curent draw and LEDs

[blanik]

simple question, i want my LEDs to eat up as little battery as possible, i usually use the 5mm 2V type (just bought blue ones at Rat Shack recently) i put 2K2 resistor ont the + (cause it's what's on in most mill byps schem i found) but i also found 500 ohm resistor in a switchbox i had lying around

my question: knowing that changing the value will change the brightness, does the value of the resistor change the current draw of the LED (or does the energy that's not lighting the LED burns anyway in heat (from the resistor)?  and if i change the value of the resist, will the mill byps still work?

the blue ones i isntalled are so bright, i can almost light up my room... lol  i wouldn't mind putting a higher resist if it can make my batteries last longer...

R.

[Seljer]

To calculate how much current a LED eats up
current = (supply voltage - LED forward voltage)/resistor


a LED is a diode, that means it will conduct with barely any resistance as long as the voltage is higher the forward voltage. For silicon diodes this is about 0.7V . For LEDs this is anything between 1.5V and 4V depending on what type of LED you have (regular red ones are about 2v, blue and white are 3v or more). In a regular setup with a LED+resistor take your multimeter and measure across the terminals of the LED and you'll get this voltage.
the resistor is there to limit the current, as if you let too much of a current and you're basically shorting your supply to ground and you end up burning out the LED. However, more current going through the LED = brighter LED.
turn around that formula up there and you can calculate the resistor needed for a certain current
resistor = (supply voltage - LED forward voltage)/current

with the ultrabright LEDs you can get away with a 10k resistor and still have it bright enough to see (thats running less than a milliamp through it), even more would probably work.

[blanik]

what are "ultra bright" leds? the small 3mm ones? or the transparent one (like the blue i've got?)

R.

[Seljer]

what are "ultra bright" leds? the small 3mm ones? or the transparent one (like the blue i've got?)

R.

I think its most of the transparent ones (well the ones that don't cost <5 cents each), size is irrellevant as you can get different types of LEDs in both 5mm in a 3mm

just hook up a led and a 50k potentiometer instead of a resistor and mess around a bit until you find how big you can go and still have enough brightness, the measure the resistance you found (and be careful not to set the resistance too low and burn out the LED) and replace with a regular resistor thats close enough. Or just swapping out resistors in a breadboard until you find it would also work.

[R.G.]

An LED can be thought of as an electrical to optical transformer. You put in current, you get out light.

Any specific LED has a conversion constant, a number that expresses how much light you get out for every milliampere you put in.  Although the conversion constant is not really a constant, it's a really useful abstraction, and a good way to think of things until you get into designs that involve the LED's linearity.

So if you put in 1ma of current, an LED gives you X amount of light. Put in 2ma and it gives you approximately 2X. Put in 100ma and it gives you 100X - right?

Kind of. The conversion gain falls off at high currents. Worse yet, the LED chip heats up and if you leav it on for more than a few milliseconds at massive currents, it burns out.

Most LEDs have settled on an average forward current of 20ma max. You can do 20ma on DC, all day long. You can put in 100ma, but you have to turn it off within a few milliseconds before the heat builds up, and then leave it off for four times as long as you had it on so that it can cool back down. This is not something that affects most people, but it's useful to know. Don't put in more than 20ma or you'll likely burn them out. From there on down to zero, light output goes down roughly proportionately to the reduction in current.

Some LEDs are very efficient - they give you a lot of light for the current. It used to be that standard LEDs would put out about 1millicandela of light per milliampere. Today, we have LEDs that will put out thousands of millicandelas per milliampere. So to know how much light you get out, you need to know how efficient the LED is and how much current you feed it.

[darron]

typically i find using a resistor around 1k5 (minimum before you burn the led) to 4k7 is good for normal led's. at the moment though i'm using clear led's which glow a colour. the cheapest ones, $2.00AU form Jaycar, are 1000mcd and I need a 56k or so resistor to get it dull enough to not suck back power and be bright enough to see it in an already light room.

[Toney]

Not sure what you mean by sucking back the power..?

But check this out: ultrabrights for 29c at Futurelec...haven't tried these but I'm gonna..


http://www.futurlec.com.au/test13.jsp?category=LEDGEN&category_title=General%20Purpose%20LEDs&main_menu=LEDS&sub_menu=LEDGEN

[gez]

Scan through supplier's catalogues and look for LEDs with a high 'Iv mcd max' or 'Lum. int. (mcd)' figure.  IIR, this tells you the approx millicandela you get at max current rating (which is usually quoted).

A number of ultra-efficient LEDs are on the market and only require small amounts of current to get a bright light.  By the look of things there's a reflector integrated into the device.  Because of this they have a limited viewing angle (step back and they become less visable), but when you go to stomp on a switch you're usually directly above the thing or within a foot (quite literally) of it so I don't find this a problem.

I use LEDs with a 5000 mcd rating and a stop resistor of 20k gives me all the brightness I need.  Before that I used 10000 mcd LEDs and a 47k stop reistor.  Rapid Electronics have just brought out a range of 'power' LEDs that have 70 000 mcd ratings (trickle of current) but again, viewing angle is limited (15 degrees).

[blanik]

but i'm still wondering if i can save power from my battery by putting the highest resistor possible (i don't care if the LED can't be seen from the back of the concert hall  , i've been playing most of my effects without LEDs), i just need to see it a bit...

so if i put the highest resistor i can (to still be able to see it) will this resistor heat up and suck the same power from the battery to produce heat or will i definitively spare my battery??
(if it's to cost the same in power, i'd rather produce light with the LED than heat from the resistor...)

