HELP! DIY moonlights not working!

[Tripoli]

OK, I tried to build my own moonlights...got it all put together, plugged it in......and nothing happened.

Here's what I did:

I've got a DC9V 500mA AC adapter....I connected a 30 ohm resistor to the positive side (the one without a colored stripe). I then connected the positive end of five 4.5V 30 mA LEDs to the resistor with wires, and the negative end of the LEDs with wires to the negative side of the AC adapter.

Is there something I'm doing wrong? If not, how can I track down the problem?

 

[MarineFlake]

Use a VOM (volt ohm meter) to check the LED, remember, they are nothing more than a diode that emits light so current should only flow in one direction. You could rig up a battery to make sure the LEDs are ok as well. I think a C or AA is close to 4V. The formula for resistance, voltage, current eludes me atm. You could also try running just a pair in parallel. Also, make sure your getting 9V DC from the supply and that the polarity is not reversed..GL.
-Paul C.

 

[24 hours? NE!]

I can't remember, but it sounds like your numbers are all good. Did you test the first led in the chain before you soldered up the rest? Try switching the leads from the adapter.

 

[JProx]

Here's the basic formula's for ohm's law.
v = i x r
r = v/i
i = v/r

i just have some basic questions..
how are the led's arranged ? are they in series or parallel ?
from the sound of what you did, you might have them in series. if they are in series then you don't have enough voltage present to lite up the leds or even get them into saturation (a fancy term, that means they turn on, or lite up). in series voltage changes per component, and current is deadlocked, so what that means is you will need to have 4v for every led you're using. however if you arrange the circuit in parallel, you will have all the voltage you require, in parallel voltage is deadlocked in the circuit, while current changes per componet.

depending on how they are hooked up in the circuit, one led might have been doa, or burned up on assembly (led's can only take a few seconds of extreme heat, from a soldering iron before they melt internally), the best way to figure it out is use a 2 AA batteries, and try out each led. i am assuming though you have blue leds, probably in the 4000mcd range, they go into saturation around 3.5v and top out at 4.3v so you need to get your supply voltage in the range, or pretty close to it, anything manufactured has a tolerance, and are never face value or follow the theoretical values. (resistors are a great example, the 4th color band is for tolerance, gold = +/- 5% ; silver = +/- 10% ; no 4th color band = +/- 20%)

so here's what i suggest you do,
1. make sure your dc transformer works
2. make sure you have the dc transformer hooked up right, if it happens to be a recycled dc transformer (from a cd player for example) the wire that has a white band running down the length of the cord, is the neutral portion (or ground).
3. if the circuit is still a no go, rig up 2 AA batteries and try out each led, like pc suggested, if they are fresh akl. batteries you should be able to squeeze by with 2, if they been used, you might want bump it up 3 AA batteries (1.5 x 3 = 4.5v)
4. if all the led's lite up with the battery, rebuild the circuit, then get access to a voltmeter (or multimeter) and run through the circuit, checking the voltage from the transformer then to the rest of the circuit. if can you spring for it.. HD has multimeters for 20, and walmart has them for 10-15. both stores have great return policies..

i am electronics major, so if you need any help let me know, i can run you through the basics, and give you some ideas how to build this circuit, provide wiring diagrams, ect ect. just let me know.

bry

 

[Tripoli]

What wattage resistor whould I use? I've got a 1/4 watt 30 ohm one on there now.

 

[JProx]

well the less commonly used formula from ohm's law..
P = watts = V x I
i am sorry for not catching this last night, i didn't do any math for this problem but your resistor pick is wrong.

let's do some math!

dc source = 9v 500mA
led forward voltage requirement = 3.7 - 4.2v, 20ma (Max)

So 9v(total) – 4v(led requirement) = 5volts
R= 5volts / 20mA (LED max Amperage) = 250 Ohms
P= v x I = 5 volts x 20ma = 100mW
so.. 1/4 = 250mW so 1/4 watt is exactly what you want for this circuit, however you will need to replace the resistors you have, with 250ohms. how exactly did you figure out, you would want to use 30ohms ?

