Current-regulated LED taillights (was: Help I am Lost)

[chopper]

I have been trying to read the thread about the Cheap Current Regulated Luxeon Star Driver and seem to begin  in the middle of the thread where they are reffering to a schematic I cannot find, what am I doing wrong? Also if a tail light is powered at half or there abouts current, and only goes to full power when brakes are applied is heat still a concern? Thanks, Carl. 

[thread title edited by Rob to assist future searchers!]

[Rob]

Okay, I'll help out however I can.

This is the mystery diagram:


The article goes through a bunch of frankly unneccessary explanation about how it works.

What you need to know to use it:

1) you need to have a target load current. The article uses 320 mA as an example.
2) You find the value of R by taking R = 1.25V/I = 1.25V/0.320A = 3.9 ohms.
3) find a power rating for the resistor by P = V*I = 1.25V*0.320A = 0.4W (use a 1/2W resistor in this case)


That's the "just the math please" version.

Now you want to arrange things so that you have an LED that accomplishes two brightness levels. (that's how I understood your earlier post anyhow).

So you figure the maximum load current, and then the fraction of that you want for the dimmer version. Let's say for discussion purposes you wanted 320mA for braking lights on, and 160mA (half that) for just running lights.

This would be done by having two resitors in parallel replace the resistor in the above diagram. The value of the resistor should be double the original value. So with one resistor in place, you have the load current = I = V/R = 1.25V/7.8 ohms = 160mA. Switching the second resistor in makes the combined resistance 3.9 ohms, and load current goes back to 320 mA.

Is that much clear? If so, then we can move on to how to switch the second resistor in and out. That's the transistor idea that JustDIY brought up.

[Rob]

Here's a quickly hacked up drawwing depicting the above scheme.

With the switch open, you get 7.8 ohms, with it closed you get half that, or 3.9 ohms. Nothing at all says the values have to be equal, by the way. That was just convenient.

[chopper]

Woo Hoo, I'm with You now! Thank You so much for Your help! Carl.

[Rob]

One thing to consider--Vregulators aren't a lot more expensive than transistors, and you get two important things that transistors lack: short-circuit protection and thermal overlaod protection.

So the more failsafe design would be to just do the original current source circuit twice (I mean the Vregs are maybe 50 cents if you get them from a surplus place in the mail), and then use rectifier diodes (about a dime) to keep the current flowing the proper direction.

Code:

running light o------[current reg]---|>|---+----[LED array]----o ground
                                           |
brake light   o------[current reg]---|>|---+

[justDIY]

HAH

I like that idea a lot better Rob, another lm317, resistors and diodes would cost like $1-2 max, compared to a 1+amp transistor which would be $2-3

I think mine is more elegant from a technically over-complicated stand point (like I said in chat, I always look for the hardest solution first).

ps,

you dont need to rely on the transistor (in my post) for any sort of protection, the load is still protected by the single LM317 ... all the transistor does is change R_ADJ, which kicks up the output current up (or it could also decrease it, if R2 > R1)

[chopper]

I totally understand the latter explanation which is either similar or exactly as You had explained it in another thread a few months back, I thought there was something to learn from this new explanation till Just diy said he likes Your's better,lol. What about my question about the heat?? Thank You, Carl.

[justDIY]

yes, heat is always a concern when dealing with more than 50-75 mW output.

the PCB you had made for your taillight is single sided right?  would it be possible to use a double sided copper clad board, and have the back side left as solid copper (surface mount your leds and resistors).  This would give a great heatsink in itself, and also give a good place to attach a heavier heatsink if needed.

any LED can be surface mounted, you don't have to deal with the tiny little grain of wheat leds sold as surface mount.   Just trim the leads on your LED as short as possible, and then put a little 90deg bend on the end (no more than 0.5 mm).  Then you just solder that bent leg down to its pad, no drilling needed.  same goes for resistors, capacitors, etc, anything with leads that can be easily bent.  if you'll be making several of these, some sort of jig might be handy to hold the leds in position while you solder... I'm thinkin a piece of scrap wood with some holes drilled in it, the size of the hole being exactly the diameter of the led, so you have to use a little force to fit lense of the LED into the hole... then once they're all soldered, you just pop off the jig, reload, and do another row.

[chopper]

more than 50-75mw? Maybe I should stay with the things I do best.. So I guess what the wizard says about my array dissipating 2948mw wont work so well on the perf board I mounted it on encased in a small oval housing..lol. What I have is 66 red leds theyare 8000mcd, forward current-20ma, and forward voltage-2.2. I did what the wizard suggested using a 10 ohm resistor in a series with 6 leds. then paralled wired the 11 arrays. Now I need to take it apart? Ok, can You post a picture with a similar pcb as You suggest they should be assembled? Btw I can custom machine a heat sink and actually make it the housing for the tail light, would this be a good idea? Carl.

[chopper]

I think I can hear Justdiy laughing at me all the way from Michigan 

[justDIY]

no need to dissassemble if you've already put it together

the #1 killer of LEDs is heat ... they can't stand it ... LEDs are mildly more efficient than incandescent bulbs, but the majority of the power they consume still gets turned into heat

but there are other things to consider ... like ambient temperature.   If you're outdoor temperature this time of year is in the single digits and teens' (on the centigrade scale), that means you have more 'play' in the thermal allowances the led will tolerate.  If you can ventiliate your enclosure so the leds can be cooled by convection, that will probably keep them well below their recommended operating temperature.

I made a mistake in my last post, and was thinking of XRay's taillight ... he's the one who made a pcb ... and of course, there is AZDave's taillight .

If you have the ability to machine an enclosure for your light, that's excellent!   I'd just take the leds as you have them, and epoxy the board right into a shallow enclosure... use some liquid steel or jbweld or something similar (with a lot of filler/pigment in it)

[Rob]

Hey Carl, I'm a little confused on your descriptions. But I think the heat thing can be addressed, and I think you'll like the answer.

If you are running 20 mA into your LEDs @ 2.2V, then each one dissipates (2.2V * 0.02A = ) 44 mW. And that wold be only at full brightness--under a lower drive current you're below that even.

My quick and dirty rule of thumb: If the LEDs are running hot to the touch, you have a heat issue. If you're pushing them at 10 mA, then I really doubt they run hot, adn periodically cranking them up to 20 isn't likely to be a problem either, since they'll have some time to cool down subsequently.

What I'm not clear on is how you are driving these--with a current-regulated driver, you usually omit the current-limiting resistor. And if you are using a current-regulated driver, you may not get a series string of 6 to light (6 * 2.2 = 13.2V, and the regulator needs just shy of 3V to do its voodoo).

Have you test-lighted this contraption yet? With the resistor-limited LED array you describe, I'd say it would be safe to hook directly to the taillight drivers, and skip the LM317 craziness.

[chopper]

Yes I have test lighted this array with numerous wall warts and with a 12v motorcycle battery, the only way I can get it to make any heat is with a walwart rated at 12v, 1000ma, and 21watts. Under load it makes 13.87v and it takes about 10 min. before the array will make much heat. I would prefer to use the resistor and diode set up to power this on the bike but am not certain which resistors or diodes  to use. Btw, I now have acess to a digital camera and plan to post pics possibly later tonight. Thank You for Your help. Carl.