LEDs

[zachtheterrible]

While i was waitin around to get started on another project, i figured that id get out my LEDs and make me a nifty little flashlight. I'm having trouble with some very simple math though.

What I want to know is how to calculate the voltage drop across a resistor. The LEDs have an absolute rating of 4 volts, but a typical voltage is 3.5. I want to power them from a 9 volt battery, but cannot figure out for the life of me how to calculate the right value.

btw, Is it alright to push them to the 4 volt limit? or will it shorten the life?

thanx :lol:

 

[Someone Electro]

You must aslo know the curent rating.

I think the eqvasion is:

(BateryVoltege - LEDVoltege) / LEDCurent = Resistance

If you have a lot of high power leds you have to have an big rasistor(1W or somting).
My 1/4 W 150 Ohm rasistor got wery wharm after driving my 10000 Mcd LED

If you drive to much in a led it will start to heat it self an this wod continue until the led overheats.So if you spot a led geting warm disconecti it and use a higher value resistor

 

[zachtheterrible]

(BateryVoltege - LEDVoltege) / LEDCurent = Resistance

The thing is, i dont know how 2 find the LED voltage either, grrrr. the current is easy. And do u do mean battery voltage minus LEDvoltage, right?

Does an LED act just like a resistor basically? Like can you measure the resistance?

I've 3 13000 mcd green LEDs. i think that theyll make 4 a very bright flashlight :lol:

2 1/4 watts in parrallel would be the same as 1 1/2 watt id imagine?

Thanx :lol:

 

[plot]

here's a nice little LED calculator thing that i've seen passed around a lot of computer MOD sites, it should be pretty helpfull.

http://metku.net/index.html?sect=view&n=1&path=mods/ledcalc/index_eng

As for the LED voltage and forward current, they'll both be found on the data sheet.


LED's are diodes, not resistors :wink:

And I'd just go ahead and get 1/2 watt resistors, no point in chancing it.

 

[Someone Electro]

Remember to duble the resistance if you use 2 resistor in paralel.

For exsample:
Two 200 ohm 1/4 W resistor in paralel is the same as 100 ohm 1/2 W
So you need to conect two 400 ohm resistor to keep the 200 ohm

LEDs are semiconductors and semiconductors change resistance on varius properys (Curent,voltege...).If you mesure the resistance of an LED you have to set your multimeter to diode test (or it will show extremly high resistance).But mesiring a LEDs resistance will only check if the LED is ok (my multimeter shows around 700 ohm on working diodes)

 

[zachtheterrible]

Remember to duble the resistance if you use 2 resistor in paralel.

duh!! how could i forget that!?!?! thanx electro, u just may have saved me a lot of pain n anguish

Thx 4 the calculator plot, it works great. When it asks for "voltage dropped across LED", how do I find that? It's not the same as continous forward voltage on the data sheet, is it? Continous forward voltage means how much voltage the pwr supply is, right?

The desired current is 30ma on the data sheet, so i already know what 2 put in 4 "desired forward current".

Obviously i want this flashlight to be as bright as it can, so can I push it 2 4 volts without gradually damaging it? It says on the data sheet that max forward voltage is 4 volts, but typical is 3.5 volts.

 

[Someone Electro]

Why dont you post an link to the datasheet or the datashet in pdf.

The LEDVoltege is supose to be the LEDs Forvard Voltege and the LEDCurent is supose to be the LEDs normal curent.I wod not risk LEDs like that so dont push the LEDs to the max

I burned one of my orange super bright LEDs (10000 Mcd at 20 mA) becose i was uncreful

 

[zachtheterrible]

http://superbrightleds.com/specs/g5_specs.htm
Thats the green one.

Chek out this yellow one. Its 10000 mcd, but it seems quite a bit brighter.
http://superbrightleds.com/specs/y3_specs.htm

 

[Someone Electro]

Lets see...
The green:
voltege: 3.5 V
curent: 30 mA (25 mA If you want to be shure the dont overheat)

and the yelow:
Voltege: 2.2 V
Curent: 50 mA (45 mA to be shure)

All you have to do now is caculate it and put evriting togeter.

