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Haru said:Hi everyone
I want to use a current mirror circuit to drive an LED.
Will a current mirror circuit (Mosfet or BJT) deliver constant current to an LED even if the LED has Capacitance?...If no, how do I go about solving this problem?
Haru said:- Doesn't the capacitance affect the constant current coming even from a current source?
Like this I previously posted?- Nigel Goodwin wrote: "Why a current mirror?, why not a simple constant current source?"
No reason, it is the first thing that came to my mind.
Other simple current sources, like what?
If there is other simple current sources y do people still use current mirrors?..How can I tell which current mirror is more suitable for an LED?
Note: The reason as to y use a constant current source for the LED is because the LED impedance changes with temperature therefore the current flowing into the LED will also change...Is my reasoning correct?
Haru said:I'm building a fiber optical communication channel, for learning purposes and it is fun.
I know I could probably buy the driver as an IC, but I want to try and build the whole system from scratch for learning purposes.
Questions:
- What would happen if you connect a bipolar squarewave signal (-V to +V) to a resistor and then to the LED?...I think the squarewave signal has to be unipolar (0 to +V) if connected in the arrangement described above…I’m I correct??
What led me to try and do something fancy was articles such as:
https://www.electro-tech-online.com/custompdfs/2005/04/82103di-1.pdf page 94
I apologize if u answeard this in ur previews post and I didn't understand:
Note: The reason as to y use a constant current source for the LED is because the LED impedance changes with temperature therefore the current flowing into the LED will also change which will change the intensity of the LED emitter...Is my reasoning correct?
Like this I previously posted?
ym2k said:Like this I previously posted?
Thanks, i look at your circuit and tried it out. It works perfectly. But lets say if the current source i need is 700ma, use to drive luxeon III leds, i try modifying with your circuit by using smaller R2, the constants current become unstable when i try changing the voltage. Is there i better constants current source to drive a 700ma load?
If increasing the current substantially, you should increase the base current as well, plus use a bigger transistor
You can use an opamp driving a transistor to make a better constant current source, at the expense of more complexity.
ym2k said:Is it safe to use lm317 to deliver constant current?