Measuring LED forward voltage

WBahn

Joined Mar 31, 2012
29,978

Thread Starter

zazas321

Joined Nov 29, 2015
936
The conventional current will flow from +Vbat ----> LED---->Resistor ----->-Vbat.
So, what is your problem ?
Yes, but during the whole circuit current flows from + to -, i cant see where it flows from - to +? or u mean - + from one battery terminal to another after the first "cycle"
 

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zazas321

Joined Nov 29, 2015
936
Hm. I think i will leave this transistor BC junction question for later, when i will have more understanding in electronics, because now it seems mystical to understand why current flows in opposite direction than from + to -.
 

WBahn

Joined Mar 31, 2012
29,978
Hm. I think i will leave this transistor BC junction question for later, when i will have more understanding in electronics, because now it seems mystical to understand why current flows in opposite direction than from + to -.
But there IS NO current flow from negative to positive!

In an active (or saturated) NPN transistor, you have:

1) Current flowing from base to emitter and the base voltage is more positive than the emitter voltage.
2) Current flowing from collector to emitter and the collector voltage is more positive than the emitter voltage.
3) NO current flowing from collector to base, even in saturation when the base-collector voltage is mildly forward biased.
 

Thread Starter

zazas321

Joined Nov 29, 2015
936
Oh okay... Very good arcticle, i will have really close look at tommorow.. Once i was scanning this article i have found out that understanding how transistor works while thinking about the electron flow rather than current flow might be easier... So i will explain and maybe you can tell me if thats a correct way to understand or nott?

So we have a emmiter and collecter with alot of electrons, and we have base, full of holes.. When the electrons flow from emmiter to base , the holes are being filled, but there are too many electrons and not enough holes to fill them all ,so they travel through the base, which is very fin, to the collector , where the positive battery terminal attracts electrons.

But when we have a saturation ( second photo)
Everything is the same, except when electrons travel through base to emmiter, there is a negative terminal at collector which should "block" the electrons flow and push them back
 

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Thread Starter

zazas321

Joined Nov 29, 2015
936
Doesn't matter. There is no current flow between the base and collector in either direction.
Yeah thats exactly the issue that i cant understand...
Im trying to visualise everything so its easier for you to understand what i dont understand^^
when the C-B junction is reversed biased, everything is fine... Electrons are pushed from the emmiter, because the Emmiter is connected to negative battery side which pushes the electrons from the emmiter to base. The base is very thing so electrons flow through the base to the collector.. The collector is is conencted to poisitive batery terminal, so it attracts the electrons, so we electrons flowing from emmiter to collector...

Everything seems okay till the transistor becomes saturated.. Then the mystical things start to happen.

What happens when the transistor is saturated, is C-B junction becomes forward biased, so electrons flow the simmilar path from emmiter to base, and then from base to collector, but as the Vcb is negative when the transistor is saturated, that means the base voltage is higher potential than the collector, so the electrons should flow from collector to base.... So i dont get how electrons , that get from emmiter to base and then to collector dont get "pushed back" from the collector and how can they travel to the opposite direction..
Really hope you understand what do i mean there..


https://www.physicsforums.com/threads/carrier-flow-in-bjt-when-saturated.778000/#post-4894261
I have read this post, and this guy have the exact same problem understanding how can current flow in the opposite direction during the saturation... I have read this whole article but didint seem to find any "easy" explanation why that happens. Maybe you can read his first post and you might understand what he means, if you dont understand my explanation .. ;/
 

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hp1729

Joined Nov 23, 2015
2,304
Yeah . Im trying to understand things slowly... Apparently i tryed doing something MikeML suggested me, but my multimeter sucks and i cant measure the accurate current.... I have built a simple circuit with a 470 Ohm resistor, 9.4Volt battery and a led. My multimer works only choose 10 option as u can see in the photo and it shows 0,02 but sometimes changes to 0,01. I cant understand why my multimeter wont show anything when i put it in 200m mode..
Not really much wrong with just battery and resistor to drive an LED. The question is what setting you had the meter on and how was it connected in the circuit. Measuring the voltage across the LED the meter should be on a voltage range (20 V DC). To measure current through the LED the meter should be set up for DC current and in series with the LED. Yes, 200 mA scale sounds right.
The suggested constant current design is superior in ways. It is worth building up on a separate tiny breadboard and keeping it handy.
 

hp1729

Joined Nov 23, 2015
2,304
I need to find the right voltage drop of the LED, but came up with problem.. Can i measure the voltage drop if i connect 4,2v batteries straight to the LED and measure the voltage across it. For some reason for the multimeter showed the same voltage across both, red and green LEDs i measured. it showed 2,1 voltage. even thought on the package label, where i bought the led is written, that red led has 2,5V drop and the green- 2.2V. Is there any easy way to find out the actual voltage drop?
http://forum.allaboutcircuits.com/threads/diode-exercises-for-getting-to-know-diodes.118760/

An exercise for diodes, including LEDs.
 
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