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?

 

Why a current mirror?, why not a simple constant current source?. There have been a number of threads about simple constant current sources for LED's.

I don't see as capacitance has any relevance in either case!.

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- Doesn't the capacitance affect the constant current coming even from a current source?

- 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?

 

No, I don't see any relationship between LED's and capacitance at all?.

Like this I previously posted?
http://www.electro-tech-online.com/download.php?id=4628
I've never heard of a current mirror being used for LED's, it's most often used in audio amplifiers to provide identical (high impedance) loads for a pair of devices.

Perhaps you should mention EXACTLY what you are trying to do?, in the vast majority of cases a simple series resistor is all that's needed for LED's.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

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:

http://www.edn.com/contents/images/82103di.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?

 

You cannot apply a lot of reverse voltage to an LED. Look on your spec sheet for "reverse breakdown voltage" and don't go near it.

A special constant current source is most helpful when the source voltage varies significantly and/or the difference between the supply voltage and LED voltage is not large. In most cases, you will do fine with just a ballast resistor. This sounds applicable to your case.

However, be aware that many logic circuit outputs do not have enough current to drive a powerful LED. Thus you may need to use an external LED driver, this might be as simple as a transistor.

 

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 :lol:

You can use an opamp driving a transistor to make a better constant current source, at the expense of more complexity.

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PIC programmer software, and PIC Tutorials at:
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I am not really good in opamp, is there any circuit i can refer to? so i can learn as i experiment with it.

 

Is it safe to use lm317 to deliver constant current?

Attached Images

constant.jpg (164.1 KB, 429 views)

 

Yes, datasheets commonly show how to use IC regulators as constant current sources.

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PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk