Jim wrote:
> This maybe such a newbie question it's untrue but how does one go about
> making your own HDD LED leads?
> I have a Fractal Design Refine XL which has a Blue power light but no
> HDD LED at all, so i have right now just rigged up and old lead (that
> was in a box somewhere) that comes of the motherboard and sits under the
> case and is red, but i'd like to make a longer lead with a Blue LED,
> given were dealing with tiny parts (compared to what I'm used to) any
> ideas what tools I'd need to get pins/wire in and out of the motherboard
> plug side of things?
>
> Hope you get my idea if not then forgive me but it's late.
>
> Jim
Normally, in a LED circuit, you place a current limiting resistor.
For example, if I wanted to build a "LED flashlight" with a blue
LED, I'd do it like this.
(+) (-)
+5V ----- resistor ------ LED ------- Ground
LEDs have a "forward voltage drop" when they're lighted. A
blue LED might be 3.2 to 3.4 volts or so. 5V minus 3.4 = 1.6 volts
across the resistor.
LEDs are also rated by maximum current they can handle. Like 20 milliamps
or 50 milliamps for a small one. Let's start small, and pump 10 milliamps
through the LED.
The resistor has 1.6V across it, and we need 10 milliamps. V=R*I
1.6V/0.010A = 160 ohms. The power dissipated in the resistor is
V**2/R or is V*I, and 1.6V*0.010A = 16 milliwatts. An 1/8th watt or
1/4 watt resistor should have the power dissipation rating needed.
So I need to buy a 160 ohm 1/8th watt resistor at the store.
So that's how you go about building a *general* kind of LED circuit.
Red LEDs have a forward drop of perhaps 1.6V, but you can look up
the value in the LED datasheet. The forward bias characteristic is
a curve, but in the example, I'm treating it as a fixed value,
for the purposes of computing a first cut at a resistor value.
The 10 milliamp value I selected, should be bright enough to verify
the circuit works. And the circuit can be later modified, if you
need more light (up to the safe limit for the LED).
*******
Now, how does the situation change in a computer case ? There is a
switch in the circuit, to turn the LED on and off. And the motherboard
already has a "resistor" in the path. The computer case, all it needs
is a LED and two wires. The LED is a polarized device, and will only
light up if the legs are turned the right way, with respect to the
motherboard header.
Cheap commodity LEDs are rated for "5 volts PIV" or peak inverse volts.
That means, an average (Radio Shack) LED can be inserted backwards, without
damage to the LED. If you plug in your LED and wires, and it doesn't light,
simple reverse the two leads and try again, and it should light.
Now, what other things would we need to be aware of:
1) What level of current, does the motherboard attempt to drive
the LED with ? I don't know the answer to that, and I'd have
to measure it to know for sure. I'd guess it is in the 10-20 milliamp
range.
2) What voltage does the motherboard use to drive the LED ?
Well, this is important. The resistor may be selected to work with
a red LED. That would be an assumption by the motherboard designer.
If you substitute a blue LED, the voltage drop across the resistor
is less. And less current flows through the circuit when turned on.
That means the blue LED could receive half the current that a red
LED would get.
Particularly important, would be a situation where the LED is powered
from the 3.3V rail, instead of from 5V. This might happen, if the chip
driving the LED, was not "5V tolerant", and then the motherboard designer
uses 3.3V (another rail on the power supply) instead. If that was the case,
practically no current would flow through a blue LED, while a red LED would
still light up. 3.3V - 1.6V would still leave some voltage drop across a
resistor, when the red LED was used. So there can be situations where a
blue LED simply won't light at all, due to the supply voltage being too
low for it.
Note that, not all LEDs are reversible when making the electrical connection.
The small and cheap LEDs you'll be using for your computer case, are 5V safe
and reversible. But if you buy expensive "illumination" type LEDs, some of
those do not tolerate reversal. In which case, a careful hobbyist checks
and double checks the wiring, before applying power. A cheap LED costs $1.00,
while illumination LEDS with 1 watt to 5 watt ratings, cost $10.00 to $20.00,
and aren't as resistant to reversal. Their performance may be compromised if
they're reversed. Again, the details are in the datasheet.
I was shopping for a LED a couple days ago, to see what my local electronics
store could manage in terms of an "eye burner". This LED is rated at 20,000 mcd
and is white in color. White means it's "up the blue end of the spectrum",
and the Vfb is 3.2V. Max current is 20 milliamps continuous (so I'd run this
one at 10 milliamps). The "reverse voltage" rating is 5V, which means the
LED can be inserted the wrong way, without permanent damage. This is to
demonstrate, that a good vendor provides a pointer to the datasheet
(i.e. by using a part number you can Google, rather than being clever
enough to give a direct URL).
http://mode-elec.com/pdf/LED/55-559UB.pdf
Now, another issue, is mounting the LED. If you take the power LED out of
your computer case, it may have a mounting ring of some sort. You could
drill another hole in the case, and insert another mounting ring. For
cosmetic reasons, you may want the size of the LEDs to match. And there
are standard mounting rings, for standard sized LEDs. You'd try and buy a
LED plus a mounting ring, and be prepared to drill out the computer
case.
These are examples of different LED sizes:
size T1 (3 mm) or T13/4 (5 mm)
This picture shows a LED mounted in a mounting ring.
http://www.builders-in-scale.com/bis...-schm-1105.bmp
This is another example of a LED mounting kit.
http://octopart.com/4304mc-chicago%2Bminiature-703471
You don't have to use a mounting kit. You could drill a
LED sized hole, then use a dab of epoxy or hot glue, to hold
the LED in place.
When you make your LED and wires, you can cover exposed areas of the
wire with heat-shrink (polyolefin) tubing. I use that, rather than
electrical tape. Polyolefin tubing comes in different diameters,
and I keep maybe four or five diameters on hand for this kind of work.
For a professional job, you can also use a plastic shell and pins from
here. This is for the motherboard end of your cable assembly.
This is a shell, with room for two pins, on 0.1" centers.
http://www.frontx.com/cpx075_8.html
If you purchase the correct female crimp pins, they fit into
the shell and "snap" into place. Bending out the "tab" on the plastic
shell, allows the pin to be pulled out again. You have to crimp a
wire into the pin, before it can be pulled out (reversed) from the
shell. Otherwise, it might be hard to get the pin out.
I sometimes crimp and solder the pin, depending on the mood I'm in.
On the minus side, soldering makes the wire "stiff" and it can
snap more readily if flexed. But on the plus side, it's less
likely to slide out of the crimp. I don't use a proper crimp tool,
which is why I do this extra step. (My crimps done with pliers aren't
always the best.)
How to crimp, is shown here. Two pairs of legs. One pair touches bare wire.
Other pair holds the insulation of the wire for support.
http://www.frontx.com/crimp.html
And this page, shows how to back a pin out of a shell, if it
needs work. Lifting the tab on the shell, allows the pin to be
backed out of its hole.
http://www.frontx.com/head_con.html
I'm sure someone, somewhere, makes an assembly with all this work
done for you. But I don't see one at the moment. Stealing the
LED out of another case, is a way to solve this problem. For
example, my oldest computer had a separate "sleep" or "alert"
LED, which could be scavenged without anyone noticing :-) That
case had too many LEDs on the front.
Paul
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