5V/12V LED's - Any need for a resistor?

[ryguy]

I've got 6 x 5V LED's I want to permanently light up and 1 x 12V LED for the trackball.  I was just going to take power from my computer using a spare Molex connector, so Yellow for the +12V and one ground for the negative, and the red +5V for the other 6 LED's and chain wire the other ground for their negatives. 
Sounded quite easy and then I heard people talking about putting in resistors into the circult?  Before I blow myself up can anybody tell me if I can just go in and wire it up like I said or is there more to it?
Many thanks,

[Ginsu Victim]

If the LEDs are the exact voltage needed, you won't need resistors....but it doesn't hurt to do what RandyT says below....

(Edited for clarity)

[ryguy]

They're 5V and 12V LED's.

Many thanks for the advice, sounds like I can keep it simple then!

Cheers,

[hatrick]

If the LEDs are the exact voltage needed, you won't need resistors.

What about amperage? I thought LED's were sensitive to amperage so you need resistors to limit the amperage to the LED.

[sarge]

There needs to be a load placed on that circuit.  If you're wiring just an LED across your + and  - supply,  you are basically just shorting the supply.

You may also have a problem keeping them close to the same brightness wiring several LED's in parallel.

[RandyT]

They're 5V and 12V LED's.

Many thanks for the advice, sounds like I can keep it simple then!

Maybe, but...

According to the calculations, no resistor is required.  However,  I have seen folks recommend using a 1 or 2 ohm resistor in this case for a bit of extra security.  The only time I have ever seen an LED used without a resistor is in those keychain single LED flashlights.  Apparently, this is acceptable because batteries, by their nature, are current limited.

But the real question is what kind of LED are you attempting to use?  Some will have a resistor built in and are designed to be used at the voltage specified without anything else.  If you have a link to a spec sheet, you should post it to make sure.

RandyT

[Blanka]

The LED is a load itself too. Otherwise it never could emit light. And current works on demand base, not on supply base. The LED just draws very little current. The computer has a switching powersupply so it adjusts very well to different loads and keeps the 5v very stable.

[RandyT]

And current works on demand base, not on supply base. The LED just draws very little current.

If you are referring to LED's in general, this statement is not true.  An LED will keep drawing current until it pops.  That's what the resistor is for

RandyT

[Samstag]

And current works on demand base, not on supply base. The LED just draws very little current.

If you are referring to LED's in general, this statement is not true.  An LED will keep drawing current until it pops.  That's what the resistor is for

RandyT

You are mistaken.  Increasing the supply voltage is what increases the current.  A resistor will also draw more current as you increase the voltage and will burn out if you exceed it's ability to dissipate the extra heat generated.

An LED just happens to be very poor at dissipating heat beyond it's design limits, but it still obeys Ohm's Law.

[ryguy]

Thought it was too simple.  I'm afriad there aren't many specs on the web page. 

The 5V ones are from Ulimarc and go with the Ultralux buttons.  They're the normal 5V ones and not the ultra bright 12V ones.  The link is http://www.ultimarc.com/ultralux.html.

The 12V one came with the Happ trackball and sits underneath.  It actually looks like a normal bulb so it may not be an LED.  Can't find anything on this one.

A photo of them is......

Any advice welcome!

UPDATE - I've just read the bottom of that link page!  It says no need for a resistor so have answered my own question, but still not sure on the 12V bulb!

[Ginsu Victim]

From the bottom of the Ultralux page:

No need to worry about resistors etc, the LED lamps are designed to be directly connected to the power source.

[RandyT]

You are mistaken.  Increasing the supply voltage is what increases the current.  A resistor will also draw more current as you increase the voltage and will burn out if you exceed it's ability to dissipate the extra heat generated.

An LED just happens to be very poor at dissipating heat beyond it's design limits, but it still obeys Ohm's Law.

This is why I gave the conditional "in general".  I didn't want folks to misunderstand and think that an LED will only pull as much current as it needs regardless of higher voltages that may be present (I.e. out of spec power supplies, which we have seen a number of in our travels.)  A power supply that is off by even .25v (5% at 5v) can shorten the life of an a LED significantly, which is why many will put a small resistor in anyway.

If you are going to skip the resistor completely, make sure that your power supply is healthy.

RandyT

[MonMotha]

An LED just happens to be very poor at dissipating heat beyond it's design limits, but it still obeys Ohm's Law.

Absolutely, positively, NOT true.  There is a (positive) relationship between applied voltage and forward current, yes, however it is not linear like Ohm's law would imply.  LEDs are not resistors; they are (surprise) diodes.  Ohm's law applies to resistors.  A standard light bulb is approximately a resistor.  If you want to model and LED, you need a model for a diode.  A suitable model is the Shockley diode equation.  Long story short, the relationship is exponential.  That is, current increases exponentially with voltage.  This is why LEDs are relatively sensitive to minor fluctuations or differences in voltage and are always driven in some current limited (either constant current, with a resistor, or from an inherently limited supply such as a battery chain) fashion.

