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Constant current supply

 
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rnielsen



Joined: 23 Sep 2003
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Constant current supply
PostPosted: Wed Jun 14, 2006 4:24 pm     Reply with quote
Since my analog expertise stinks, I thought I would ask you guys on your opinion.

I want to drive some super bright LED's. Their rated current is 20mA. I will be using a 12V battery as the supply. I will have three LED's in series and then use a resistor(or whatever) to limit the current and drop the remaining voltage. My dilema..

As the voltage drops the current will also drop and reduce the brightness of the LED's. I want to be able to keep the current at 20mA regardless of the voltage of the battery. What IC's or descrete components would work for this?

Thanks
Ronald
treitmey



Joined: 23 Jan 2004
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PostPosted: Wed Jun 14, 2006 4:51 pm     Reply with quote
Try this.
http://www.discovercircuits.com/PDF-FILES/NewPDF/CONSTANTCURRETLED1.pdf

If you drive more than 1 then there are special circuits for that.

http://rocky.digikey.com/WebLib/Texas%20Instruments/Web%20data/TPIC2810.pdf
and the TPIC6C595 for relays

and this one looks good.
http://focus.ti.com/lit/ds/symlink/tps61060.pdf

Last edited by treitmey on Wed Jun 14, 2006 5:00 pm; edited 1 time in total
Kenny



Joined: 07 Sep 2003
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PostPosted: Wed Jun 14, 2006 4:57 pm     Reply with quote
LM317L connected in constant current mode, an old trick.
http://www.onsemi.com/pub/Collateral/AND8109-D.PDF
SherpaDoug



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PostPosted: Wed Jun 14, 2006 8:21 pm     Reply with quote
My favorite for driving one or two LEDS from 12V just uses a bipolar transistor (2N3904) and a resistor if your PIC voltage is regulated. Drive the base from the PIC pin and connect a resistor (220 Ohms for 20mA and VCC=5V) from the emitter to GND. Then connect the LEDs in series from +12V to the collector. Three LEDs may have a problem if the LED voltage is high or the +12V is low. You will have to do the math for your application. Things will drift a little with temperature, but not enough for the eye to see.
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Humberto



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PostPosted: Thu Jun 15, 2006 6:33 am     Reply with quote
Quote:

As the voltage drops the current will also drop and reduce the brightness of the LED's. I want to be able to keep the current at 20mA regardless of the voltage of the battery. What IC's or descrete components would work for this?


The key for this is to use a constant current source that deliver the expected constant 20mA AND a couple of voltage comparator wired as window comparator that enable the whole functioning only if the battery voltage is within an expected adjustable range, ie:
An upper comparator that watch if the battery voltage is under Vmax(adj).
A lower comparator that watch if the battery voltage is over Vmin(adj).
Each comparator with open colector output and OR wired with a pull up resistor tied from 5V. If the battery voltage is outside of the expected value, the comparator output will generate a Shut-Down level to switch off the whole circuit.
This way the circuit should keep a constant brightness of the LED's regardless of the battery voltage drops.


Humberto
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