β is a bit unstable (especially from temperature) you need to put feedback or control mechanisms using resistor networks to keep it stable.

BTW, I ran into this problem when I was 15. Confused the heck out of me, they looked so similar, and yet not.

14. ### bigcape Thread Starter Member

Sep 18, 2009
158
0
13 and 16?

Well, I must read like an idiot to you (if so, terribly sorry for that) For I would hope that one does not have to be that young to learn new things.

I'm old enough to hold a Phd, old enough to be a father, old enough to hold several awards and in several professions, old enough to be part of an invention you use everyday, and old enough to be polite too.

If you look at my home page you'll see one of several
company's I started over 13-16yrs ago.

Some of my "hobbies" fruits have been displayed in galleries, boght and sold, even written up in internatonal magazines as well. One of such was self taught less than 1 year ago.

I'll take a young mind over young at heart anyday

Bill, I salute you!

Thank you for the indepth explination as well, very kind of you.

Circuits will not be "logical" to me until I know how the components work. Once I know how they work...... who knows.... you might just read, hear, or better yet buy something I did.

Last edited: Oct 3, 2009
15. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
OK, your enthusiasm and not wanting to climb the learning curve is what had me fooled. I'm more of a hands on kinda guy, I was building circuits before there was understanding. When I went to college a lot of stuff made sense after the fact.

We have a good text book on this site, it is worth the read.

I'm in the process of doing an update on my LEDs, 555s, Flashers, and Light Chasers, though it is slow going.

Sep 18, 2009
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17. ### pkennedy Active Member

Feb 27, 2009
44
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you may want to check www.flashtopics.com if you look in the digital section there is some animations on semiconductors they may help give you some better understanding on transistors

18. ### bigcape Thread Starter Member

Sep 18, 2009
158
0
I will, thank you.

Don't get mad at me when I start reading and "frying" I.C.'s! = MORE QUESTIONS!

Sep 18, 2009
158
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