The purpose of the resistor when lighting an LED like is commonly done is to attempt to swamp the LED's exponential I/V relationship with the linear nature of the resistor.  That is, one wants to allow to resistor's behavior to "make up" for minor imperfections in power supply.  Most LEDs also have inconvenient "typical" forward characteristics in terms of voltage (1.8-2.5V is common for non-superbright types), so the resistor also lets you drive it from a more "normal" rail like 3.3V or 5V.  The idea is that one treats the diode as a constant voltage device (above a certain point, they are very close) and then sets the current using the resistor.  A constant current power supply (an item commonly found on EE lab benches) can also be used, but a resistor and constant voltage supply is a lot simpler and easier to come by.  Note RandyT's comment about even relatively small differences in power supply specifications substantially affecting lifetime: that's the exponential behavior of the LED at work.

In this case, your LED is actually an incandescent replacement module and therefore has everything it needs built in.  Just apply 5V at the shown polarity.  Your 12V lamp is an incandescent light bulb.  Apply 12V AC or DC as you please.

I really feel like I've said this sort of things several times over the past week.  Perhaps a sticky FAQ on LEDs is in order?

[SavannahLion]

Early on (ie yonkers ago, before the Internet) I had a hard time understanding how and why LED's behave the way they do and why you had to include a resistor in the mix. I kept blowing, burning and melting LED's in my early experiments. MonMotha's relationship to diodes is absolutely correct but doesn't translate well to someone who is just starting to understand LED's (and by extension diodes).

If you understood MonMotha's explanation, great, stop reading my crap and move on to the next post. If you didn't, here's what I was once told (it's kind of corny really).

An LED is like a goldfish. If you keep feeding it food (electricity) it will keep eating and eating until it pops. Resist the temptation to feed them too much.

Now I feel stupid for writing that. 

I really feel like I've said this sort of things several times over the past week.  Perhaps a sticky FAQ on LEDs is in order?

How about a wiki entry? It might be more useful and easier to include it as a link in posts?

[MonMotha]

To say what I was saying in the context of your analogy, resistors are like goldfish: if you offer them more voltage, they'll just keep taking whatever you offer them (in the form of current) until they eat too much and pop.  LEDs are like a mouse: if you offer it an extra cookie, it won't stop taking things until your whole house is cleaned out.

As far as a wiki, one could be written.  The wikipedia entry on diodes is actually fairly good, though.

[SavannahLion]

To say what I was saying in the context of your analogy, resistors are like goldfish: if you offer them more voltage, they'll just keep taking whatever you offer them (in the form of current) until they eat too much and pop.  LEDs are like a mouse: if you offer it an extra cookie, it won't stop taking things until your whole house is cleaned out.

Mice don't pop, they breed. 

I guess I had the description wrong or whomever passed it to me passed it to me wrong. Worked well enough as a learner to mate resistors to LEDs. 

[RandyT]

To say what I was saying in the context of your analogy, resistors are like goldfish: if you offer them more voltage, they'll just keep taking whatever you offer them (in the form of current) until they eat too much and pop.  LEDs are like a mouse: if you offer it an extra cookie, it won't stop taking things until your whole house is cleaned out.

Mice don't pop, they breed. 

I guess I had the description wrong or whomever passed it to me passed it to me wrong. Worked well enough as a learner to mate resistors to LEDs. 

Heh.  I made that up (at least I thought I did) and printed it in the LED-Wiz documentation.  It wasn't meant to be terribly technical, just provide a simple way to characterize the behavior of an LED compared to something that most folks have had some experience with.

MonMotha is "the man" if you really want to know how they work.

RandyT

[MonMotha]

Well, it's still perfectly valid to say that if you feed your mouse (LED) too much, it'll pop.  Just that a goldfish (light bulb) will do that, too.  The difference is that while the goldfish takes what you offer, the mouse runs right past you and cleans out your pantry if you offer it a few extra crumbs.

[SavannahLion]

To say what I was saying in the context of your analogy, resistors are like goldfish: if you offer them more voltage, they'll just keep taking whatever you offer them (in the form of current) until they eat too much and pop.  LEDs are like a mouse: if you offer it an extra cookie, it won't stop taking things until your whole house is cleaned out.

Mice don't pop, they breed. 

I guess I had the description wrong or whomever passed it to me passed it to me wrong. Worked well enough as a learner to mate resistors to LEDs. 

Heh.  I made that up (at least I thought I did) and printed it in the LED-Wiz documentation.  It wasn't meant to be terribly technical, just provide a simple way to characterize the behavior of an LED compared to something that most folks have had some experience with.

It's probably where it came from. I'm pretty certain I don't remember what I heard/read 100%. I had to struggle to remember what goldfish had to do with LED's and had to piece together the saying from scraps that I could recall. 

Everything before 1996 was an educational ---smurfing--- nightmare anyways.