The Right Resistor

[diroga]

i want to use a 9v batter for a power supply to a LED array. what is the right resistance to use. the LED's are going to normal red and green. they operate around 2 to 3 volts.

 

[jrz126]

Do you have a more exact forward/operating voltage? Or the normal current that the can handle?

To find your resistor, just do (9-(forward voltage))/.020 (most leds can handle 20 mA continous.
There are online calculators that will give you a standard resistor value too. Just google for LED series calculator.

 

[diroga]

ok, here is my general idea. any pointers

 

[evandude]

pick a current and use R=V/I.

if each LED needs 15mA of current, and has a forward operating voltage of 1 volt, and you have 4 in parallel, then you need 60mA of current at 1 volt. with an 9v battery, you would have R=(9v-1v)/(.06A) = 133 ohms.

for quickly estimating resistors, as a general rule of thumb, i use at least 330 ohms for one LED at 5 volts. doubling the voltage means doubling the resistance, and adding more LEDs in parallel means halving the resistance. so for 9v you'd about double the resistance to 660, then divide by 4 for the number of LEDs and get about 165 ohms.

either way, most LEDs will light just fine on 10mA or less, so you should be fine using a resistor somewhere around 200 ohms.

 

[TheOne]

Just remember to pair only the same colored led's, as the voltage drop is not the same for all colors.

 

[zachtheterrible]

if the forward voltage on the LEDs is 2-3 volts, y not just use some 2 AA or AAA batteries? they'll last waaaay longer 2.

 

[jrz126]

It isn't good to put leds in parallel with one current limiting resistor.
Due to the variance while manufacturing, some properties of the leds are different, so one LED could get more current than the rest and burn out quicker.
I'm not saying that it wont work, but if your leds burn out kinda often, this is probably why.

 

[diroga]

if the forward voltage on the LEDs is 2-3 volts, y not just use some 2 AA or AAA batteries? they'll last waaaay longer 2.

i was going to use a 9v for the convienace of it's size and no need for a larg battery clip. what i'm trying to make is a flashing light device for wearing when i run at night. i woould rather build one then pay $20 - $35 for one. of course my version of such a device will probably be twice as large and more combersome than the store bought version.

It isn't good to put leds in parallel with one current limiting resistor.
Due to the variance while manufacturing, some properties of the leds are different, so one LED could get more current than the rest and burn out quicker.
I'm not saying that it wont work, but if your leds burn out kinda often, this is probably why.

so i should put a resistor on each LED insted of having just one?

 

[Nigel Goodwin]

so i should put a resistor on each LED insted of having just one?

You should put LED's in series when you can, with a 9V battery you could probably manage four in series with a single resistor (this will minimize waste in the resistors). You shouldn't put LED's in parallel, unless they each have their own current limiting resistor, it's extremely poor practice.

 

[jrz126]

so i should put a resistor on each LED insted of having just one?

Yes, and like Nigel said, you should put them in series, then you can get away with only using 1 resistor.

I found out the hard way with needing a seperate resistor for each led in parallel. And unfortunitly my leds were mounted in the headliner of my car, so changeing them would be a pain. I have redesigned it since then and I've been running the leds for a couple months now with no problems.
I have some pics posted on page 2 of the www below this post. but it appears to be down at the moment.[/url]

 

[audioguru]

Hi Diroga,
Zach is right. AA cells last much longer than a 9V battery. Have you seen the tiny size of the 6 cells that are inside an expensive 9V battery?

I have a chaser project that blinks bright LEDs in a circular sequence. It uses 1.7V to 1.9V red LEDs and only 2 AA alkaline battery cells that last "forever". It uses a 74HC14 IC that works down to a voltage lower than the LEDs need and the LEDs are still pretty bright when the battery needs replacing (2V).
I use a diode/resistor to make a short (30ms) "on" duty-cycle for the LEDs to conserve battery power.
You could make a simple oscillator from 1 inverter of a 74HC14 like I did but dispose of its speed control. The rest of the inverters can be paralleled like mine and used to drive a transistor more powerful than mine, to drive all your LEDs. You can also dispose of the inverter that I use for PWM brightness control, and dispose of the 74HC4017 sequencer.
Using 3V to 3.5V ultra-bright LEDs and a 4-AA cells battery (the max supply voltage for the 74HCxx series is 7V), it can be seen for miles and is awesome! My schematic is here:

 

[diroga]

audioguru-

where did you get your LED's? the radio shack LED's aren't vary bright. i also found a site where i can buy UV LED's! i was going to use a transitor capasitor self trigoring thingy to cycle 180 so one let of LED's on the other off. yeah i dont know the techincal term for it.

