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Constant current circuit

Amit Dhiman

Oct 14, 2017
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hi i am new here and know little about electronics and electronic circuits
so i need explanation on this circuit, means how this circuit will make a constant current on load resistance
i know that transistor will be on when have supply on base .please help me in this circuit
 

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Harald Kapp

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Welcome to EP.

We will not solve your homework or assignemnts, we will only guide you in finding the solution yourself.


First check the orientation of Q4. I think you have it backwards: emitter should go to ground (0 V), not towards R1.

Simplify the circuit for easier understanding:
  • replace R5, R6 by a single resistor R56 = 500 Ω
  • remove Q2, R3. These are obviously there to share the load (power dissipation) between transistors Q1 and Q2. This is not required for understanding the circuit (in a real circuit it may be necessary to protect the transistors from overload).
The load current (current through LOAD R) will flow throughR56 (see above) and create a voltage drop at the base of Q4. This voltage drop will turn on Q4, thereby allowing current to flow through R1 and Q4. The current throughR4 creates a voltage drop at the base of Q1 which in turn decreases the voltage at the base of Q1.

Now follow up the chain of reasoning: A lower voltaeg at the base of Q1 means . ..?
This in turn has which consequence on the load current?
A change in load current leads to which effect at the base of Q4?
...
 

Amit Dhiman

Oct 14, 2017
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yes orientation of Q4 was wrong .
how load current will flow through R56 means voltage divider??
1. low voltage at the base of Q1 turned it off .
then i dont know the consequences ,,sorry

and yes it was not a homework i need to attach LEDs at the load
 

Audioguru

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You said LEDs in plural. LEDs have different forward voltages even if they have the same part number.
If you parallel a 2.8V white LED with a 3.2V white LED then the 2.8V LED will take all the current and might burn out but the 3.2V LED will not light until the other one has burned out then it will also burn out.

Chinese flashlights have many LEDs in parallel because they buy thousands of LEDs and have a kid measure and sort them into piles with the same forward voltage. You can doo dat too if you want, then throw away the hundreds of LEDs that do not match.
 

Harald Kapp

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ohk i got it up to this ,but how it will make current constant at load
What happens to Vbe of Q4 when Iload rises?
What happens in turn to the base voltage of Q1?
And consequently to the current trough Q1?

I updated the image for further clarification (ignore R4 for the sake of simplicity):
upload_2017-10-16_17-31-18.png
Part of Iload will flow through R4, but the regulation happens via Q1/Q4.


Where did you get this circuit from? Wasn't there a description on that webpage?
 

Amit Dhiman

Oct 14, 2017
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You said LEDs in plural. LEDs have different forward voltages even if they have the same part number.
If you parallel a 2.8V white LED with a 3.2V white LED then the 2.8V LED will take all the current and might burn out but the 3.2V LED will not light until the other one has burned out then it will also burn out.

Chinese flashlights have many LEDs in parallel because they buy thousands of LEDs and have a kid measure and sort them into piles with the same forward voltage. You can doo dat too if you want, then throw away the hundreds of LEDs that do not match.
yes there are 8 leds to be fed with a constant current and are of same ratings
 

Amit Dhiman

Oct 14, 2017
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What happens to Vbe of Q4 when Iload rises?
What happens in turn to the base voltage of Q1?
And consequently to the current trough Q1?

I updated the image for further clarification (ignore R4 for the sake of simplicity):
View attachment 36832
Part of Iload will flow through R4, but the regulation happens via Q1/Q4.


Where did you get this circuit from? Wasn't there a description on that webpage?
actually it was an project for driving a leds for a vehicle rear leds and i am new to this project and got this circuit from there but struck here that how this circuit makes constant current on load where leds are connected
 

Amit Dhiman

Oct 14, 2017
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Which project team? Which data files?
If the project is well documented, you should find the description and the equations for dimensioning the components in those files.
I not, the project is not a good project.

This is essentially the same circuit, also this one, including some explanation.
thanks for the links ,I dont know this is the circuit they have made and i just wanted to know the working ,but thanks for the circuit ,i think need more basic knowledge first
 

Audioguru

Sep 24, 2016
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yes there are 8 leds to be fed with a constant current and are of same ratings
You did not understand what I said. You cannot buy an LED with a certain forward voltage.The forward voltage rating of an LED part number is not a single voltage, it is a range of voltages (an example is 2.8V to 3.2V) because an LED is not a certain length of very hot wire, it is a diode and each one is different. If you buy a few hundred LEDs with one part number then you must test their forward voltages and group them so that all LEDs in a group have the same forward voltage then they can be paralleled.
 
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