Dimming an IR LED

Log in or register to post comments
Go To Last Post
6 posts / 0 new
Author
Message
aor_dreamer
aor_dreamer's picture
Level: Wannabe
Joined: Thu. Dec 2, 2004
Posts: 67 View posts
Location: Greece
#1
Posted by aor_dreamer: Tue. Feb 26, 2008 - 03:04 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I'm using a SEP8706 IR LED in pairs with a SDP8600 in a label sensing application and I'm trying to control the current of the former. In the past I was using a rheostat in series with a 270 Ohm resistor which was working fine.

So, I thought it would be a good idea to dim my LED using PWM, but using a small resistor in series with the LED and supplying it using the pwm output of a mc didn't quite work, since I need to control the current flowing through the LED, not it's voltage.

Do you think that it's plausible (using a small signal FET maybe) to control the current? Any ideas? The LED has a max Vf=1.7V and max If=50mA.

Thanks

So, now I know how ICs work:
they work with smoke.
Cause every time you let the smoke come out of an IC, it stops working.
I can prove it, too...

Tags:

Learning and Information, General Electronics
Top
snarflemike
snarflemike's picture
Level: Hangaround
Joined: Sun. Jan 6, 2008
Posts: 355 View posts
Posted by snarflemike: Tue. Feb 26, 2008 - 03:22 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

In your application using PWM sounds very problematic. While your average current will indeed be lower, your sensor is going to see the IR output as BRIGHT-OFF-BRIGHT-OFF... I doubt that's the result you want in a label-sensing application, since you'll be modulating the desired label data with the undesired PWM data.

Mike

Top
aor_dreamer
aor_dreamer's picture
Level: Wannabe
Joined: Thu. Dec 2, 2004
Posts: 67 View posts
Location: Greece
Posted by aor_dreamer: Tue. Feb 26, 2008 - 03:35 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Actually I'm using them as an ON/OFF sensor, (I see a label/I don't see a label), a switch, sorry that I didn't make it clear earlier. I want to control the LED's current according to the label's transparency.

So, now I know how ICs work:
they work with smoke.
Cause every time you let the smoke come out of an IC, it stops working.
I can prove it, too...

Top
sgomes
sgomes's picture
Level: Resident
Joined: Wed. Feb 19, 2003
Posts: 919 View posts
Posted by sgomes: Tue. Feb 26, 2008 - 03:36 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Add a cap and filter out the PWM frequency.

Go electric!
Happy electric car owner / builder

Top
ezcomp
ezcomp's picture
Level: Hangaround
Joined: Sun. Dec 28, 2003
Posts: 348 View posts
Location: Oklahoma
Posted by ezcomp: Wed. Feb 27, 2008 - 12:39 AM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

If you really want a true current source, you need to do something like this, or use an LM334, or even a normal voltage regulator slightly tweaked. There are plenty of other ways to do this, but that should give you a good shove in the right direction.

www.beleted.com

Top
aor_dreamer
aor_dreamer's picture
Level: Wannabe
Joined: Thu. Dec 2, 2004
Posts: 67 View posts
Location: Greece
Posted by aor_dreamer: Wed. Feb 27, 2008 - 11:09 AM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I think that the LM334 is what I was looking for.
Thanks a lot ezcomp!

So, now I know how ICs work:
they work with smoke.
Cause every time you let the smoke come out of an IC, it stops working.
I can prove it, too...

Top