anyway since the leds were getting alot more than 4 volts and more than 20mA of current, the led's you have are probably toast (5v / 30ohm = 166mA, most led's can only take 20mA before they burn up). so it looks like you will need to replace them, if i may suggest a vendor.. go to www.lsdiodes.com they have blue led's @ 4000mcd for 45cents a piece, with 2dollar shipping (untill you spend more than 20bucks). if you want some more help, or wiring diagrams, or more explantions plz email ijedi@hotmail.com

just for reference, led's, diodes, some types of capacitors, are polarity specific. they will only work in one fashion, the longer leg on an led, is the anode (positive side) the shorter leg is the cathode (negitive). the cathode leg needs to be put on the ground (netural) wire of the power source. with out getting completely technical, if you don't install the led's in the correct fashion they can be destroyed. resistors though have no polarity, and can be installed in any fashion.

hope that helps..
bry

 

[Tripoli]

Turns out the polarity of the adaptor was just backwards. Stupid noobie electronics mistake. My 30 ohm resistor seems fine and everything's working now.

 

[Tripoli]

Well, after plugging it in and letting it run for about 5 minutes, the resistor gets so hot it nearly burned me. Any idea why and what I can do about it?


edit:
I think I have an idea....
I used a 30 watt resister because I'm using 1 resistor for all 5 LEDs...if it were 1 resistor for each LED it would be 150-180 ohms.

I did some more math of my own and discovered that with the multiple-resister setup I needed 1/4 watt resisters.....but with my single-resistor setup, I need a resistor that's ~ 1.125 watts.

So, I believe my problem is solved. I'll head to the electronics shop tomorrow morning and grab a different 30 ohm resister and see how it works. 8)

 

[JProx]

P= v x I = 5 volts x 20ma = 100mW
so.. 1/4 = 250mW so 1/4 watt is exactly what you want for this circuit,

heat = power = wattage. if the resistors you have are hot enough to burn you, or start to change color, then you don't have a high enough wattage. there is only 2 fixes for that problem
1) replace the power source with a lower voltage (extreme i know..)
2) replace the resistors you have, with ones with a larger wattage rating. Again use the formula for power p = V x I then grab a resistor with a rating above what you calculated. (if wattage calced. is 270mW then get 1/2 watt resistor, or safer yet 1watt)

and sorry.. i was a bit confused on how you wired the circuit (parallel or series) and over looked some keywords. 5v / 30ohm = 166mA / 5 # of leds in circuit (i am guessing parallel now) = 33mA, so you're practically within their max rating, personally though i would use a higher ohm rating, just to reduce the current and get under the max value (anything over max either burns it up in a few seconds/minutes of operation, or shortens life span of the component, led's are put into saturation by voltage, not current so lowering the current will not reduce their lighting output)

using a current limiting resistor (1 resistor in series with the load) is about the most efficient way to build the circuit, and the best way to build an led driver, when all the led's are the same. however power generated becomes a problem (as you learned) since radio shack sells resistors in a 5pack for 99cents.. it doesn't hurt to give each led their own resistor, just to avoid the heat/power problems.

personally though, i have built my moonlight with a resistor on each led, just to make the circuit a bit safer, and less prone to break, resistors can be killed running at hi temps for extended periods of time. instead of just breaking like an led, or an IC, or the toys we get at x-mas, a resistor can start to leak. Leaking (nothing to do with liquid or releasing anything) as in losing resistance, running at hi temps for a long time, can make the resistor burn up internally, lowering it's resistance thus letting more current and voltage flow into the led, and eventually killing the led. how likely is that to happen though? with a higher watt resistor, unlikely to happen. but just thought i would pass on extra knowledge and get on tangent.

bry

 

[Tripoli]

Well, I put in a 1 watt resistor and it's better, but it's still mightly hot. Plenty hot to burn me in 1-2 seconds. I'm trying leaving it plugged in for a while to see if the resistor starts to melt, but I'm starting to see the wisdom of using one resistor per LED. Would that make this much less of an issue?