Teel me how good it works wen its done. my single 10000 Mcd LED proved it self good as a flash light and so did my UV LED (yep its a led that radiats strong ultraviolet light) (small viewing angels make leds good flash lights).

 

[zachtheterrible]

The next thing that i wanted to look into was cutting down the duty cycle. If the LED flashes 60 hz, and u cant tell the difference, then it is using less power. Is there anything that I should know about this? Heres the circuit that i plan on using to do this. The diode is not needed.

 

[zachtheterrible]

As led forward voltages:
Red and green; 2 volts
Blue and white; 3.5 - 4 volts

i was just lookin @ the calculator a little bit more, and realized it says that.

I think that the calculations are way off cuz my LEDs are getting warm, and my resistor is also.

 

[Someone Electro]

I think the diode is neded (becose its thurned the oporite of the curent flow).

If you flash a diode fast it will apear a litle draker than normal

I think il have to make an order of components.(At Conrad agen becose i cant find an beter suplyer in my area)Gota get some Op amps ,LEDs,germanium diodes,caps,bords,radio stuf...

LEGO Tehnic bricks (for mehanics) and electronics(controling the motors and stuf) go well togeter

PS:
If you have some old,broken uneused electronics (radios,TVs...) take them a part you can find somtimes neet stuf (thats why i like to take things a a part)
Today isent my day!my multimeter has sudenly gone coo coo(good thing i got it working agen) :?

 

[zachtheterrible]

yeah, i just realized that it is a little darker.

i luv takin things apart 2. My multimeter's amperage reading is screwed up, so if i cant replace the fuse, i get 2 take it apart!!

I ordered a yellow LED along w/ a bunch of other junk for my radio receiver project, so im gonna try it out. ill let ya no how it went. I thought that the human eye is more sensitive 2 green than yellow, i wonder why a 10000 mcd yellow led is brighter than my 13000 mcd green one . . . unless the camera is more sensitive 2 yellow . . .

oh yeah, n the diode isnt needed. u just leave it disconnected and everything works great :lol:

 

[Someone Electro]

I once orderd 200 mA fast fouses for my multimater(for thre low curent conector).But if i conect more than 20 A to the high curent conector my multimeter wod be toast.

I figured you that my multimeter isent working wen i tryed to mesure curent and the reding was 0.00 (so i tought wtf) and wen i turend tha diall to diode test it stil vas 0.000 .But the beper shod go of if the conacts are shorten on diode test. so i kenw somting was wrong. and screwd the botom of and there wasent any smell of anyting burnt burnt. and wen a screwn ti back togeter it worked

 

[audioguru]

Hi Zach,
If you operate your green LED anywhere near its absolute maximum continuous current rating of 30mA, its internal temperature will probably also reach its maximum of 100 degrees C or more. Sure the LED will be warm and it won't last long. They spec its voltage at 20mA so why not operate it at 20mA?

How close to their power rating are you operating the resistors?
What extremely high temperature do they reach at max power rating?
Derate the resistors because you want light from the LEDs, not from red-hot resistors. Use a 1/2W resistor if you calculate 250mW dissipation.

Since the voltage of the 9V battery will drop down to 6V during its lifetime, if you put the LEDs in series for a voltage across them of 5.7V, a fresh 9V will create a current through them of 20mA with a 165 ohm resistor. But the current when the battery drops to 6V will be only 1.8mA! Very dim.
Instead of a resistor, use a low-dropout voltage regulator connected as a current regulator of 20mA. The LEDs' brightness will remain the same until the battery is dead.

Like all other electronic devices, LEDs have a tolerance, but it is not shown on your datasheets. Your specs show only typical brightness, not min nor max. They are not all the same, some are brighter than others, especially if they were made late on a Friday or 1st thing on a Monday! You are correct about the eye being more sensitive to green. So maybe your "10,000mcd" yellow LED is actually 15,000mcd, and the "13,000mcd" green one is 9,000mcd.

