Homebrew Bike Lights - probs fitting the lens?

[nigelb]

Hi

I've got some CREE 7090 led's and the lenses to fit.

Look absolutely superb when you get them, but when you solder the wires onto the led's the lens won't sit flat any more.

So, my question is, do I use some very fine wire (and suffer the voltage drop), or stick the lens in place slightly higher off the pcb (and risk not getting the right beam)?

Any ideas welcome

Nige

ps, boy are these things bright!

 

[gbb]

Same problem here Nigel...
You're not using these are you ?...
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=6452182

I nibbled off the tabs at the base with a pair of pliers and a file where the tabs sit over the solder points, then bonded it with plenty of araldite to the heatsink. I roughed up the heatsink surface to get a good key for the araldite. I even ran plenty of araldite up onto the side of the lens.
I was concerned that it may not like the extreme heat generated by the Crees, but no problems encountered all last winter. Still going now.

 

[nigelb]

Cool, think I'll use a Dremel to make small holes where the wire attaches, to the lens will sit flat on the pcb, then stick the lens in place.

Out of interest, how much power are you putting through the leds, how many are you using, what battery packs, and what heat sink?

Nige

 

[gbb]

• Cool, think I'll use a Dremel to make small holes where the wire attaches, to the lens will sit flat on the pcb, then stick the lens in place.
  
  Out of interest, how much power are you putting through the leds, how many are you using, what battery packs, and what heat sink?
  
  Nige

  
  I've done three with different LEDs, but i think the last one was a double Cree MC-E, around 430 lumens per LED...cant remember if they're 3 or 5 Watt each.
  Heat sink, i have access to loads of old electronic equipment so i just butcher something to suit, old PC heatsinks can do as well. Here's the last one i made, cut and filed to suit the housing...

http://i146.photobucket.com/albums/r269/gbb_photo/cree/DSC02145.jpg
(bloomin picture insert buttons not working)

Battery ?...an Enix 15v Li-on 2.2ahr. Quite expensive, but really quite compact. Easily fits in a small saddlebag..and i had to buy a proper charger of course...none of its cheap

 

[the_mikey]

I've always wondered about current limiting on these super bright leds, must need one mighty resistor. I took apart a cheap chinese made lamp I had and found there was only a 100 ohm 1/4 watt resistor supporting a 1w LED with a 4.5v supply (I don't know the forward bias voltage), but even some quick calculating with a few common forward bias voltages thrown at it suggests my cheap lamp will be toast sooner or later!

 

[gbb]

I've always wondered about current limiting on these super bright leds, must need one mighty resistor. I took apart a cheap chinese made lamp I had and found there was only a 100 ohm 1/4 watt resistor supporting a 1w LED with a 4.5v supply (I don't know the forward bias voltage), but even some quick calculating with a few common forward bias voltages thrown at it suggests my cheap lamp will be toast sooner or later!

You need a DC/DC LED driver mikey...moderately cheap on ebay etc. The current is fixed then, there are some that have selectable currents (usually 350ma/700ma, even 1000ma).
I use a PCB mounted version i just happen to have access to, 350ma (the lowest usually available) and they're still very bright, very compact too. Mines about 20x10x10mm.

You'd usually use something like this..(although there are some coin sized PCBs usually about)
http://cgi.ebay.co.uk/Power-LED-Dri...isure_Cycling_Bike_Lights&hash=item3a5f793209

Input voltage is irrelevent of course as long as its higher than the LEDs VF (up to the drivers rated maximum of course), so in my case, 2x 3.75v LEDs running at 350ma, powered by a 15v battery = loads and loads of run time.

 

[the_mikey]

Thanks for that, after reading it looks like the driver does all the work for you pretty much, it also seems to ensure the LED is driven with a constant power output, so as your battery supply reduces the led output does not suffer, where with my 1W led and resistor combination, the LED progressively becomes dim as the battery drains. Sorry for this post, it's just I didn't know about these 'driver' circuits until now...

 

[Coco]

You'd usually use something like this..(although there are some coin sized PCBs usually about)
http://cgi.ebay.co.u...=item3a5f793209

+1 for these. Used them in all my lights. Easy to use and work well. And so tiny...

 

[gbb]

Yes indeed, the driver is running the LED at a fixed current...its the current thats important. Most are very easy to install, a +V and -V from the battery, and a + and - feed to the LED...simples...although i have been known to blow a driver wiring it up wrong

 

[nigelb]

Yes indeed, the driver is running the LED at a fixed current...its the current thats important. Most are very easy to install, a +V and -V from the battery, and a + and - feed to the LED...simples...although i have been known to blow a driver wiring it up wrong

In fact, I'm using an LM317 and a couple of resistors, does the business.

Downside is efficiency (and heat dissipation) .....

If say my led's are drawing 500mA, and dropping 3.5V each, that means together they're dropping 7V.
Now, if you're running from a 12V battery, that means the regulator is having to drop 5V (12V - 7V) at 500mA, which is about 2.5W.

The device is small, its rated to 150 deg C, but personally I absolutely do not want something that hot flapping about in the dark, near my hands, melting holes through my gloves!

For me, the answer is to keep the voltage dropped by the regulator to a minimum, and bolt the regulator to something solid (heated handlebars anyone?).

However, I have now ordered a couple of the buck-mode regulators, hopefully they'll keep cooler as they're burning less power themselves (and should thus be more efficient).

Nige