hi
i am new to this forum and need help on this circuit ,consider me as a newbie
i need a constant current on load resistance or leds more precisely and found this circuit through my friend but i am in bit confusion like i know when BJT turn on and Ic current starts flowing please hep me to understand this.

 

I don't care for that circuit because it's too complex for just lighting an LED.

Let's start with what your goal is. What is your power source and what are the specs of the LED? The simplest solution is a single resistor for limiting current, and many people never do anything more.

 

hi
i am new to this forum and need help on this circuit ,consider me as a newbie
i need a constant current on load resistance or leds more precisely and found this circuit through my friend but i am in bit confusion like i know when BJT turn on and Ic current starts flowing please hep me to understand this.

What is it you are trying to accomplish? Just light up an LED as an indicator?

What current do you want? How much can it vary before that is a problem? What is the supply voltage? How much can it vary? Does the current need to be adjustable?

 

I don't care for that circuit because it's too complex for just lighting an LED.

Let's start with what your goal is. What is your power source and what are the specs of the LED? The simplest solution is a single resistor for limiting current, and many people never do anything more.

the project i am working is driving leds of a car rear end leds so i just wanted explanation that how this circuit makes a constant current

 

the project i am working is driving leds of a car rear end leds so i just wanted explanation that how this circuit makes a constant current

Where did you get such a horrible schematic?

Many of the components make no sense at all. D1, C1, R7 are unnecessary. R5 and R6 in parallel is unnecessary; just use a single resistor. Just use a higher power transistor instead of paralleling Q1 and Q2.

It might be a current source, but I'm not going to bother trying to analyze it. I keep getting distracted by all of the unnecessary wire jogs...

What current do you require? Why do you need a current source in the first place?

 

the project i am working is driving leds of a car rear end leds so i just wanted explanation that how this circuit makes a constant current

I don't know how tail light LEDs are driven in vehicles. I speculate that they might use a DC-DC converter with constant current output. This would achieve two things: Constant brightness as the vehicle voltage varies, and high energy efficiency. The opposite end of the spectrum is the resistor I mentioned earlier. This is usually less efficient and does not protect the LEDs from varying vehicle voltage, which can include transient spikes. I doubt this is used in cars but I honestly don't know.

There are many other techniques you could use in between these two extremes.

 

Is this a home made led lighting system or is it after market?

Also, does the car have turn signal flashers?