You don't get anything for nuthin'. If you cut the average current consumption by flashing the LEDs at a duty cycle frequency that is too fast to see flickering, then you will have what is commonly called a light dimmer circuit. Who says that you can't tell the difference? They probably mean that you can't see those dimmed LEDs flickering.
Those LEDs won't dim the same. If the specs are for the same operation (maybe the green one was tested operating continuously, causing heating which reduces light output, but the yellow one was pulsed), then when the duty cycle is 50-50, the green one will be at 60% light output but the yellow one will be only 30%.

C'mon, put red and blue LEDs in your flashlight (torch?) also.

 

[Someone Electro]

Not a bad idea to put red,green and blue leds to make wight light.I thnink blue leds are orund 2000 mcd .You cod also use white LEDs.I Seen a Flash light whith super bright white leds.

Meby puting a Variable Regulator.So you cod set the brithes of your flash light or a beter solutin wod be an timer that you wod set its duty cicle(to save power).

If you wod have 3 regulators and red,green and blue LEDs you cod get a large nuber of colors,That wod be cool, but you wod need red,green and blue LEDs

arent water clear LEDs somtimes anoying.You have to conect it to power to se what color is it.(But almost all super bright LEDs are water clear)

By the way Zach how much experience do you have in electronics?

 

[zachtheterrible]

By the way Zach how much experience do you have in electronics?

Not enuf!! Ive taught myself everything i know through books(other than wut the nice people on this forum have taught me) Never taken any classes, although i was in an electronics class in highschool for 3 weeks, but then we moved :cry: .

I just want the green LEDs. i want this 2 be simple and not have a bunch o colors.

I think that what i really need to know is how to calculate the voltage drop across a resistor. Thanx 4 the equation electro, but i dont know if i did the math wrong, or the equation was wrong, but i ended up coming out with 9 volts!! Which is my supply voltage. If I know how much voltage a resistor is dropping, then I know how much voltage the LED is getting.

Thanx :lol: Its amazing how so simple a project can become so complicated :lol:

 

[audioguru]

the eqvasion is:

(BateryVoltege - LEDVoltege) / LEDCurent = Resistance

It is that easy to calculate. For your 9V battery and a single 3.5V green LED, the resistor will have 5.5V across it (the subtraction part of the equation). and Ohm's Law says that for 20mA to flow in the resistor (and also to flow in the LED because they are in series), the resistor value is (voltage divided by current) 275 ohms. Use 270 ohms which is a very close stock value, and the current will be 20.4mA.
The power in the 270 resistor is calculated as the voltage across it squared (multiplied by itself) divided by the resistance, so 5.5 squared is 30.25, divided by 270 is 112mW. A 1/4W resistor will be slightly warm.

When the battery voltage drops down to 6V, the current is calculated to be 9.3mA, but it will actually be a little more because the LED's curves show that it will have a voltage drop of 3.35V at 10mA.
So 6.0 - 3.35 / 270 = 9.8mA. It will be almost as bright as at 20mA, because the eye's response is logarithmic.

A 9V battery doesn't have much power capacity, and when it drives a 3.5V LED most of its power is wasted in the current-limiting resistor. That's why small commercial LED flashlights use two AA cells (4 to 5 times the power capacity of a 9V battery) and a voltage step-up IC. The IC has regulation to keep the LED current the same, as the battery voltage drops.

A little key-ring LED flashlight doesn't even have a circuit. Just an LED (3.5V white one) and 3 tiny battery cells. The internal resistance of those tiny cells drops their 4.5V down to about 3.5V at about 20mA. Those tiny cells don't last long.

Think again about using only green. If you shine a green LED on a red stop sign in the dark, then you will see nuthin'.

 

[jrz126]

Whenever I need to know the R-value i just plug it into this site. **broken link removed**

(I know it is easy to calculate, but using this site will give the standard resistor value)

 

[zachtheterrible]

Well, im just about to get my shipment in for my other tda7000project. this 1 was 2 keep me occupied while i waited :lol: . I will return though :lol:

Thanx