Too much work? How so?

Manufacturers provide that information:

 

Manufacturers provide that information:
View attachment 96349

Hello again,

That's nice, but many of the LEDs you can get from various places for very cheap dont mention the manufacturer so a test or two is of course in order. A curve tracer will help here too or just measure two or three points and do a curve fitting using the diode equation.

I found this thread interesting because i was just doing this over the past few weeks when i purchased 100 5mm LEDs and 100 3mm LEDs of various colors for use for Christmas decorations.
Several made a nice Nano controlled Christmas tree

The tri color ones are nice too, but of course you have to measure all three internal LED voltages.

 

I understand the KVL like that: You choose a starting point, lets say R1. At the beggining of the element we take + and in the end of element we take -, so if we are moving clockwise, we are gonna do same for every element. So lets say we are moving clockwise and in the beggining of R2 we are gonna mark + , and - in the end, and we are gonna do same to R3, but if it was like i said, the R3
marks would be opposite

 

In saturation both junctions are forward biased.
View attachment 96325

after reading 10 articles about KVL i think i almost understand this. Just one thing what is not clear for me, why collector has negative mark? Teacher meantioned that base and collector has to be positive, and emmiter - negative.

 

I understand the KVL

Test your knowledge by wiring up some circuits so you can verify your understanding experimentally.

 

Just one thing what is not clear for me, why collector has negative mark? Teacher meantioned that base and collector has to be positive, and emmiter - negative.

Positive and negative relative to what? Post a schematic.

Was your teacher not able to teach you KVL?

 

Positive and negative relative to what? Post a schematic.

Was your teacher not able to teach you KVL?

In a simple words, i wanted to ask if transistor works like a diode when its saturated ant current flows from B to E and from B to C?

 

In a simple words, i wanted to ask if transistor works like a diode when its saturated ant current flows from B to E and from B to C?

No, a diode has no current gain.

 

It might be clearer from this schematic:
View attachment 96323
\( \small V_{R1}+V_{DCB}=V_{LED}+V_{R3}\)
or
\( \small V_{R1}=V_{LED}+V_{R3}-V_{DCB} \)
Corrected equation.

But why it has same marking as a diode?

 

http://forum.allaboutcircuits.com/threads/calculating-resistor-values-for-transistor-circuit.88409/
Its also strange that the voltage across CE is kinda high

 

But why it has same marking as a diode?

The junctions in a transistor are PN junctions and share many characteristics with diodes, but diodes can't be substituted for transistors. In the schematic I used to illustrate KVL, the diodes were to help visualize voltage drop and the reason for it.

 

Its also strange that the voltage across CE is kinda high

The transistor wasn't being driven hard enough to go into saturation. If you're trying to learn, be careful when reading or watching stuff from people who don't know what they're doing; that's a classic example of the blind leading the blind (no offense intended to persons with impaired vision). There's all kinds of misinformation on the internet. This forum is better if you get multiple people responding because Peer review might root out any mistakes.

I learned this stuff 40 years ago and for most of my working career, I was working in different aspects of the field, so it isn't beyond the realm of possibility that I'll recall something incorrectly.

 

The transistor wasn't being driven hard enough to go into saturation. If you're trying to learn, be careful when reading or watching stuff from people who don't know what they're doing; that's a classic example of the blind leading the blind (no offense intended to persons with impaired vision). There's all kinds of misinformation on the internet. This forum is better if you get multiple people responding because Peer review might root out any mistakes.

I learned this stuff 40 years ago and for most of my working career, I was working in different aspects of the field.

Yeah i tottaly agree with that, but there is voltage measured acros BE , and when the LED is lightining, the voltage across BE is 0,7V , doesnt that mean that transistor should be saturated? but there is over 1V voltage across CE

 

Yeah i tottaly agree with that, but there is voltage measured acros BE , and when the LED is lightining, the voltage across BE is 0,7V , doesnt that mean that transistor should be saturated? but there is over 1V voltage across CE

The voltmeters in that simulation show the C-B junction being reverse biased, so the transistor is in it's active region.

 

If i understand the reverse biased good, that means that the emmiter base and collector is connected to the reverse battery terminals, but how is that possible if in that circuit there are no additional batteries? And how can a LED light up when the transistor is reverse biased? I thought that the transistor must be forward biased to allow current flow through CE and light the LED

 

If i understand the reverse biased good, that means that the emmiter base and collector is connected to the reverse battery terminals, but how is that possible if in that circuit there are no additional batteries?

The transistor is biased by the components it's connected to. Using a battery or power supply to bias the transistor junctions wouldn't be very useful.

C-B junction is reverse biased by 1.43V - 0.71V = 0.72V.

And how can a LED light up when the transistor is reverse biased? I thought that the transistor must be forward biased to allow current flow through CE and light the LED

You're being sloppy with your terminology and need to do some study so you can better express your thoughts.

We don't speak of a transistor as being reverse biased, that isn't a meaningful description. What is reverse biased? Transistors have 2 junctions, so there are 4 possible combinations of junction bias.

When a transistor is saturated, the BE and CB junctions will be forward biased, and Vce will be low. When a transistor is operating in it's active region, the BE junction will be forward biased, the CB junction will be reverse biased, and Vce will be whatever the components dictate.

 

http://forum.allaboutcircuits.com/threads/calculating-resistor-values-for-transistor-circuit.88409/

isnt the R2 and R4 in series with each other? So the current should be the same for both? but in this article says that across R4 it will be 8,75uA and R2- is 38. Why would he even sum those current like that?:
The total current through R2 when the transistor is ON is thus 8.75μA + 30μA = 38.8μA.

 

isnt the R2 and R4 in series with each other? So the current should be the same for both?

Why do you say that? Don't you see something in parallel with R4? What does KCL say?

 

Is thats a correct way to understand that place?

Anyway, i tried to calculate everything in the circuit to prove that if there is a light , the circuit will not work, but i dont know where to go from there.. Assuming that the voltage to the transistor will not be enough to turn transistor ON, so how do transistor will act that the LED will not light up? :/ I have calculated that if i have Rldr=1k Ohm, the BE voltage will be only 0,05V.. Using the KVL equation and taking the loop Vldr and Vbe i get that Vldr will be also 0,05V. Assuming that Vr1+Vldr=5V ( parralel with battery) , so we get that Vr1 will be 4,95V.. I can calculate the currents through the resistors, but i dont know where to go from there

 

I have made alot of reserach about transistor modes. So in my led circuit, i have posted one message before we have forward biased BE junction and reverse biased CB junction.. If we have positive voltage at the Collector and Negative and Emmiter that means the transistor is operating as amplifier and 1 junctions is forward biased (BE) and other junction is reversed biased (CB). What is not clear for me, how can a transistor turn into saturation mode, if we wont change the polarity of the battery, becuase the both junctions has to be forward biased if i want transistor to be in saturation