Constant current led driving

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theasus

Joined: 06 Jun 2009
Posts: 26

30 Oct 2009 15:12 Constant current led driving

I want to use IR led and photodiode in my project as a sensor. And for stable working I have to assure constant current for IR Led.How can i do this operation?Could you post a simple circuit and its usage?

Naveed Ahmed

Joined: 31 Aug 2008
Posts: 22
Location: Pakistan

30 Oct 2009 16:00 Re: Constant current led driving

You can use a transistor biased circuit to make the current constant as required. Below is the link follow it and click on "Shunt Constant Current Source"

http://www.discovercircuits.com/C/constant.htm

Regards
Naveed Khowaja

theasus

Joined: 06 Jun 2009
Posts: 26

01 Nov 2009 14:38 Re: Constant current led driving

Could you propose more sensitive circuit ?
I think this is not a sensitive.And this one more sensitive but i couldn't calculate the compenent values(transistor,resistor..)
Could you help about this circuit?And also for this operation we can prefer fet to transistor.But as i said i didn't calculate the right value for one led driving.

Constant current led driving

dick_freebird

Joined: 04 Mar 2008
Posts: 313
Helped: 45
Location: USA

02 Nov 2009 1:47 Constant current led driving

How constant does it really have to be? With a high enough
and tight enough voltage supply, the diode forward variation
can be small relative to headroom and a resistor will be fine.

So how good, and why?

theasus

Joined: 06 Jun 2009
Posts: 26

02 Nov 2009 18:18 Re: Constant current led driving

I want to use an IR led for a sensor application.And for this application IR led's current must be constant for stable working. And I think,to end of my voltage regulation circuit which is consists of 7805, connecting a constant current source for IR led is neccessary. And My problem is searching a good circuit for this operation.

kender

Joined: 19 Jun 2005
Posts: 1231
Helped: 88
Location: Stanford, SF Bay Peninsula, California, Earth, Solar System, Milky Way

02 Nov 2009 20:30 Constant current led driving

How much current do you need to run through your LED?

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theasus

Joined: 06 Jun 2009
Posts: 26

03 Nov 2009 19:55 Constant current led driving

18 mA

kender

Joined: 19 Jun 2005
Posts: 1231
Helped: 88
Location: Stanford, SF Bay Peninsula, California, Earth, Solar System, Milky Way

04 Nov 2009 2:40 Constant current led driving

18mA is a relatively small current. An output of common OpAmp can source or sink 18mA.

Here's an example of an OpAmp constant current source: http://www.faqs.org/docs/electric/Semi/03039.png. This is just one of many topologies. LED can be hooked up instead of a load resistor.
Here's a small collection of OpAmp current source and sink circuits: http://www.maxim-ic.com/appnotes.cfm/an_pk/3869

- Nick

vistapoint

Joined: 20 Feb 2005
Posts: 91
Helped: 2

04 Nov 2009 15:17 Re: Constant current led driving

This is how you calculate:

The input+ has a voltage set by the zener diode=Vz. Input- = Input+ due to feedback. So Vsense=Vz. The current through the resistors=Vz/Rsense

This is a good current sink, better than other designs in this thread.

__________________________________________
Low-price resistors, capacitor, and LEDs: http://store.engeecorp.com

theasus wrote:

Added after 2 minutes:

or current source, depending where you put the load

__________________________________________
Low-price resistors, capacitor, and LEDs: http://store.engeecorp.com

vistapoint wrote: