# Circut with transistor analysis

#### Human2

Joined Dec 22, 2017
51
Hello everyone I hope I'm not posting this in a bad place, anyways, this circut:

I just don't understand how come no current is flowing through a resistor R2, can someone

Moderatores note : removed white space and showed picture full size

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#### dl324

Joined Mar 30, 2015
12,256
Welcome to AAC!

Is this homework?

#### Human2

Joined Dec 22, 2017
51
No, Its from one youtube video I was watching (Ben Eater)

#### Dodgydave

Joined Jun 22, 2012
9,867
Because the transistor is turned on and shorting it out!!

#### dl324

Joined Mar 30, 2015
12,256
The video is wrong. Even if the resistor values are appropriate for saturating the transistor, the saturation voltage will only be zero in an ideal transistor.

#### Human2

Joined Dec 22, 2017
51
There was a switch just before R3, and there was not R2 there was a led diode.
Led diode was on when switch was off, and when switch was closed led diode went off.

@Dodgydave can you be more specific

#### dl324

Joined Mar 30, 2015
12,256
There was a switch just before R3, and there was not R2 there was a led diode.
Led diode was on when switch was off, and when switch was closed led diode went off.
That's a different circuit.

Again, assuming the values for R1 and R3 are appropriate for the transistor to be saturated (when the switch you didn't show is closed), the saturation voltage of a real transistor won't be 0V.

But, in the case of an LED instead of R2, the saturation voltage will be insufficient for the LED to turn on.

Next time, show us the actual circuit. It will waste less of our time and effort.

#### absf

Joined Dec 29, 2010
1,949
OK, here is how your circuit connected...

When the switch is OFF, the transistor DOESN'T conduct, so most of the current is flowing through the LED (8.18mA). Only very little leakage current (1.15uA) flows through the C-E of Q1.

When the switch is ON, the transistor CONDUCTS, making voltage Vc very low. Majority of the current flow through the CE junction (14mA) and very little current is flowing through the LED (5.55uA) not enough to light up the LED.

Allen

#### Human2

Joined Dec 22, 2017
51
Thanks thats really useful information, but the guy in the video said something like
when the switch is ON the e. potential is equal at a dot between transistor collector and R1 and at the other dot
right of the transistors emmiter.

This is this video, at 02:55 - 03:30 (
)

But it confuses me because:

1. How can those 2 points be at the same potential when we consider the voltage drop on the transistor ? Don't transistors have voltage drops like resistors ?

2. Shouldn't the voltage after the transistors emmiter be = GND = 0V ?

#### dl324

Joined Mar 30, 2015
12,256
the guy in the video said something
He's wrong. No content on YouTube is vetted. You need to use your knowledge and experience as a filter.

If you don't have any, you should use more trustworthy sources.

#### ArakelTheDragon

Joined Nov 18, 2016
1,350
Voltage drop applies on serial connections, not on parallel ones.

In your circuit the button is only to turn the transistor on (Proteus not youtube). The idea is when you turn the transistor more current flows through the transistor, because the current like the student passes through the lowest resistance more. The resistance of the transistor will depend on the channel NPN which should be in the datasheet.

You have 5V/330Ohms which is enough for a normal diode. 15ma is something acceptable. When you turn on the transistor most of this current passes through the transistor, leaving too little for the diode to be on. Its a basic key transistor mode or common base.

Your yourube video is making logic gat.es, which is not as your circuit and this is most likely homework for a high school project

#### Human2

Joined Dec 22, 2017
51
Well the only diference between my circut and youtube video circut
is that i didn't draw a button switch because I wanted to ask a question about switch being closed anyways.

And I thought that it would be the same if I'd draw a resistor or a LED.

So basically he lied when he said that there is no difference in potential between before colector and after emmiter

#### MrAl

Joined Jun 17, 2014
7,891
Hello,

Stating that no current flows through the resistor from collector to emitter come from an approximation often used for switching transistors. The approximation is that the transistor collector to emitter drop is exactly zero when the transistor is turned 'on';

This approximation is used in a lot of educational material so it is nothing new. The idea is to introduce the concept of a switching transistor in the simplest possible way. Some time after that the material may go more in depth about the actual operation of a transistor as a switch such as some voltage drop C to E.

In many applications the approximation is valid anyway, and is also used to develop theory of switching networks too.

Consider a 1k resistor from C to E, and the transistor turned fully 'on'. The drop could be 0.4v with 10 amps current flow through the transistor. How much in the resistor? 0.4/1000=400ua current. So the resistor has 400ua and the transistor has 10 amps, so the ratio is 0.000400/10=0.000040 which is 0.004 percent, which is in words FOUR THOUSANDTHS of a percent.
See how that works? The amount that the resistor conducts is so little compared to what the transistor conducts sometimes it is just referred to as ZERO even though there is some voltage drop across the transistor in a real transistor.

In switching network theory the transistor is approximated in this way also, and it's only later when we start to elaborate on the losses that we start to consider the actual values. Before that we just need a basic working knowledge of what is happening.

#### dl324

Joined Mar 30, 2015
12,256
Well the only diference between my circut and youtube video circut
is that i didn't draw a button switch because I wanted to ask a question about switch being closed anyways.
What made you want to draw the schematic so unconventionally? At least the guy in the video got that right.
And I thought that it would be the same if I'd draw a resistor or a LED.
Resistors and LEDs aren't interchangeable and have different characteristics.
So basically he lied when he said that there is no difference in potential between before colector and after emmiter
Saying he lied is rather harsh.

He definitely misspoke and could have used more precise wording. In RTL (Resistor Transistor Logic), a logic LOW voltage just needs to be low enough to not turn on the transistor it drives. He got into trouble when he started implying that the saturation voltage of a transistor was 0V.

There's so much wrong with that video that it should be taken down. The circuit that he called an AND gate is actually a NAND gate. He was driving the LEDs low side, so the output needed to be inverted for it to turn on.

I don't know if he was over simplifying, leaving out information on purpose, or just doesn't know what he's talking about. You should make a mental note when viewing other videos from him.

#### Human2

Joined Dec 22, 2017
51
Thanks everyone, everything is more clear now

#### Audioguru

Joined Dec 20, 2007
11,249
Why are you confusing the video saying that the collector voltage of the turned on transistor is the same as the voltage at the emitter when a datasheet shows almost the same?
The datasheet of a 2N3904 transistor shows that when it is turned on with a collector current of 10mA and a base current of 1mA then its collector voltage is typically a little less than only 0.05V!

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#### ArakelTheDragon

Joined Nov 18, 2016
1,350
You should remember this:

There is always a voltage drop over the transistor. The voltage drop over the transistor is the current through the transistor ( meaning the collector current) multiplied by the resistance of the channel collector emitter (this is in the datasheet).