Replacing power and hd led?

[TheKeith]

I'm replacing the leds in my pc case with red and blue for power and hd,
respectively. My case uses what appear to be 3mm leds and I'm just have a
few questions. Most of the pc mod leds commonly available seem to be 5mm,
not 3. Could I just as easily use the 5's, or is it necessary for me to use
the 3's? Where is a good place to purchase them? Exactly what voltage led
shoudl I get? I tried powering the ones I have (the old green and orange)
with 4x AA batteries and that works but they do get warm--probably too much
juice right? Any help and info would be appreciated--thanks.

 

[Gary Tait]

I'm replacing the leds in my pc case with red and blue for power and hd,
respectively. My case uses what appear to be 3mm leds and I'm just have a
few questions. Most of the pc mod leds commonly available seem to be 5mm,
not 3. Could I just as easily use the 5's, or is it necessary for me to use
the 3's? Where is a good place to purchase them? Exactly what voltage led
shoudl I get? I tried powering the ones I have (the old green and orange)
with 4x AA batteries and that works but they do get warm--probably too much
juice right? Any help and info would be appreciated--thanks.

LEDs are current limited devices, the limiting resistors on the
motherboard. Regular 3mm and 5mm LEDs are electronically compatible.
High brightness ones may have special current/voltage needs.

 

[TheKeith]

LEDs are current limited devices, the limiting resistors on the
motherboard. Regular 3mm and 5mm LEDs are electronically compatible.
High brightness ones may have special current/voltage needs.

thanks but I'm a little confused. When you say they are current limited
devices--what exactly do you mean? Thanks.

 

[Bill Turner]

thanks but I'm a little confused. When you say they are current limited
devices--what exactly do you mean? Thanks.

_________________________________________________________

It would be more correct to say that LEDs need to have external current
limiting included in the circuit design. LEDs have a very nonlinear E
vs I response: As the voltage across them is raised from zero, little
current flows until you reach a certain point, then as the voltage
continues to rise, current increases dramatically. If there is no
external current limiting (usually a simple resistor), the LED can be
destroyed very quickly.

 

[ric]

thanks but I'm a little confused. When you say they are current limited
devices--what exactly do you mean? Thanks.

LEDs are diodes - Light Emitting Diodes. As such, they have a near constant
forward drop, and rely on a series resistor to limit the forward current
through them.

For example, with an LED with a 2.0v forward drop and a 5v source voltage.
A series dropping resistor of 300 ohms would yield (5.0 - 2.0)/300 or
10mA of current. A 150 ohm resistor would give the LED (5.0 - 2.0)/150
or 20mA of current, making it a brighter LED.

You should either know the voltage and current specifications of the LEDs
you are replacing (and try to match that), or know the MB source voltage
and dropping resistor value.

 

[Christopher Pollard]

You should either know the voltage and current specifications of the LEDs
you are replacing (and try to match that), or know the MB source voltage
and dropping resistor value.

But having said that, pretty much any LED will work just fine, if a little
outside its ideal parameters.


Chris Pollard

 

[Conor]

thanks but I'm a little confused. When you say they are current limited
devices--what exactly do you mean? Thanks.

In laymans terms it means the supply voltage doesn't really matter too
much but the current supplied does.

 

[Bill Turner]

In laymans terms it means the supply voltage doesn't really matter too
much but the current supplied does.

_________________________________________________________

What Conor should have said is it doesn't matter too much as long as the
voltage is higher than the turn-on point of the LED. That may be
obvious to Conor but not necessarily to a newbie.

 

[Ralph Mowery]

thanks but I'm a little confused. When you say they are current limited

In laymans terms it means the supply voltage doesn't really matter too
much but the current supplied does.

In real laymans terms it means , don't mess with things you don't know
anything about. You will let all the smoke out of the box and it will quit
working.

 

[TheKeith]

....it doesn't matter too much as long as the

voltage is higher than the turn-on point of the LED.

Is the voltage number provided with most LED the turn-on point of the led or
is it the suggested voltage you should be running the led at?

 

[Eric]

But having said that, pretty much any LED will work just fine, if a little
outside its ideal parameters.

Yes and no. Some LEDs have very large forward drops. Others have internal
dropping resistors. These will be very dim or won't light at all in PC
cases.

 

[Guest]

I'm replacing the leds in my pc case with red and blue for
power and hd, respectively. My case uses what appear to be 3mm
leds and I'm just have a few questions. Most of the pc mod leds
commonly available seem to be 5mm, not 3. Could I just as easily
use the 5's, or is it necessary for me to use the 3's? Where is
a good place to purchase them? Exactly what voltage led should
I get? I tried powering the ones I have (the old green and orange)
with 4x AA batteries and that works but they do get warm--probably
too much juice right?

Electronics suppliers are usually cheaper than computer shops for
parts like this, and some are www.bgmicro.com, www.jameco.com,
www.jdr.com, www.digikey.com, and www.mouser.com.

Size doesn't matter nearly as much as the type of material the LED is
made of.
LEDs try to maintain a constant voltage across them, usually around 2V
for most colors, but most blue and white ones work at around 3.5V,
although some blue ones work at 5V. A big change in current will
cause only a small change in this voltage, but too much current will
destroy them, so there has to be a resistor in series with the supply
voltage. The motherboard has the needed resistors, and they should be
fine for blue and white ones, although they may be a little dimmer.
Try to keep the current at roughly 20mA or less, and the needed
resistance will be (Vsupply - Vled)/.020A. In the case of your 4 AA
cells, Vsupply would be 6V, while motherboards typically use 5V (but
already have the needed resistor). Never, ever connect a voltage
source directly to an LED or it'll burn out immediately.

 

[Bill Turner]

Is the voltage number provided with most LED the turn-on point of the led or
is it the suggested voltage you should be running the led at?

_________________________________________________________

Manufacturers are free to spec it either way. You have to find out,
either from them or from a carefully controlled experiment.

 

[Conor]

In real laymans terms it means , don't mess with things you don't know
anything about. You will let all the smoke out of the box and it will quit
working.

THere are no +ve DC voltages inside a PC that'll fry an LED.

 

[Muttley]

Connect a LED directly to 12V with no current limiting resistor and see what
happens!!!!!!

John S.

 

[Bill Turner]

THere are no +ve DC voltages inside a PC that'll fry an LED.

_________________________________________________________

Unbelievable. Conor, take up some other hobby. Please!!

 

[ric]

Same could be said of +5v or +3.3v, or *any* of the PSU outputs, not to
mention the internal +15v or +300v rails.

 

[Conor]

Connect a LED directly to 12V with no current limiting resistor and see what
happens!!!!!!

Do you not understand the difference between voltage and current?
Also the motherboard frnt panel headers already have the current
limiters built in. The case front LEDs have nothing to limit the
current so the OP would be able to connect any LEDs he wishes to use on
the front panel to the motherboard without worrying.

 

[Conor]

Same could be said of +5v or +3.3v, or *any* of the PSU outputs, not to
mention the internal +15v or +300v rails.

And of course we all open up the PSU to connect to the internal power
rails.

 

[Conor]

Unbelievable. Conor, take up some other hobby. Please!!

Another HAM radio wanker who's electronics knowledge fits on a postage
stamp.
Bill, please learn the difference between current and voltage. Also
read up about potential differential. If you connect the -ve side of
the LED to -15V and the +ve side to +15v you have a PD of 30V which, if
the current isn't too high, is still within the limits of an LED.