Question on LED's ?? again :-(

[jeffrey j]

Hey guys,
This is what I need to ask someone, I picked up several small key chains that have 2 small 3Volt lithium batts in them with one LED (batterys are stacked on each other). When I check these with my meter it reads 6V's, like it should, right ! What I want to know is can I take these same LED's (using 3 of them) and use a 6V 4amp hr. gel cell type battery on them. I did try one and they seem to work ok, but will they last using the larger capacity battery? or should there be some kind of resistor on them?? Thanks alot for any info,, I am electrically challenged in most , if not all respects! Jeff Jones sc#258

[wayne frey]

Jeff,
A good point. And

Jeff,
A good point. And it is something coming up on my mind. I want to put lights on most of my hulls to be done. Leds vs wheat lights, and the question of capacitors is a good one. And, should you run an led rated for higher voltage, so it will not burn too bright, so as to stay in scale? How about a brief debate of information on those that have been there, done that?
I am also aware of some leds coming "ready" with the mentioned capacitor built in, yes?
Good questions. Would love to see a lot of input here, because I sure dont't know, but would like to.
Mr Merrrimen, ideas? Anyone?

[JWLaRue]

Hi Jeff,

......hooked in series or

Hi Jeff,

......hooked in series or parallel?

The amp-hr rating of the two lithium buttton cells will be a good deal less than the 4 amp-hr gel cell that you noted.....so I'd expect them to be able to run significantly longer.

-Jeff

[KevinMC]

Batter up!

Jeff (Jones)- I'm not

Batter up!

Jeff (Jones)- I'm not honestly sure how it is that those keylights with the lithium cells don't burn out LEDs without having a series resistor. I think it may be a combination of 1.) internal battery resistance, 2.) output voltage that's close to the Vf of the LED, and (as you suggest) 3.) the LED itself may have an internal resistor.

The capacity of the battery you're using won't adversely affect the life of the LED. Having said that if the gel cell has a lower internal resistance than the lithium cell you're replacing, the resulting higher current could shorten it's life. Is the brightness of the LED under "lithium power" comparable to the output on the gel cell?

From all the playing around I've done with LEDs (which only amounts to a half dozen LEDs on my first sub) I've found that the LEDs are way to bright when run at anything close to their rated current. The solution is to add an external resistor in series with the LED to dim it down to a "pleasing" level. Unfortunately finding the right resistor value for a "pleasing light output" can only be found by trial and error. See if you can come by an assortment of resistor values between 100 ohms and 4.7k ohms you should be set.

Wayne- Capacitors won't limit the flow of current through an LED- you need a resistor for that.

Jeff (LaRue)- Although not very efficient, you get the best control over LEDs when they each have their own (series) current limiting resistor, then wiring each LED/resistor set in parallel.

KMc

[jeffrey j]

...

Thanks guys,
As usuall I came to the right place for help! I think I will just play around with these things and see what happens. I might even take a spare and hook it to the larger batt. , let her run just to see how long it will last. The brightness of the LED was about the same with both battery set ups. Thanks again, you guys gave me some good ideas to look at this weekend. If they end up on the brighter side, that would be ok. Will keep you posted. Thanks Jeff Jones

[JWLaRue]

Kevin,

Yup.....the LEDs on my Skipjack

Kevin,

Yup.....the LEDs on my Skipjack are wired in series using resistors.

-Jeff

[tknofile]

Hi guys,

all of the

Hi guys,

all of the info up above is correct and if I may add one more thing... If you are trying to get that perfect glow and are trying to select the right resistor, you should put a potentiometer in-line (in series) with the LED. this way you can dial in exactly the brightness you desire. Any physical size pot would work but for space constraints, use the smallest one you can find in the right range. The VALUE of the pot is sort of critical. If you use a huge-value pot, say, 1 mega-ohm then the portion of the turn that will let current flow thru will be very tiny and you won't be able to adjust very well.
Most LED's run off of a circuits TTL logic level voltage (5 volts). Most of us use 6 volts and up. This is totally fine to do. There is an equation called ohm's law that relates the voltage, resistance and current flow.

V = I*R

Voltage = current x resistance

Voltage is in volts (not millivolts) i.e. 6 volts
Current is in amps (mot milliamps) i.e. .020 amps (same as 20 milliamps)
Resistance is in ohms(not kiloOhms) i.e. 330 ohms

Most LED's run fine up to 40 milliamps (that I can remember) but are rated somewhere around 20-30 milliamps for maximum light output. To achieve this desired current output and to determine the right resistor to use, simply use the above equation. 6=(.030)*R
R ends up being 200 ohms. To me, this seems a little low. Typically, I use a 330 ohm 1/8th watt resistor when running from 5 volts. I think you're supposed to subtract the Vf of the LED from the battery voltage to be more exact, but I never do. Mostly because the LED's I use are second hand and I don't know what that value is. I guess you could measure it.

Anyway, I'm rambling, sorry. The point is that the value of the pot should be around 1,ooo ohms, or 1K. This is what I look for in the smallest physical package. Trim pots are really small and work fine for this. A safeguard for this technique is to have some minimum resistance already in line with the LED in case you dial the pot down to zero. This will save your LED. I'd put a 150 ohm in line for this. With a relatively small resistance value of 1k, you can adjust the current pretty precisely. When you get what you like, you can remove the pot (without turning it) and measure the value and replace it with a single resistor. I could see a use for two settings, one for daytime and one for night running. These two separate resistors could be selected with a hidden switch or some other internal electronics. Anyway, hope this helps..

-Rick
ps. last note about pots and I'll shut up.... Two electrical types to choose from: linear-taper are good, audio-taper are bad.
Two physical types that get confused with these. A rotary pot is what you have all over your transmitter(sticks and dials and sliders). A linear pot is usually rectangular in shape with a tab sticking up out of a slot. These are found on mixing boards in music studios and such. Use a rotary or a smaller version, a trim pot.

[bob the builder]

For those of you who

For those of you who really don't want to do the math and are running 12V subs (like me! ), Walmart here carries 12V Leds in white, red, green, and blue. They come with built in resistors to run directly with 12V. A little pricey, but saves on hassle for those of you who don't have the time or energy to play with getting resistors right.

Saves on the calculations.


Bob