 

[JProx]

Yes, it will completely remove this problem, of heat disipaction. resistors don't melt like ice cream, but they will burn up the carbon inside of them, and throw off their resistance value.

 

[Tripoli]

Well, I let them run for about 2 hours and everything seemed ok, except that the resister stayed pretty hot.

I've decided I'd like to get different LEDs, though. The 5 I've got have a 60-degree dispersion and even when it's pitch black in the room, they barely give off any noticable light in my 10-gallon tank. From the pictures I've seen on these forums, most folks' moonlighting is substantially brighter, and they use the super blue LEDs that are more focused (I think 15-degrees).

So, now I need to find somewhere that sells them (I think Fry's might), and then do some more calculations to see what resisters I need. I think I'll go with the 1 resister per LED setup this time.

 

[JProx]

lol i am glad i finally won you over. the ones i have used have a 30degree angle, and are quite nice, and generate a good amount of lite in the tank. if you have a paypal account/credit card i suggest you get the next set of led's from www.lsdiodes.com they are Probably cheaper than frys (in ohio we have no such store, but according to the forums on fatwallet they rock) and they have 30degree lighting angles, and are 45cents a peice, they do sell resistors (in limited choices though) so you could get everything you need from one place, if that interests you.

 

[dralarms]

you can get some good one's at lebos.com 4000 mcd. bright enought to make a difference and only 95 cents.

 

[JProx]

the ones from lsdiodes, are 4000mcd too, and at 45cents a piece beat lebos.com

 

[Tripoli]

Well, I may just take a set off Isdiodes...those seem to be what I'm looking for....

I noticed the LEDs have Min, Typ, and Max specifications....which should I base my resister choice on? If I use the max values, my math says I should get 130 ohm 1/4 watt resistors. Does it hurt to use a higher wattage resistor than is needed?

And on another note....just how bright SHOULD my moonlights be?? This is just a little 10-gallon tank, so with focused LEDs I'm concerned about getting the light spread out enough. That's why I started with five 60-degree LEDs. With these brighter, more focued LEDs, I was thinking of turning the LEDs upside down and aiming them into my reflecters, to help spread the light out better.

 

[JProx]

ok first things first..

it's completely fine to use a higher wattage resistor, the higher the wattage, means they can take more of a beating. you should almost never base resistor choices on max anything (voltage, amps, etc) you should use the middle of the road to calculate stuff. so if max is 30, and min is 20, use 25. that way if something goes wrong you have a safe zone of operation.

the brightness is really user specific, the best way to do this.. is get the brightest leds you can, then use another method of toning them down to your liking. there is a few ways to do that. you could build the circuit as is, then slightly sand the leds, to get a frosted effect. however some people say that's a bad idea because you have cause deep scratches, that reflect the light back into the led and severely limit it's lighting ability. another way to diffuse the light, is coat the led in aquarium sealant. that will diffuse the lite and alot of people prefer doing that, opposed to other methods. next you could take a trip up to hd and get some lighting diffusion planes. it's basically a plastic sheet that diffuses light, you see them alot in commerical buildings that use florescent lighting, alot of people also prefer that method, since it really dissipates the light evenly through the tank and doesn't require you to physcially change the led.

my personal opinion though is use to the plastic lighting planes from hd, i think they sold for 4-5bucks and you get a huge pain, 2' x 4'. you cut them to fit your application and they do the best job of diffusing the light.

 

[Tripoli]

Hmm...the plastic lighting planes sound interesting....

You think just cutting a very small piece (say, 1" square) and mounting it in front of each LED would do the trick?

 

[JProx]

that's exactly what you need to do. just make sure you either use a sharp utility knife, or a sharp pair of scissors.

 

[Tripoli]

Almost starting to sound like I need to build an entire "sub-fixture" within one of my fixtures. Maybe get a 1/2" diameter piece of PVC cut in half with the LEDs mounted in it and the reflector covering the open side. That'd be cool.

Too bad it's such a pain to find LEDs for sale locally....and they're always like $1.50 each. :|