Led Constant Current driver help

This is the place for any magazine-related discussions that don't fit in any of the column discussion boards below.
Post Reply
Tparker
Posts: 40
Joined: Tue Apr 29, 2003 1:01 am
Contact:

Led Constant Current driver help

Post by Tparker » Tue Apr 20, 2004 9:22 am

I need some help in designing a circuit to add to constant current LED driver module available here. http://www.leddynamics.com/LuxDrive/Page_Buck.html I want to use a single 5 watt Luxeon Star LED as a tail/stop/turn signal. When the tail/running lights are turned on the output from the module would be reduced to say 350 ma. by adding a fixed resistance between the control and Ref. pins And when the stop/turn is activated, the resistance would be removed to get the full 700 ma. output. I am having trouble visualizing what I would need to accomplish this. Any help would be greatly appreciated.

User avatar
Edd
Posts: 885
Joined: Sun Jan 27, 2002 1:01 am
Location: Dallas Tx
Contact:

Re: Led Constant Current driver help

Post by Edd » Tue Apr 20, 2004 10:37 am

One thought……Looking at your modules Da Da sheet fig 9 how about using a 5 k pot and finding the resistances settings required for getting the two current values desired and then measuring the resistance values required so that two fixed resistances can then be incorporated in the pots place and then use a 12v coil reed relay to make its reed switch shunt the resistor that shifts the modules output to the higher current level. One end of the the reeds coil is grounded and the other lead is connected to the cathodes of two 1N4002 steering diodes and the separate anodes go to the stop light line and respective turn signal indicator. Minumal component use and maximum interfacing isolation. Reliability ……..reed relays used to be the workhouse components in telephone switching operations with millions of operations MTBF. <p>73's de Edd
[email protected] .........(Interstellar~~~~Warp~~~Speed)
[email protected].........(Firewalled-Spam*Cookies*Crumbs)
;) ;)

Tparker
Posts: 40
Joined: Tue Apr 29, 2003 1:01 am
Contact:

Re: Led Constant Current driver help

Post by Tparker » Tue Apr 20, 2004 11:24 am

Edd,<p> Thought about using relay, but had reliability issues. But I guess its better to KISS. I think I can get by with one DPST relay, one resistor, and 2 diodes. The resistor would be wired to the NC contacts to reduce current drive to the LED when park/running lights were selected. When Stop/Turn signal was present, relay would pull-in opening circuit and allowing LED to go to full brightness.

User avatar
Edd
Posts: 885
Joined: Sun Jan 27, 2002 1:01 am
Location: Dallas Tx
Contact:

Re: Led Constant Current driver help

Post by Edd » Tue Apr 20, 2004 11:38 am

You noticed initially,thats why I was sure to harp on the reliability issue, since the reed units contact components are hermetically sealed in glass. Gold contacts and the current carrying issue at the levels involved certainly wont be a problem. Thats where problems usually creep up if they experience them, the level of current used on the reeds contact points.
73's de Edd<p>[ April 20, 2004: Message edited by: Edd Whatley ]</p>

dyarker
Posts: 1752
Joined: Fri Aug 22, 2003 1:01 am
Location: Izmir, Turkiye; from Rochester, NY
Contact:

Re: Led Constant Current driver help

Post by dyarker » Tue Apr 20, 2004 7:09 pm

Have you seen Figure 12 of the page you gave the link for? The pot sets low current, when the transistor is on it shorts around the pot.
Dale Y

User avatar
jwax
Posts: 2205
Joined: Mon Feb 09, 2004 1:01 am
Location: NY
Contact:

Re: Led Constant Current driver help

Post by jwax » Wed Apr 21, 2004 5:00 am

Don't forget the capacitor shown in Fig. 17, too!

Tparker
Posts: 40
Joined: Tue Apr 29, 2003 1:01 am
Contact:

Re: Led Constant Current driver help

Post by Tparker » Wed Apr 21, 2004 8:47 am

Dale Y,<p>I saw that but I am not sure how that helps in my situation. When the transistor is on you say it shorts the cntrl/ref. circuit, the way I see it that would turn the current off. Not sure what i would be gaining?

Tparker
Posts: 40
Joined: Tue Apr 29, 2003 1:01 am
Contact:

Re: Led Constant Current driver help

Post by Tparker » Wed Apr 21, 2004 8:48 am

Dale Y,<p>I saw that but I am not sure how that helps in my situation. When the transistor is on you say it shorts the cntrl/ref. circuit, the way I see it that would turn the current off. Not sure what i would be gaining?

dyarker
Posts: 1752
Joined: Fri Aug 22, 2003 1:01 am
Location: Izmir, Turkiye; from Rochester, NY
Contact:

Re: Led Constant Current driver help

Post by dyarker » Wed Apr 21, 2004 8:16 pm

Hummmm oh-oh. Oh yeah, then put the transistor in series with the pot instead of parallel.
Dale Y

Post Reply

Who is online

Users browsing this forum: No registered users and 31 guests