R.

[Toney]

Get Superbright LED.
Connect to 20k pot and bat. DONT TURN DOWN BEYOND 1/4!start at 1/2 way point.
Adjust to taste.
Measure resistance. Replace with resistor.

[gez]

but i'm still wondering if i can save power from my battery by putting the highest resistor possible (i don't care if the LED can't be seen from the back of the concert hall  , i've been playing most of my effects without LEDs), i just need to see it a bit...

so if i put the highest resistor i can (to still be able to see it) will this resistor heat up and suck the same power from the battery to produce heat or will i definitively spare my battery??
(if it's to cost the same in power, i'd rather produce light with the LED than heat from the resistor...)

R.

If you make the stop resistor larger in value, less current flows.  Less current won't cause your resistor to heat up.  The point I was trying to make is buy LEDs that are incredibly efficient, then you can select larger value stop resistors so that little current used.

[amz-fx]

The response of the human eye is not linear nor does it respond to all wavelengths of light equally.  The eye responds to green the most, red the next and blue much less:



Did you know that most white LEDs are actually blue LEDs with an interior yellowish phophur coating that emits white light when excited by the blue light of the internal diode?

If you do an experiment, you will notice that you can see a big difference between an LED at 5ma vs. one at 10ma, but not a lot of difference between 10ma and 20ma.   

I use efficient LEDs and never need more than 2 to 5 ma to run them at an effective brightness level.

Here is my online LED-resistor  calculator:  http://www.muzique.com/schem/led.htm

regards, Jack

[Stephen]

Stupidity SORRY

But if a led is rated 2.2 forward voltage you can still hook up 9 volts to IT

Adjust a resistor to light taste???

[R.G.]

You can still power it from any voltage bigger than the 2.2V if you limit the current somehow.

The 2.2V rating is the average forward voltage when it's being driven with a current. Like any diode, an LED doesn't conduct much at all until very near its forward voltage rating, then it conducts a lot of current but the voltage doesn't change much. So you have to provide more than the rated voltage to get it to turn on at all, then you have to limit the current to less than the maximum rated current to keep it from burning up.

They tell you that forward voltage so you can size the current limiting resistor. For instance, if you have an LED that's rated for 20ma max and 2.2V average forward voltage, then if you power it from 9V, the diode eats up 2.2V of the 9V just to turn on. You'll then need a resistor between 9V and the LED, and that resistor must have 9V - 2.2V = 6.8V across it.

From there, it's simply Ohm's law to figure out the resistor value. If you want 20ma, then the resistor is (9-2.2)/.02A = 340 ohms. So any resistor bigger than 340 ohms will let the LED work from 9V, and limit its current to less than the 20ma max that might burn it out.

[blanik]

nice calculator Jack, from what i see anyway it seems that most LED have a voltage around 2V and with a resist of 2k2 (like i have) average current draw is around 2-3 mA wich is nothing anyway, so i might have made a big deal of this for nothing... 

it's just that i made the decision to use batteries only (much less problems live and so far batteries last months in my rig, so buying 4-5 batteries 2-3 times a year is ok for me) so i wanted to maximize my batteries life for my DIY projects... my worries was my rebote 2.5, i read somewhere that it's curreny draw was under 30 mA so 2-3 mA more for LEDs (and that's when i'm actually using the delay) isn't much...

thanks

R.

[Seljer]

However some pedals use VERY little (a wah wah uses only 0.5mA, a simple distortion pedal about 4 or 5, a fuzz face type pedal 1 or 2mA) and adding a LED can easily halve the battery life (if you use the pedal all the time).

[Mark Hammer]

The fundamental question is "Can you see it as much as you need to?"  Getting the status LED to be functionally and optimally visible can involve a few things.  As noted so far in this thread, that can involve how much current is pumped in, the supply voltage, the wavelength, the size of the LED, and the efficiency (mcd per ma).  It also involves the surround and ambient lighting conditions.  Many LEDs will be more visible against a dark background, where contrast is enhanced.  That dark surround could be JUST the bezel but can also include the colour of the chassis, the "glare" or finish of the painted surface on the chassis (hi-gloss will make all ambient light shine off it more, reducing contrast and visibility), and the extent to which the LED protrudes or is recessed into the chassis.

In general, the more things you can do to enhance the contrast between the LED and all other potential reflected light sources, the less bright that LED has to shine to be seen.  Do all of that, and use a superbright LED that makes use of human visual sensitivity, and you can easily "get away with" sticking 15k and more between the supply line and the LED, dramatically dropping current requirements.

One of the benefits of reducing current needs for an LED is not just longer battery life but reduced risk of switch-related popping.  Anything that suddenly draws current away from the circuitry at the same time that the circuit *needs* current will likely produce an audible pop.  There have been several discussions about how to cure this, but the most convenient and reliable solution is simply to keep your current needs for the LED to a bare minimum.

[Gilles C]

To complement Mark's comment, you could use this kind of socket easily available at Radio Shack to make it more visible.

http://www.radioshack.com/product/index.jsp?productId=2062559&cp=2032058.2032233.2032297&parentPage=family

I love them because you can use any Led with them, even the bi-color ones.

Gilles

[gez]

+1, they increase the viewing angle of ultra-efficient LEDs.