[Madame Sprocket]

So,,I am trying to wire up two 5mm UV LEDS to backlight my vasaline marble monster eyes. I have done LED's before with no issue but for some reason these little devils appear more tempermental. I think the heat is getting to them during soldering since they work just fine temporarily wired with a 330Ohm resistor in place. I usually use half a dozen pennies clamped around the stem of the LED as a heat sink and try to be quick about heating the connection but its no use...they are done for by the time I get a decent bead. Anyone have any ideas where I am going wrong? These are 380um UV LEDs 3.5-4.5 VDC wired in series and I am running them off a 9v battery with a 330ohm resistor (until I install them that is).

 

[DexterSinister]

I can't speak to the LEDs other than they may be particularly heat sensitive. If you can't resolve that, you could just use heat shrink tubing to attach the leads - that's usually pretty secure.

Also, just noticed you're in my neck of the woods... Howdy

 

[Hooked_on_Scares]

Some thoughts...

Are you using the same circuit for testing, as you are after you solder the LEDs together? The polarity on one or both could be reversed. If they're in series, and only one is reversed, neither will light no matter what you do.

The forward voltage you call out (3.5-4.5v) would potentially prevent you from being able to light them with 9V. The current-driving voltage you have to play with will be...
9V (supply) - Vf * 2 (since you have two in series)

If Vf ends up being 4.5V, you'd be out of voltage headroom, and wouldn't be able to drive any current. If I remember right, though, typical on these is 3.5V, so this would give you about 6mA, which should light the LEDs. I think this is probably not the issue.

If you're having trouble getting the solder to wet the leads, try getting some flux on the leads first. I can't be sure it's never happened, but I can't remember ever having fried anything by soldering it too long. I know it's possible, but it seems to me less likely than a wiring issue.

- Hook

 

[hedg12]

I've never burned up a LED by soldering it, but anything's possible. What wattage soldering iron are you using, & about how long are you holding it on the lead?
I tend to agree with Hook - check your polarity.

 

[DaveintheGrave]

I've solderered probably over a hundred LED circuits together and I've never burned one out yet. And I use a 30 amp soldering iron.
You're doing them the same way I do--with a 330 ohm resistor and a 9-volt battery.

Are you sure you have the polarity correct? The negative lead of the battery should go to a short leg on one of the LEDs, then I solder the resistor to the positive (long) leg of the same LED, then other end of resistor to the negative leg of the second LED, then positive battery wire to the positive (long) leg of the second LED.
I've bought LEDs from many different sources over the years and never had a problem doing it this way.

 

[Madame Sprocket]

Thanks for all the great feedback. I don't think I am going to figure out what went wrong with the first attempt. I *thought* I had double checked all variables, polarity, resistance, voltage etc. but you never know what you miss because well, you miss it. I sat down and basically did exactly the same procedure with circuit 2 , except I added material to the heat sink, and the darn thing didn't work Again. Same issue, worked fine during test hook up..same circuit soldered gave me nothing. But, 40 min later as I am sitting there mindlessly touching the contacts to the battery and thinking of other options lo' the darn thing came to life. It's seems to be a good connection too because I surely did test it by cramming it repeatedly into the sculp. Who knows, maybe the first circuit would have worked had I been patient enough. I have no explanation for what was going on.

 

[CreepyCreations]

I think you're cooking the LEDs with too much current. Do you know what the LEDs are rated for voltage-wise? You may want to try using a larger resisitor.
The calculation is fairly simple:
I = Current the LED typically needs (usually 20 mA or so)
Vd = Voltage dropped across the LED (usually in the 2 volt range, but can vary
Vs = Voltage source (in your case, 9 V)
R = (Vs-Vd)/I
So, in my example, (9-2)/0.02 = 350 ohms, which would be about right (330 is close enough). But if you have the LEDs wired together in parallel or in series, everything changes. In series, add the Vd for both LEDs. In parallel, add the I for LED. Then, do your calculation.

 

[CreepyCreations]

Being UV LEDs, how are you testing them?

 

[dariusobells]

sounds like a connection problem some where. if your leads are clean and you are using sufficient flux the solder joint should flow very quickly.

I don't use heat sinks when I am soldering up the leads on arrays and (besides the occasional flipped polarity) I don't have any problems. I can't say I am any where as good with electronic formulas as CreepyCreations, I use this web site - plug in the numbers and it spits out wiring diagrams and the resistor sizes needed. it hasn't failed me yet. http://led.linear1.org/led.wiz

 

[DaveintheGrave]

Maybe cold solder joints?? Seems strange to have a delayed reaction like that.

Could they be haunted??? I love UV LEDS, by the way. So many things you can do with them.

 

[Xane]

I just use alligator clips as heat sinks. Or better yet, the alligator clips on an "helping hands" stand like this:

http://www.harborfreight.com/helping-hands-319.html

I've never had this problem even though it should have happened since I'm so incredibly bad at soldering things that I'll heat up the entire helping hands in the process of trying to solder one arm. Maybe you have bad resistors? Some variable is off. I would try wiring 2 AA batteries together or even just a button cell, and use the ends to try and light it past the soldered area (3v should still work). If it lights up, then you know something else is wrong.

 

[The_Caretaker]

I've soldered over 100 UV leds with no issue, what type of soldering iron are you using

 

[dariusobells]

Perhaps you are using to much of a heat sink. you may be cooking your leads trying to get them hot enough for solder to flow and introducing carbon to the joint.

 

[Madame Sprocket]

Ha! haunted LEDs would likely improve my prop. Not sure what happened but I suppose this is the same reason you don't want to buy a car with electrical issues. On the plus side after I positioned the lights to the side of the marble eyes the effect was worth the overall irritation. I will try to get a photo up soon. I think I am also going to dig up the fried LEDs and see if they are actually fried...if not then the plot thickens...