LED help

[Scootie]

hey guys,

 

this is my first post on the site. its probably a newbe question and im not sure if this is were it should be.... but here it is....

 

I am making some modifications to my computer case and i want in install some LED's. I know that when installing LED's you typically need to use a resistor to drop the voltage. I have two green LED's rated at 2.1V (2.8V Max) 30mA each. Could i simply solder them in series and connect them to my 5v form my PSU? I have been told this will work. I have also been told this will not work and to use resistors ranging from 1 ohm to 600ohm.

 

im just looking for a little clarification, thx.

[Thomas Kirby]

Use a resistor. LEDs will draw current until either their voltage drop equals the supply voltage, or they burn out. The power supply you are hooking this up to can run as high as 5.35 or so actual, and your LEDs together may only drop 4.2 volts. For the worst case the voltage difference is about 1.25. If you want 20 milliamps max, use 50 ohms per volt. I think Radio Shack has something like a 68 ohm resistor, but if you have a 100 or a 120 ohm resistor, I think you'll find that the LEDs shine brightly enough using that.

[Douglas]

Try one LED with a 510 ohm resister in series (at 5V). If it's sufficiently bright, put the 2nd LED in parallel with the other. If you have another 510 ohm resistor, put it in parallel with the 1st.

 

NOTE: for testing purposes any resistor from say 240 ohms to 1k is ok.

[Scootie]

I guess im trying to get away from resistors altogether. I will have another circut with two 3.7v (4.5v Max) and two 1.7v (2.4v Max) powered by the 12v side of the power supply. Do you think i could use voltage regulators (7505 and 7812) to ensure the voltage is correct and forget the resistors?

[Thomas Kirby]

You can't get away from the use of resistors. Trying not to use resistors means that your LEDs will face unpredictable currents. Then they won't last as long, could burn out, could catch fire. It isn't safe and it isn't that desirable. It's also a lot easier to just put a resistor on the line than to try to use regulators, which are just another form of resistor anyway.

[Douglas]

TK is right, you can't use a voltage regulator, though you could use a properly configured constant current source (current regulator)

 

Bite the bullet and use the resistors.

[mmalluck]

Use a resistor. Why do you want to do otherwise?

 

It's not like resistors cost a whole bunch. If you get them in bulk it's less than half a cent (US) a piece. If you get them at radioshack it's like 10 for a $1.

 

If you can't spend a dollar to mod a computer should you be modding it??

[Scootie]

resistors it is

 

thanks guys

[Scootie]

..... just so i can walk away having learned something... would i be correct in saying "LED's are not like a light bulb or any other load on a circuit in that they have no resistance" that is why a resistor (or current regulator) is needed to keep the current under control.

 

Highschool physics is the extent of my electronic knowledge but im looking to learn more.

[Thomas Kirby]

LEDs have resistance. The resistance varies in such a way that the LED has a more or less constant voltage across it even when the current varies. The trouble with that is that the LED's resistance will drop the more current you put into it. Hook it up to a power supply that can deliver the current without a resistor and it will burn up.

 

This is actually a good thing because it makes the current usage very predictable when you use a resistor. You can make the LED brighter or dimmer over a very wide range without having to change the power supply. This also lets you save power. You can cut the LED current, using a resistor, to a level that lets a 9 volt battery last for a long time. Unlike incandescent bulbs, they don't seem to change color when you change the current. I've built an illuminator using six white LEDs that is easy to read by, using about 5 milliamps of current from a 9 volt battery.

[Lance]

An LED is more like a diode that heppens to emit light.

[Thomas Kirby]

That is exactly what it is, but it doesn't like much reverse voltage. I've seen an LED die while being powered by AC through an appropriate resistor.

[mezarashi]

An LED is more like a diode that heppens to emit light.

 

LED stands for Light Emitting Diode after all

 

You generally need resistors in any circuit, especially one with LEDs. LEDs are non-linear devices. Once the bias voltage exceeds their rated voltage, say for example 2.1 volts, the current rises exponentially and quickly reaches some insanely high value which will in effect destroy the poor diode. If there is no resistance in the circuit, you are relying only on internal resistances which is very low and your LED will forcefully take the entire voltage drop.

 

A voltage discrepency of 0.1 volts can create an excess current of say 50mA or more.

[Douglas]

That is exactly what it is, but it doesn't like much reverse voltage. I've seen an LED die while being powered by AC through an appropriate resistor.

I don't know of any application where one would or could use AC to power an LED.

[Thomas Kirby]

In case you just wanted an indicator light on the low voltage side of a power transformer to let you know that it has power. Experimenters who have low budgets have a real interest in saving a few cents here and there. In this case, you want to spend the few cents for the rectifier diode. Another alternative is to wire the two LEDs in parallel but not in the same direction. There are uses for indicators like this, including if you want to test for polarity or if you have AC or DC. They are also good for RS-232 lines which can run up to plus or minus 30 volts.

[YT2095]

I have a board here with an array of over 100 LEDs in series, I can (and have) plugged this directly into household mains (240vac) for hours at a time, I use No resistors or anything, just the LEDs and mains wire with a BS1363 at the end. never had an ounce of trouble with it either

 

btw, 3 LEDs in series across the output of a 7805 works perfectly fine without a resistor also! the only thing he`ll need to do is put the 7805 on the 12v supply rail, as it requires several working volts ontop of it`s output rating, that`s all

[Douglas]

Hey YT, it takes some balls to plug 100+ LED's into 240 VAC. I wouldn't have done it.

[YT2095]

well I was GIVEN a whole box full of brand new ones, and wondered what would happen if I used a few of them to make a nice red light for a room, so I set about drilling holes in a sheet of fiber board, glueing alu foil onto it and fitting and soldering the LEDs.

there was no technical reason it shouldn`t have worked, and I had nothing else better to do with a box full full of them , so I used a few

 

the only disappointing part was that it wasn`t as fantastic a room light as I`de hoped, BUT you CAN feel a heat from the glow up to about 36 inch (3foot) away, That part was quite surprising

[DV8 2XL]

I put an LED white spotlight up on my eves, and yes they ship a lot of heat when they run.

[YT2095]

if I ever get lucky enough to be given a box of 1000 White LED`s THEN I`ll certainly do the same again and rip out all the normal fillament Bulbs in this place.

sure save a heck of alot of electricity!

[mmalluck]

If just one of those LEDs go dead, so will the rest. That's gotta be crappy to debug should that ever happen.

 

Right now I'm building a 32 LED * 32 LED matrix (thats a 1024 LEDs). If I ever get it done, I'll post some pictures.

[YT2095]

If just one of those LEDs go dead' date=' so will the rest. That's gotta be crappy to debug should that ever happen.

[/quote']

 

actualy, it`s easier than you think, all you need is a simple meter and hold one end on the live then sweep the board until you lose the power, THERE will be the offending article

[DV8 2XL]

If you want to save power, go for compact fluorescents. the LED's don't save that much elecricity, but they do last much, much longer.

[YT2095]

IIRC, you can buy something like that to fit inside a PC case too, they look like mini strip lights, I`m fairly sure you don`t have to do any significant "wiring up" either.

[Lance]

I would go for cold cathode lights if you want it to look cool.