LED dilemma: adding 6v types to a 3v circuit

MillenniumFalsehood · Post by **MillenniumFalsehood** » Sat Dec 10, 2005 2:38 pm

I have completed the lighting circuit for the interior of the Engineering section of the 1:2500 Enterprise E and have sealed the hull, leaving wires set in grooves on the warp engine pylons, similar to the setup for the Fiber Optic Enterprise D kit as per it's original instructions. The problem is that the new LEDs I ordered to put in the Warp Engines are rated for a minimum voltage of 6v, and the LEDs I have already installed are rated for a maximum voltage of 3v.

I didn't realize this until I tested them. The only parts not glued onto the hull are the outer engine halves and the upper saucer half. I am contemplating cutting off the top of the engineering hull to access the wires inside, but I don't know if I can work in that small a space (<1in/<3cm), and to complicate matters, I have already attatched the lower saucer, so I can't take appart the entire lower hull. Please help me! I hate the smell of burnt LEDs. . .

en'til Zog · Post by **en'til Zog** » Sat Dec 10, 2005 4:36 pm

Um.... LEDs rated at a MINIMUM of 6 volts? Very odd. Those must have built in resistors to run at 6 volts.

You could run everything at 6 volts (TEST those LEDs FIRST to make sure they'll run at 6 and not fry) and use a resistor (or bunch) to drop the 6 volts down to 3. If you're familiar with Ohms' Law, that is fairly simple. (Right.)

Lessee are the 3 volters in series or parallel? Did you put resistors in anywhere?

Best idea - send me the schematic for what you've got, and I'll try and figger out the best approach.

(Oh, worst comes to worsted - can you use LEDs other than the 6 volt ones, or are those installed? You could try and run everything at 3 volts and see how bright (dim) the 6 volt ones are.

Model ON!

MillenniumFalsehood · Post by **MillenniumFalsehood** » Sat Dec 10, 2005 5:10 pm

Yes, I already put a small resistor on the interior circuit. I'm not entirely sure how I'm going to put another one in. I have all the LEDs wired in paralelle, so the only way to put another resistor on the circuit would be to tear apart the entire model and hope that I don't rip anything. I think the new LEDs must have built-in resistors, because they won't glow on anything less than 6 volts. They glow very dimly on 3v, not enough to light the warp engines.

The idea of this circuit is very simple; I just inserted two wires into the hole in the bottom, with a single resistor attached to the positive end, then wired every LED one at a time onto those wires, adding four lengths to be run up the pylons and into the engines.

Madman Lighting · Post by **Madman Lighting** » Sat Dec 10, 2005 5:29 pm

You wired LEDs in parallel? Ugh. OK, thats a big no-no. It might be OK for the ones you've got because they sound like they have current limiting resistors built into them but generally, NEVER wire LEDs in parallel.

The reason is this: LEDs are semiconductors which means the voltage across them varies a little from part to part. If there's two in parallel, the one with the lower voltage will get more current than it should and blow eventually. Then the remainder will get 2x the current it should and blow too.

One day I'm gonna write all this stuff I know about lighting down and get it published for you guys....

-John C.

Warped Speedster · Post by **Warped Speedster** » Sun Dec 11, 2005 6:39 am

I'm not gonna scold ya Millennium cause I'm sure you're already beating yourself up for this. But you could always get some 3 volt LEDs for your engine nacelles. Superbrightleds.com has several white or blue ones that'll work good for you ship. (I think they're 3.2V) Even blinking ones. Cheap too. I just bought another bunch last month. I really wouldn't mess with the six volt bulbs if your ship's already buttoned up, it's not worth taking a chance.

>SELF BLINKING LEDs<

I don't build that much ST stuff but I mainly use self blinking LEDs for all my engine effects. Lots of em. I never use more than a three volt power supply myself and I always use bulbs rated for at least that much voltage (3V to 5V) so none have ever suffered evil if any other bulb goes out. (Which has never happened yet in the hundreds and hundreds of LEDs that I've used so far)

ALL my models have had all their blinking LEDs wired in parallel with each other. They've been burning for seven or eight years without a glitch. It's a totaly random lighting display, extremely active and almost animated. No resistors needed, and no other components other than a battery pack and a bulb, or bulbs. It's as basic as a circuit can get, looks good (hypnotic even) and I highly recommend it for beginners or "Oldginners."

>NON BLINKING LEDs<

Now, like the Madman said, for >non blinking< bulbs wired in parallel there is a slight danger of one bulb being over driven if another bulb has a slightly different current characteristic, and if they're being driven by a voltage source greater than their individual voltage rating, which is usually the case. (that was a long winded sentence) Now maybe I've been lucky but I've almost never had identically rated LEDs not work indentically the same, so series or parallel hook-ups have never created any problems. I always test them all with a volt meter anyways before permanently installing them to be sure of this.

But if this is a concern though then just use a separate resistor in series with each individual LED. Then wire all of those directly to your power source (a series-parallel circuit). It will protect your bulbs no matter what current rating they each individually have and no matter what happens to another bulb.

You can also use this same series-parallel method when using a six volt or higher power source. (re-calculate your resistor of course) But a lotta folks like to wire a group of identical bulbs in series and then calculate a single resistor for that higher voltage circuit. Both work great, I just like using big "D" size 3 volt battery packs and ~three volt bulbs~ and wiring everything in parallel.

Warped Speedster · Post by **Warped Speedster** » Sun Dec 11, 2005 7:02 am

Millennium. If you haven't already, checkout these bulbs.

http://www.superbrightleds.com/

Buy here:

http://www.superbrightleds.com/cgi-bin/ ... 451&next=0

en'til Zog · Post by **en'til Zog** » Sun Dec 11, 2005 11:13 am

Can you open the model up just enough to put an additional resistor on the 3 volt buncha LEDs? Leave the 6 volters in place, but work to increase the protection on the others? That might be your best solution (out of some not very good ones....).

Sparky · Post by **Sparky** » Tue Dec 13, 2005 3:23 pm

You still use one resistor for the 'parallel' circuit, remember that the total current used by all the LEDs in the parallel wiring will be gonig through the resistor. So you have
x number of LEDs each drawing a current I
3volts/(x*Ieach)= resistance needed
The sizes resistor (in watts) is I (total) * 3 volts. Be sure and get a big enough resistor or it will burn up/get real hot maybe even burn out the LEDs down line.