wiring up chinese LED torches

[Andrew]

Hi folks.  I've been wiring up a red 'Cree' LED and a 1300 white Cree LED for my bike. I bought a volt converter too (all China goods).  I've got the converter in a choc box and turned the potentiometer down to 4.3 volt output according to my volt meter.    The red lght worked fine on its own when  tested. I then paralleled the white light and both lights lit up for about 3 seconds and then both went off and now they don't work.

Can  anyone give an explanation?
do I need to turn up the potentiometer due to the parallel wiring? or is it likely to be something else?   

The battery source was 39v
the volt converter is 63v max input
both Cree red and white LED's are 3.7 to 4.2 volt each
wire used 5 amp rated

Andrew

[Sangesf]

Sounds like you prolly put too much voltage into one or the other which caused a spike and then more voltage and/or amps and blew up the LEDs..
I've been known to do that once or twice, myself..

[GM Canada]

Have you tested the bulbs with a meter?

Gary

[Andrew]

thanks for the replies. Using a volt meter there is voltage going into the DC/DC step down converter but there is no longer anything coming out the other side.  there was when I first set it up as I set it to just over 4 volts and it lit the LED.  The converter is isolated in a plastic choc box and cannot move as the wires are screwed down, so i'm sure it's no fault of my own i.e crossing poles on the converter(as I did on the last one).   

i've used a large multi connecter cube(30 or 50 amp compatable) on the out put side of the converter to then wire up the lights.  Can these cause a large draw of current that could kill the voltage converter   Or have I been unlucky with the DC/DC converter?

I have not tested the LED bulbs alone, how do I do this?

[e-lmer]

You did not specify the output current rating of the
converter or of the LED assemblies.

Two lights in parallel will draw twice the rated
 current of  light.

If you can't read an output voltage then you probably
toasted the DC-DC converter. 

I would be curious to see the docs on the converter.

[Andrew]

here's the link to the exact purchase, the description states 2A to 3amp max http://www.ebay.co.uk/itm/250941599114?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649


I don't know the the output current of the lights only the voltage (3.7 - 4.2)

here's the lights... http://www.ebay.co.uk/itm/390349046056?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

http://www.ebay.co.uk/itm/1300-Lumens-UltraFire-CREE-XM-L-T6-C8-5-mode-LED-Torch-Flashlight-/110810217755?pt=LH_DefaultDomain_0&hash=item19cccdbd1b

Hope this helps.

How can I measure the current draw from the LEDS?


Also, I dn't know if this would have made any difference to the current draw.... but I removed the curcuitry from the White LED torch and soldered straight to the + and -  next to the LED.  In other words the torch is permanently turned on with no differing modes.

[Bikemad]

Unfortunately Andrew, as Elmer has already suggested, you have more than likely blown the converter by applying an excessive load.

Here are some specifications for the White LED:
CREE XM-L T6 Emitter
Cree XMLAWT-0000T6051
Viewing Angle:    125°
Height:    3.02mm
Width:    5.00mm
Depth:    5.00mm
Emitting Color:    white
Lumen min.:    280
Lumen max.:    975
Kelvin min.:    5700
Kelvin max.:    6100
mA test.:    700 mA
mA typ.:    700 mA
mA max.:    3000 mA
V typ.:    2.9 V
V max.:    4 V
Reverse Voltage:    5 V
Watt:    2,03 W

If you had not bypassed the driver circuit by soldering dirctly to the LED (or kept the output voltage below 3.6V) your voltage convertor would probably have survived.

Unfortunately, an LED's current increases greatly as the voltage rises over and above the typical forward voltage across the LED.
Here's a graph showing the results of voltage variation on a 5mm 40,000mcd white led that I tested a few years ago:



If you look at the curve you can see just how quickly the current starts to rise as the voltage exceeds the point where the recommended typical forward current is achieved.
I only tested the LED up to its 3.8V maximum rated voltage, but if I had carried on to 4.1 volts it would actually have been drawing more than 5x the recommended typical current!

The driver circuit incorporated into your torch not only controls the different lighting modes (low, High, Flashing etc.) but it also regulates the current going to the emitter (LED). This ensures it doesn't suddenly change from a very bright LED into a useless DED (Dark Emitting Diode).

I tested my 800 Lumen torch powered by a fully charged 4.2V lithium cell, and the voltage measured directly across the LED on full power is approximately 3.6V.
I measured the current at the battery on each of the three modes and the results are as follows:

• Low brightness = 0.25 Amp
• Flashing = 0.5 Amp
• High Brightness = 1.25 Amps

As you put 4.2 volts directly across the LED, I reckon the current drawn by the white LED alone would be at least 3.5-4 Amps, and I would expect the red LED to be at least 1-1.25 Amps when wired through its driver circuit, and a lot more if wired directly to the LED.
This amount of current is at least 50% more than the stated maximum output of your convertor, so it's hardly surprising that it's blown.

I would expect the forward voltage across the red LED to be much less than the White LED.

I've recently purchased one of these, which should hopefully arrive soon:


I'm hoping it will be more than up to the job of powering this LED floodlight that has a 12V 10W LED:


and possibly a couple of these as well (or instead of):


I'll let you know the outcome if I can find some time to wire it up and test it when it arrives.

To measure the current draw from the LEDs you would need to connect an Ampmeter in series with each of the LEDs in turn, on the output side of a working convertor.

Alan
 

[Andrew]

Alan, thanks for answering all my quieries .  I can cope with the fact that I killed the dc/dc converter(again ),  and understand that taking away the torches 'driver curcuitry' would change things but I had no choice due to mucking up other approaches on the white torch
anyway it was fun to try out and i'll definately do it again . I may just buy a lower lumen white LED and two dc/dc converters and wire the red and white seperately . 
The challenge is to get both lights working off te main batt pack

At least I now know where I went wrong! 

Here to learn,

Andrew 

P.s  it was a lot easier with the magic shine lights, though not as bespoke

[Andrew]

Is there still a way I can still use both of these torches with one dc/dc converter?  taking into account the white 1300Lm torch has no driver curcuit and the red torch is just 150 Lm's?

Any links welcome,  advice too

[SydneyCommuter]

I thought I was bad with my 700 lum helmet spotti.
10W Led spotti, all you need is a truck horn and they'll think its a convoy commin!!
with the lounge room on the back and you sittin on the soffa with your down light reading a book?

 

[Andrew]

it must be said I think I went a bit OTT with 1300Lm

I'll give it some time and maybe just buy two cheap 200lm cree torches. But as i've already blown 2 dc/dc convertors I may gives things a rest for a while.

[SydneyCommuter]

If you ride on poorly lit roads and really need to see, then stay with your 1300. My experience with my work commute is. You need at least that much to see any crap on the road far enough ahead to avoid it. My commute in Syd u can use the breakdown lane of the highway, and they have large section with no street lights. So add rain and cars lights off/on and you need all you can get. Its the stuff that bounces off tradies utes lunch boxes bash hats an entire flouro light unit you name it. My fit out for those occasions I have 3 900single lead a ay up twin wide beam 700 and an ay up narrow spot beam on my helmet. Thinking about it now I have serious ampere hours I might start looking at bike mads 10watter.