 

[jrz126]

I get mine from ebay in 100+ quantities, most of the people that put them in their cars get them from LSDiodes.com.
I have 62 4000MCD blue leds in my roof. **broken link removed** I had a fog machine going for that pic...

I wouldnt go with the UV leds, unless you wear white clothing or something, the UV's are more reflective than bright (white stuff glows purple).

 

[audioguru]

I have been using HLMP CB15 for narrow angle and HLMP CB30 for wide angle, blue LEDs originally made by HP but now Agilent. They project a perfect pattern, their brightness keeps increasing and their prices keep dropping. I get them from Newarkinone (owned by worldwide Farnell now), they have them selected for brightness and charge less if you buy more.
I tried some cheap Indian and Chinese blue LEDs but their pattern is different for each one and brightness varies too. Frequently their colours are mixed-up. I had to test them one-by-one in the store, what a nuisance. The next guy got my rejects, or they went to Ebay.

I looked at LSDiodes site. Their LEDs' brightness are rated at max, not a guaranteed min like everyone else, and some have their brightness rated at a melting 80mA instead of 20mA like everyone else. Instead of a discount for quantity, they give freebies (stickers and stuff that I don't need).

 

[diroga]

what color would be best? blue or red? how about green or white? white is more expesive.

 

[audioguru]

The color of the LED depends on your battery voltage and the LED's voltage. Ordinary red LEDs are about 1.8V. Ordinary green LEDs are about 2.5V. Ultra-bright blue, green or white LEDs are 3V to 5V.
Someone on a post reported that he connected his LED directly to a 6V battery without a current-limiting resistor! It didn't burn out! (yet)

White LEDs shouldn't cost any more than an ultra-bright blue or green one.
The new UV LEDs cost more. I haven't seen a truly purple one yet.

 

[Juglenaut]

Diffused Leds are cheap even for blue, they really can't get to expensive for standard size and MCD. The most I have seen 20-40mcd bright translucent diffused LED go for is 20cents each.

Where is starts to get ridiculously expensive is this gasagap or something like that where the lens is clear and incredibly bright, but these LED cost just as much to make as diffused, in mass manufacturing setting. Cost varies by size, tolerance, chemicals, and process, the brightness has little to do with actual value or cost of an LED.

White is actually a LED that emits RGB at the same time, an actuall white single emitting diode is still not on the market. A purple LED would emit a mix of RB.

Unless it is a 6 volt LED that he hooked to a 6 volt battery, it still would get rather hot at the terminals.

 

[audioguru]

White is actually a LED that emits RGB at the same time, an actuall white single emitting diode is still not on the market.

Nope. A white LED is a blue one with a yellowish phosphor. They don't appear very white, right? But you can buy RGB LEDs for pixel screens and stuff. I don't think they will ever have a true white LED because white is wideband and LEDs are very narrowband.

A purple LED would emit a mix of RB.

Sorry, nope again. Purple is radiation at a frequency between blue and UV (remember learning about a rainbow?). Your eye or brain gets tricked when colours are combined. They now make LEDs operate at the extremely high frequency of UV, so purple (maybe called violet) should be easy.

 

[Oznog]

Luxeons man.
https://www.luxeonstar.com

Expensive, but they're designed for more power than you can imagine. Some take as much a 5 watts and put out the light of hundreds of normal LEDs in a pkg smaller than a pencil eraser, but require careful heatsinking since they're not a whole lot more efficient (around 15% or so) and dissipate the rest as heat. The die cannot take high temps due to poor heatsinking, it will lower the light output and degrades it over time.