Alas. Not as bright as it should be

[ibbleobble]

Hey, I'm in the process of building a 5up xpg bike light, and I'm just not getting the brightness I was when I first hooked it up.

I'm using this driver from dealextreme: http://www.dealextreme.com/details.dx/sku.26106

When I first connected it, it lit up a whole room, and got really hot. I never left it on high for very long, I figure when I get it going on the bike heat won't be such a problem.

Now its just not that bright. Heres what happened:

I tried testing the current coming out of the driver (ie. between the driver and the leds) and I think I had the multimeter on the wrong setting. The light went out, and the prongs were sort of stuck in the wires, so I didn't get them out quick enough, and the driver started smoking. I pulled them out, and the light went back on but not very bright. I have since replaced the driver, and it was brighter for a bit, but then went dimmer again. Its possible I shorted the out leads on the driver again:sigh: and it was still not as bright as it originally was.

My question is, do I keep trying to replace the driver or have I damaged the LEDs?

I have been using it as a bedside lamp, and its still pretty bright, but I'm clearly not getting the full power of the xpgs. Interestingly, I am getting great battery life at the moment, which makes me think there's nothing wrong with the LEDs. And the colour of the light is still good - the same as it always was. It also doesn't get hot at all.

I was always thinking I might get a Maxflex once I got it working on the cheapo driver. Maybe I should try that if the cheap ones are not very reliable. Or maybe I would just kill the maxflex too.

Here are some pics:

http://picasaweb.google.com.au/jlegge/BikeLight?feat=directlink

 

[HarryN]

Hi, I will give you my impressions based on the info you have provided and some educated guessing.

LED thermals
- At a minium, you really need 1 sq in per watt of LED power for moving the heat when it is going on a flashlight, and perhaps 4 sq in when your body is not carrying away the heat. You might get away with less, but based on my guess that your optics are 20 - 25mm dia, and you have perhaps 1/2 of the needed thermal path area.

LED heat damage happens really fast. Guess how I know. I don't know if yours are damaged or not, but its possible.

LED Electrical Isolation
- Consider to read the data sheet on those LEDs again and see if they need electrical isolation in the thermal path. Some do, some don't. I don't know if that one does or not.

Driver
- Consider that you are asking a sub $ 5 driver to do the work that one might expect to be challenging for a $ 25 driver. $ 5 barely covers the cost of a decent coil for that much power, much less the rest of the circuit. The reason I happen to be aware of these parts prices is that I have done some recent design work on LED drivers in this range.

It is highly unlikely that a circuit like this can really take 8 volts in and drive 5 XP-Gs. This would require a driver to deal with 25 watts input, so at least 3, if not 4 amps comfortably. One of the most powerful boost drivers I have seen is the taskled hyperboost, and it is just enough for the task.

Even if you use a taskled hyperboost, then it still needs a thermal path of its own to work. The reviews of the driver you are using indicate perhaps 60 - 70% efficiency, so the need for a healthy thermal path just for the driver are significant. at leat 4- 5 watts) by itself.

Battery Supply
I can't really tell what battery supply you are using, but hopefully you are running at nearly the max allowed voltage if you want any chance of getting enough power into that LED string.

It looks like an intersting project, perhaps it is just time to beef it up a bit.

 

[HarryN]

Consider to lower your expectations for this setup. Try to see how it work driving just 3 leds, and provide a thermal path for the driver. Let us know if that is more normal behavior for current management.

 

[ibbleobble]

Response*

Thanks so much for the responses, I really appreciate it. The driver I'm using has a max output of .8amps, which works out to about 12 watts with 5 LEDs. I know this is far from the 'full' power of the xpgs, but it's still insanely bright, and I thought it would be thermally manageable. Going by the one inch per watt rule, I have a 5*1" piece of square tubing that the LEDs are stuck to, and the tubing obviously has four sides, so there are 20 square inches of surface area. There is a good thermal path, I know, because when it was working properly the whole thing got really hot. Probably when it's mounted on the bike it will be enough, but indoors it's clearly not.*

The driver itself is stuck to the end of a piece of aluminium tubing for a thermal path, but I didn't have anything attached to the coil on the top which may have been a mistake. It didn't seem to get really hot though. I've ordered another one of those drivers, I'll try one more before I go up to a taskled driver. I don't know where you're from, but I'm Oz the maxflex is over $70 with delivery and soldered leads. I really don't want to wreck one.*

As for trying three LEDs, it was such an effort to solder them together, I don't want to desolder them. I'm going to make a 3up prawn light, so maybe I'll try the old driver on that.

Thanks again


By the way I'm using 2 18650's - 8.4v is the max the driver can do.

 

[HarryN]

Hi, it is pretty darn bright and you are on a good path.

Let me make a slight clarification about what I mean on the surface area required. In your example:
- 1 face is used up by the LED and optic - so that does not count, as there is no real air flow exposure.
- 1 face is down on the table, so again, no air exposure, so it does not count

That leaves you with more like 2-3 in sq per led, each one putting out around 3 watts, or about 1 watt per in sq.

The reason you can do that when it is "in your hands" is that your blood circulation acts like a water cooler system, drawing the heat away and sweating it off. This works up to about the 15 - 20 watt region, then your body just can't deal with it any more. (unless you are holding a cold drink. )

When the light bar is just sitting there with only passive air movement, then you need about 4x that much exposed area to keep the LEDs within their thermal specs.

What you probably are seeing is the LED output dropping with temperature, just like they all do. Lumileds packages are a little less sensitive to this than most, and Cree are about average, but it all sort of balances out in the end.

