Today I did a mockup of the lights I want to put in the RH. I used this array wizard and used the suggested resister on the four lights in the series. The lights get duller the further you go along the string. I did not expect this with just four lights and am wondering why. Is the array wizard just wrong or am I doing something else wrong? I am glad I did the mockup and tested things, methinks the only way to get even light from the LEDs is to put a resister on each LED.
Today I did a mockup of the lights I want to put in the RH. I used this array wizard and used the suggested resister on the four lights in the series. The lights get duller the further you go along the string. I did not expect this with just four lights and am wondering why. Is the array wizard just wrong or am I doing something else wrong?
I am glad I did the mockup and tested things, methinks the only way to get even light from the LEDs is to put a resister on each LED.
Brent
"All of the world's problems are the result of the difference between how we think and how the world works."
Are you using a 15 volt power supply ? Seems like a rather odd voltage for a source. (?)
Mark.
¡ uʍop ǝpısdn sı ǝɹnʇɐuƃıs ʎɯ 'dlǝɥ
Hi Brent,
Too much for my brain! By the time you figure it all out you could have soldered a resistor to each LED and had the lights half installed.
Dave
I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!
Mark R. Are you using a 15 volt power supply ? Seems like a rather odd voltage for a source. (?) Mark.
Yes, it is an old adapter from something. I save them all when I throw the devices out. The meter shows 15.4v when I tested it. Close to a DC transformer.
hon30critter Hi Brent, Too much for my brain! By the time you figure it all out you could have soldered a resistor to each LED and had the lights half installed. Dave
It's a good thing I order a smorgasbord of resisters from Digi-Key, I am ready to go that route. Instead of wires going everywhere I have used copper tape along the roof support beams, I then only have the #38 magnet wire hanging down with the Ngineering light shades from those beams. It actually looks really good, really clean. Once the soldering is all done I will paint the copper tape and you will never see it. It is easy to solder the SMD resistors to the copper tape just bridging a little nick with the blade.
BATMANIt is easy to solder the SMD resistors to the copper tape just bridging a little nick with the blade.
I'm not quite sure that I understand what you mean by that. Can you elaborate please?
If it is a wall wart, they usually measure higher when not loaded at the rated current. Look at the rating on the pack. Many times these packs are not regulated. I have seen that many times and others here have noted that. Just a thought.
I normally use 12 vdc. to maybe slightly higher. Many times I have made LM317 voltage regulators and trimmed them to about 12 vdc with a trim pot. If I used a couple fixed resistors, they might be around 12.5 vdc or so, not a big deal. Depends on what I had in my stock of fixed resistors.
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
BATMANThe lights get duller the further you go along the string.
do you mean that within each group of 4 LEDs in series, the LEDs get duller further from the +15V , or that an entire group of 4 LEDs and resistor is duller further from the +15V?
i find it hard to believe that the LEDs in series would have different intensity since the same current passes thru them all.
in the later case, is the voltage across the group, measured at the LED connectde to +15v and the resistor lead connected presumably to the ground of the +15V supply (not shown), the same for all groups? If the value isn't the same across each group, is there a drop between the +15V and the LED?
greg - Philadelphia & Reading / Reading
The four lights in the series (each individual light) get duller as they get away from the resistor, they have all come off the same spool so they are the same.
Here is the copper tape with the lights hanging down, once painted you won't see the tape and it will look pretty good IMO. Forgive the soldering mess as I have had many things on and off as I have been experimenting.
Here you can see the resistor, I was playing around with the location of it, thus the mess.
Now if I can only get it working with my limited knowledge of such things.
so the LEDs in series have different intensity even though they are in series and therefore the same current.
can you verify the voltage across each LED and then in pairs to make sure the pair voltage is the sum of the individual voltages?
can you swap the LEDs nearest and further from the resistor to see if the LED furthest from the resistor is dullest?
Thanks Greg, I'll check those things out, is it possible the copper tape can't carry enough juice?
Mel, I tested each LED with the meter touching the copper tape on each side as I soldered them on and they all were bright. I have a slit in the tape and the magnet wire from the light is connected on each side of the slit.
BATMANMel, I tested each LED with the meter touching the copper tape on each side as I soldered them on and they all were bright. I have a slit in the tape and the magnet wire from the light is connected on each side of the slit.
what do you mean "each side" and "slit in the tape"? suggests you're creating two conductors with the copper tape and wiring the LEDs in parallel.
there should be a single conductor between each LED and the LED and resistor. If you remove any one LED or resistor, there should be no current flow and nothing should light.
LEDs in parrellel can have different intensities
if the copper tape has some resistance, each piece contributes to the total resistance and affect the current that should be common to all LEDs
thanks Mel, hadn't read it that way.
in that case, simply "slitting" the tape may not completely separate the two ends. Any partial contact may provide a higher resistance path for some current, which could explain the problem except for the part that the LEDs are dimmer further from the resistor.
it would be better to cut the copper tape twice and remove a section
Haven't had a chance to get to the trainroom yet but Mel is correct in how I am hooking them up and I did make two slits and removed the piece between the slits. Thanks for your help so far guys.
Edit; if you click on the pic to enlarge it you can see the gaps.
RR_MelAll the LEDs from the same batch should draw the same current or be very close.
if wired in series, the same current flows thru all components in the path.
Okay, I finally made it to the trainroom, with the probe either side of each individual LED on the copper tape, here's the poop. Left to right, brightest light next to the resistor.
Light #1 (the brightest) 3.28v
Light#2 (the second brightest) 3.55v
Light #3 (now the fourth brightest) 2.95v
Light #4 (now the third brightest)2.62vW
Do you think it could have something to do with my crappy soldering? They were my first efforts, the bright light was much later in production. I did get much better after about twenty of them.
Could be - either overheating them, or a bad joint resulting in an additional resistence.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
As Mel suggested, there shouldn't be that much variance in the voltage across the LEDs. I suspect that the LEDs are themselves the culprit. Where did you get them?
I have two suggestions:
1. Buy some new LEDs from a source that you can trust. 'Wehonest' on eBay usually receives favourable comments on the forums.
2. Forget wiring the LEDs four in series with one resistor. Use a resistor for each LED. Tried and true. Sorry to repeat myself, but if you are like me and don't understand the intricacies of electronics, then do what works.
rrinker Could be - either overheating them, or a bad joint resulting in an additional resistence. --Randy
I think the overheating might have been an issue as I was having trouble with the early ones. When I switched from liquid flux to a paste, the paste helped hold the magnet wire in place and I was in and out fast.
What if there is some residual paste on the LED causing a shorting issue, is that possible?
I think I am going to hang some of my later efforts and see if that solves the problem.
What temp should I use to solder the resistors to the copper tape? It seems to take longer for the solder to bond with the resistor than the copper tape even though I am tinning the resistor.