CCS C Software and Maintenance Offers
FAQFAQ   FAQForum Help   FAQOfficial CCS Support   SearchSearch  RegisterRegister 

ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 

CCS does not monitor this forum on a regular basis.

Please do not post bug reports on this forum. Send them to support@ccsinfo.com
Constant current supply

 
Post new topic   Reply to topic    CCS Forum Index -> General CCS C Discussion
View previous topic :: View next topic  
Author Message
rnielsen



Joined: 23 Sep 2003
Posts: 852
Location: Utah

View user's profile Send private message

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
Posts: 1094
Location: Appleton,WI USA

View user's profile Send private message Visit poster's website

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
Posts: 173
Location: Australia

View user's profile Send private message

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



Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA

View user's profile Send private message

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.
_________________
The search for better is endless. Instead simply find very good and get the job done.
Humberto



Joined: 08 Sep 2003
Posts: 1215
Location: Buenos Aires, La Reina del Plata

View user's profile Send private message

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
Display posts from previous:   
Post new topic   Reply to topic    CCS Forum Index -> General CCS C Discussion All times are GMT - 6 Hours
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group