I guess in theory, it should put out more light if you stand it on end and blow a fan on it. If you don't see an improvement from that, then perhaps your driver is doing something funny. Do you happen to have a volt meter to meaure the actual current?

 

[ibbleobble]

Hi, I do have a multimetre, but I'm afraid to use it to test current because the driver started smoking last time I did that. I really can't figure out which settings to use. I found a photo of the one I have here:

I should measure the current, because I'm convinced that the reduced brightness is because of reduced current. Nothing is getting hot at all, not even the circuit boards that the leds are attached to (from cutter). And the batteries last a really long time. When I first plugged it in it got hot, and was much much brighter.

I'm starting to think it would be better to run leds individually from separate drivers wired in paralell from the battery.

 

[ibbleobble]

Link didn't work. The pic of the multimeter is here:

http://www.google.com.au/imgres?img...mage_result&resnum=5&ct=image&ved=0CEQQ9QEwBA

 

[HarryN]

Hi, for $ 70 you can do a lot, especially if you are willing to run in a single mode. I guess I am spoiled living local to taskled.

The buckpuck driver could run 2 of those in series from that pack. ($ 15 each). There is actually a very nice list of drivers in the electronics section. I think it is in the "stick" threads of interest area.

 

[ibbleobble]

finally worked out the multimeter!

Thanks for link to the info on the drivers. The Taskleds are still my preferred, particularly the maxflex, but they're just so expensive in Australia, especially because I want to get the leads soldered on for me. I'm pretty clumsy with the soldering iron.

I have a number of plans to improve heat management, various aluminium bits and pieces i can stick to the light. I'm thinking of making a permanent bike mount from aluminium that would transfer heat, and would mean I don't have to take it off my bike all the time.

on the brightness front, The driver is only giving 130mA to the leds on high. 60 on medium, 30 on low. As I suspected, there is something wrong with the driver. At least I know that the leds are probably ok. Don't know how I damaged this driver though. maybe heat issues. I'm going to try another one. I really don't think this setup could handle 1.2A from a better driver anyway. Maybe I'll make a new light when the XMs come out. If they're not too expensive.

I did not realise that you have to put the multimeter into the circuit in series to measure the current. There you go!

 

[roadie]

hmmmm

1. what type of reflector are u using with the leds ?

2. did u place/attach a heatsink (w/fan) on/around the driver?

3. what type of batteries u using?

 

[ibbleobble]

Hey,

I'm using plastic dx lenses with the leds, they are still not stuck on though, just sitting on top because the first ones i got were no good for crees (apparently).

On my first try I tested the light a bit with no heat sink
I have the new driver attached to the end of a piece of aluminium tubing, and a PVC cap over the top. I think this might be an issue, because the potential heat path is going from the flat side of the board, when i think it should go from the big copper coil on the top. That seems obvious, but its much more convenient to do it this way.
When I get the new driver I will find a way to have the coil attached to the alu tubing, and maybe a copper wire heat pipe as well, that could also function to attach the driver/switch module to my bike.

The Driver didn't really seem to get super hot, so I didn't think it was a major concern. Obviously I was wrong.

I'm using two 18650's in series. According to the product page, 8.4 is the max voltage it can take

 

[pe2er]

...I have the new driver attached to the end of a piece of aluminium tubing, and a PVC cap over the top. I think this might be an issue, because the potential heat path is going from the flat side of the board, when i think it should go from the big copper coil on the top.

Cooling the driver from the flat bottom is OK. You do not need to cool the inductor on top - you need to cool down the electonic parts on the PCB, so mounting it like this will do:

(Pic from Let's design a road front light beam thread).

 

[ibbleobble]

Cool, I was worried about sticking it to alu like that because I thought that would short circuit the power-in contacts. I'm just using the thermal silicon from dx, I don't know how good an electrical insulator it is.

The other problem is that i've been soldering the leads to the bottom, so it isn't flat. That why I stuck it to the end of a piece of tubing, so the aluminium was only touching the negative outer ring, not the positive inner one, and the lump of solder is in the middle. That means there's not that much actual contact I guess.

 

[pe2er]

My answer was incomplete.

You DO need to worry about shorting out the positive and negative feed.

- Either use an insulator in between the PCB and the aluminum.
- Or modify the board.

I Drilled out the two Via's that connect the upper and lower tracks on the PCB. This is a different board, but it shows what a Via is; a metal-lined hole through the board, connecting the track on the PCB top and bottom.

In the PCB in the first picture, there were 3 vias in the positive feed. These connected the part marked SS14 (a diode) and the feed for the integrated circuit through the positive feed contact on the bottom. After breaking the Vias with a small (2mm) drill, I needed to restore this connection. Therefore the extra wire on the top side was added (marked with red dashed line).

What needs to be done depends on the board used. Recent versions of the Dx Sku 3256 drivers I use have only two vias and breaking them does not break any other circuit...

But that may be purely theoretical. If you have already soldered leads to the bottom of the PCB, it will be difficult to find all the vias underneath the solder

 

[HarryN]

Hi, with wire solder, my results are really poor. I switched to using solder paste and it is like night and day even with a simple soldering iron. Even the modest "solder it" stuff is MUCH easier to use.

There is a guy in the mod and custom section that is attempting to use a DX driver for his 4x and 5x custom light and somehow is making it work, or believes that he is. I think it is van isle diesel or similar, making a P60 drop in.

I don't think he has said what sku driver he is using, but probably similar to yours.

I know your pain on the driver aspect. I am working on a 4S LED project myself and I was forced to make a custom driver - or more correctly - I am deep in the middle of the project.