# Confused about the Collector Emitter Apparent resistance.

#### arishy

Joined Apr 26, 2014
123
In a common Emitter configuration, and the circuit is acting as switch; Let us say you design the circuit to provide 100 ma at the collector.
If I plug in a led ( 20 ma) with NO resistor to protect it; Will the transistor Ic provide ONLY 20 ma or I loose the led ?????
There is a lot of talk about transistor dynamic resistors and "early voltage" that I do not understand.
I am hoping for a circuit that can provide such "Intelligent" response that performs according to the load.

#### ericgibbs

Joined Jan 29, 2010
18,979
hi arishy,
The 20mA LED would fail.
All the 100mA would try to flow through the LED.
E

#### AlbertHall

Joined Jun 4, 2014
12,384
hi arishy,
The 20mA LED would fail.
All the 100mA would try to flow through the LED.
E
Assuming the supply voltage is greater than the LED voltage at 100mA plus whatever the collector to emitter voltage is at 100mA for this circuit.

#### ericgibbs

Joined Jan 29, 2010
18,979
If I plug in a led ( 20 ma) with NO resistor to protect it; Will the transistor Ic provide ONLY 20 ma or I loose the led ?????
hi AH,
I posted reply based on the information given by the TS.

If you don't agree with my reply, tell the TS , not me.

E

#### arishy

Joined Apr 26, 2014
123
May be Constant current circuit with Variable resistor at the emitter ???? Some sort of semi intelligent work around
How About LM317....Will it does the "semi intelligent" part by using adjustable resistor.

If you are asking What the hell he (me) wants??

#### BobTPH

Joined Jun 5, 2013
9,109
We cannot tell you How to do it until you tell us what you are trying to do.

Bob

Joined Mar 10, 2018
4,057
A switched constant current source will accommodate load changes as long
as you maintain current source compliance needs.

A 317 has high differential requirements, so if you are in a LV circuit, consider
using LDO

http://www.ti.com/lit/ug/tidu922a/tidu922a.pdf

Regards, Dana.

#### arishy

Joined Apr 26, 2014
123
We cannot tell you How to do it until you tell us what you are trying to do.

Bob
You are right...I have quite few High powered LEDs with different voltage and current requirements. I am really asking for "an approach" rather than a specific circuit. That is why I did not mention the specifics.

For the sake of clarity; I am looking for a way (approach) to deal with dynamic load. It seems this is a complicated issue that needs a complicated circuit design. A simple transistor or sophisticated IC is not the answer.

#### arishy

Joined Apr 26, 2014
123
A switched constant current source will accommodate load changes as long
as you maintain current source compliance needs.

A 317 has high differential requirements, so if you are in a LV circuit, consider
using LDO

http://www.ti.com/lit/ug/tidu922a/tidu922a.pdf

Regards, Dana.
Thank you for your info....I will look into it in more details.
Since my requirements entails relatively high voltages ( up to 36V, 3 Amps) ; how about two 317's

#### AnalogKid

Joined Aug 1, 2013
11,121
It is not complicated, but you keep using a term that has no recognized standard meaning. What is a "dynamic load"?

IF you mean you want a circuit that delivers a constant current of 20 mA to different LEDs with different forward voltages (Vf)
THEN say that.

Dribbling out your requirements wastes everyone's time. Is this homework, a work project, or something else? Is this for a test setup, industrial lighting, or something else? You've jumped from 20 mA in post #1 to 3 A in post #9. Why?

Please tell us what you are trying to achieve.

ak

#### BobTPH

Joined Jun 5, 2013
9,109
An LED is not a dynamic load. It needs a constant current driver because the voltage to get the correct operating current will vary over a range, and the correct voltage to drive one of them might blow the next one.

An LM317 and 1 resistor will make a fine constant current souce as long as you have st least 2V more than the LED requires. That is not what I would call complicated.

Bob

#### arishy

Joined Apr 26, 2014
123
My apology for anyone who wasted his time with this post.
I started by the word confusion....so, I thought I should clear my mind on the concept of collector current behavior.
And you kindly responded.
Then, some of you asked me what you wants to do. Obviously I am in a very early stage.
As for using "dynamic load" With constant current circuits ; You can add led's ( dynamic load ), in MY understanding If that offended you my apology.

Lesson learned: You cannot get a loan unless you have the money that guarantee the loan..... Just kidding.....

Joined Mar 10, 2018
4,057
Never tried "paralleling" two LM317 operated in constant current mode. Have to
think about that, stability issues, etc....

This is a simple approach, and you scale the transistor for current needs -

Iload = (Vd1 - Vbe) / R1

Regards, Dana.

#### AnalogKid

Joined Aug 1, 2013
11,121
Lesson learned: You cannot get a loan unless you have the money that guarantee the loan.
That is not even remotely close to what is being taught.

ak

#### AnalogKid

Joined Aug 1, 2013
11,121
Please drop the use of the word dynamic.

My guess is that you want to power a varying number of LEDs (types unknown, colors unknown, current requirements unknown) with a constant current source so the current through the LEDs does not change as you add and remove devices. If that is correct ...

A constant current circuit for 20 mA and one for 3 A are two entirely different things, especially for a novice. Also, a circuit that can deliver 3 A at 36 V requires a source power supply of over 100 W. All of that heat has to go somewhere. Please refine your requirements fora clear starting point.

ak

#### arishy

Joined Apr 26, 2014
123
Never tried "paralleling" two LM317 operated in constant current mode. Have to
think about that, stability issues, etc....

This is a simple approach, and you scale the transistor for current needs -

Iload = (Vd1 - Vbe) / R1

Regards, Dana.
This is the way I will go.......Thank you Dana