High voltage drop LED on computer MB

R

Robert Murphy

Guest
Hi all,

This is all a bit non-consequential, but thought I'd dive in anyway.

I'd like to replace the standard power LED on a computer with a white one,
(DSE part Z-3980), purely for aesthetic reasons. I don't want to drive it
at full brightness - the spec sheets claim 2000mcd with 30mA and 4V, I'd
settle for about 1/1000 of that brightness.

I haven't tested this, but I suspect that the power / hard drive LED headers
on a motherboard won't give 4V. My thought was using the signal from the MB
to drive a FET, fed from the +5V PSU rail via a dropper resistor to drive
the LED.

My problem is not knowing enough to be able to choose a suitable FET, or
values of resistor suitable to bias the gate, or the dropper resistor. The
FET would only need to be hard off or on.

The dropper resistor should be easy; 4V across the LED, 0.5V (say) dropped
across the FET when fully on, and maybe 5mA for a non-lethal brightness. Fed
from 5V that leaves a resistor dropping 0.5V with 5mA through it ie 100ohm.

Just wondering if anyone's done this before, or any suggestions ?

Thanks
Rob
 
"Robert Murphy" <okami1,westnet,com,au> wrote in message
news:42377cb4$0$257$61c65585@uq-127creek-reader-03.brisbane.pipenetworks.com.au...
Hi all,

This is all a bit non-consequential, but thought I'd dive in anyway.

I'd like to replace the standard power LED on a computer with a white one,
(DSE part Z-3980), purely for aesthetic reasons. I don't want to drive it
at full brightness - the spec sheets claim 2000mcd with 30mA and 4V, I'd
settle for about 1/1000 of that brightness.

I haven't tested this, but I suspect that the power / hard drive LED
headers
on a motherboard won't give 4V. My thought was using the signal from the
MB
to drive a FET, fed from the +5V PSU rail via a dropper resistor to drive
the LED.

My problem is not knowing enough to be able to choose a suitable FET, or
values of resistor suitable to bias the gate, or the dropper resistor. The
FET would only need to be hard off or on.

The dropper resistor should be easy; 4V across the LED, 0.5V (say) dropped
across the FET when fully on, and maybe 5mA for a non-lethal brightness.
Fed
from 5V that leaves a resistor dropping 0.5V with 5mA through it ie
100ohm.

Just wondering if anyone's done this before, or any suggestions ?
Click to expand...
It won't hurt anything to try driving it directly from the motherboard.
Locate the
series resistor on the motherboard and replace it with a suitable resistor
to work
with your white LED. What your LED polarity.
 
"Lord Garth" <LGarth@Tantalus.net> wrote in message
news:CWLZd.19798$YD4.269@newssvr12.news.prodigy.com...
"Robert Murphy" <okami1,westnet,com,au> wrote in message

news:42377cb4$0$257$61c65585@uq-127creek-reader-03.brisbane.pipenetworks.com.au...

Hi all,

This is all a bit non-consequential, but thought I'd dive in anyway.

I'd like to replace the standard power LED on a computer with a white
one,
(DSE part Z-3980), purely for aesthetic reasons. I don't want to drive
it
at full brightness - the spec sheets claim 2000mcd with 30mA and 4V, I'd
settle for about 1/1000 of that brightness.

I haven't tested this, but I suspect that the power / hard drive LED
headers
on a motherboard won't give 4V. My thought was using the signal from the
MB
to drive a FET, fed from the +5V PSU rail via a dropper resistor to
drive
the LED.

My problem is not knowing enough to be able to choose a suitable FET, or
values of resistor suitable to bias the gate, or the dropper resistor.
The
FET would only need to be hard off or on.

The dropper resistor should be easy; 4V across the LED, 0.5V (say)
dropped
across the FET when fully on, and maybe 5mA for a non-lethal brightness.
Fed
from 5V that leaves a resistor dropping 0.5V with 5mA through it ie
100ohm.

Just wondering if anyone's done this before, or any suggestions ?


It won't hurt anything to try driving it directly from the motherboard.
Locate the
series resistor on the motherboard and replace it with a suitable resistor
to work
with your white LED. What your LED polarity.
Make that "Watch your LED polarity".
Click to expand...
 
For reference, to top post and to answer my own question
http://www.web-ee.com/Schematics/LED_AMP/LED_AMP.htm

"Robert Murphy" <okami1,westnet,com,au> wrote in message
news:42377cb4$0$257$61c65585@uq-127creek-reader-03.brisbane.pipenetworks.com.au...
Hi all,

This is all a bit non-consequential, but thought I'd dive in anyway.

I'd like to replace the standard power LED on a computer with a white one,
(DSE part Z-3980), purely for aesthetic reasons. I don't want to drive it
at full brightness - the spec sheets claim 2000mcd with 30mA and 4V, I'd
settle for about 1/1000 of that brightness.

I haven't tested this, but I suspect that the power / hard drive LED
headers on a motherboard won't give 4V. My thought was using the signal
from the MB to drive a FET, fed from the +5V PSU rail via a dropper
resistor to drive the LED.

My problem is not knowing enough to be able to choose a suitable FET, or
values of resistor suitable to bias the gate, or the dropper resistor. The
FET would only need to be hard off or on.

The dropper resistor should be easy; 4V across the LED, 0.5V (say) dropped
across the FET when fully on, and maybe 5mA for a non-lethal brightness.
Fed from 5V that leaves a resistor dropping 0.5V with 5mA through it ie
100ohm.

Just wondering if anyone's done this before, or any suggestions ?

Thanks
Rob
Click to expand...
 
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