How to mod LED fans to control LED's illumination

Lookingwest · Jul 26, 2011

I have a bunch of 120 mm PWM LED fans. Each fan has 4 LED's which are not switched. They go on when the fans are powered. The fans are powered by either motherboard headers or through a fan controller.

I would like to install a device to manage fan illumination. Ideally, I'd like to reduce the illumination or actually turn the LED's off while not disturbing the rotation of the fan blades.

Inspecting the fans, each of the two leads from the LED's go to a circular PCB around the hub of the fan.

I have two ideas:
1. I can cut one of the two leads from each of the LED's and bundle all these ends to one side of a rheostat. Take the other ends of the cut leads, bundle them and place them on the other side of the rheostat. This is very messy and admittedly not very elegant.

2. The other idea was to possibly isolate the power from the PCB that powers the LED's and place them in a rheostat controlled circuit. However, I don't know if I can isolate the power going to the LED's without also interrupting the power going to the motor windings.

The approaches I've mentioned are hack quality and I'm looking for a better solution. I welcome a better plan.......please!

Psycogeec · Jul 26, 2011

2 different methods that can be used in Fan LEDs, must determine method used first.

1) Parellel with resistance:

the led powering is rather simple. even on the pcb itself.
it will be the 12v power going in and a simple resister to keep the total power to the led low.

+ ----<|-----{~~}-------- - something like that.

and multiples just repeat the same thing
+ |----<|-----{~~}--------| -
+ |----<|-----{~~}--------| -
+ |----<|-----{~~}--------| -
+ |----<|-----{~~}--------| -

if you split off the led itself OR between the led and the PCB, your still looking at 2 wires somehwere.
unless it was to just alter them ONE time. you could easily add a extra resistance in and reduce the brightness

+ ----<|-----{~~}--- ADD in resistance -{~~}----- - something like that.

hmmm,, neither is a solution, which leaves getting it down to 1 wire

by analising the connection, pulling ONE side of each led, and using "ground" or the + at another location to finish the connection.
with a parellel configuration, you could pull 4 leggs off and wire them together, and get your power and resistance elsewhere (like up by your controller)

at any rate, make sure you dont feed 12V in direct, the leds use from 2.2v-3.3V about depending on the color, and a resister is in place between them to reduce the total power to very small ammounts for driving them.
leds are a "current driven device" and the added restance keep the total current to them low.

and watch out for shorting to the case, or shorting out the + and -'s cause you dont want to lose a fan ports power.

===========================================================================================
you know it uses series style connection, if you pull ANY ONE led leg and they all go out.

2) SERIES with or without added resistance:
it is also possible that they could have Series the leds, say like 2-4 White or blue leds would run off of 12v. if they are series, then it is simpler again.

+ ----<|-----<|-----<|----<|----- -

+ ----<|-----<|-----<|----<|---{~~}-- -

a 4 led fan could have easily used a series configuration, if they were so inclined to think about it at the time .
this would be very simple, a bit of measuring off the led legs should be able to tell you the method used,

then you just pull the last leg off and send it to your "rheostat" and the other side of the rheostat to the line of the power it was originally connected to down there.
this would be a piece of cake, and take only 1 change on the fan, and only 1 wire from each fan item, assuming you feed the other connection at some other location.

more caveats: while messing with this stuff, dont forget that +12 and all the metal of the Case can be the - , a short could cause some big problems.
using a very small rated fuse (like .125 or .25 amp) would be usefull for the short term connecting/testing and even in the long term.

.

Lookingwest · Jul 27, 2011

Psycogeec said:
2 different methods that can be used in Fan LEDs, must determine method used first.

1) Parellel with resistance:

the led powering is rather simple. even on the pcb itself.
it will be the 12v power going in and a simple resister to keep the total power to the led low.

+ ----|>-----{~~}-------- - something like that.

and multiples just repeat the same thing
+ |----|>-----{~~}--------| -
+ |----|>-----{~~}--------| -
+ |----|>-----{~~}--------| -
+ |----|>-----{~~}--------| -

if you split off the led itself OR between the led and the PCB, your still looking at 2 wires somehwere.
unless it was to just alter them ONE time. you could easily add a extra resistance in and reduce the brightness

+ ----|>-----{~~}--- ADD in resistance -{~~}----- - something like that.

hmmm,, neither is a solution, which leaves getting it down to 1 wire by analising the connection, pulling ONE side of each led, and using "ground" or the + at another location to finish the connection.
with a parellel configuration, you could pull 4 leggs off and wire them together, and get your power and resistance elsewhere (like up by your controller)

at any rate, make sure you dont feed 12V in direct, the leds use from 2.2v-3.3V about depending on the color, and a resister is in place between them to reduce the total power to very small ammounts for driving them.
leds are a "current driven device" and the added restance keep the total current to them low.

and watch out for shorting to the case, or shorting out the + and -'s cause you dont want to lose a fan ports power.

===========================================================================================
you know it uses series style connection, if you pull ANY ONE led leg and they all go out.

2) SERIES with or without added resistance:
it is also possible that they could have Series the leds, say like 2-4 White or blue leds would run off of 12v. if they are series, then it is simpler again.

+ ----|>-----|>-----|>----|>----- -

+ ----|>-----|>-----|>----|>---{~~}-- -

a 4 led fan could have easily used a series configuration, if they were so inclined to think about it at the time .
this would be very simple, a bit of measuring off the led legs should be able to tell you the method used,

then you just pull the last leg off and send it to your "rheostat" and the other side of the rheostat to the line of the power it was originally connected to down there.
this would be a piece of cake, and take only 1 change on the fan, and only 1 wire from each fan item, assuming you feed the other connection at some other location.

more caveats: while messing with this stuff, dont forget that +12 and all the metal of the Case can be the - , a short could cause some big problems.
using a very small rated fuse (like .125 or .25 amp) would be usefull for the short term connecting/testing and even in the long term.

.

Psychogeec
Several thoughts. With regard to the parallel circuit, If I could just pull off the ground end of each of the 4 LED's of each fan, that would be quite easy. As you said, I could just bundle them together and ground them all somewhere else with a switch between the bundle and ground. Under these conditions, I wouldn't have to worry about adding in resistance since the resistance is on the positive leg. Since the entire case is acrylic, it would be easier to isolate ground.

Regarding a series circuit, I would pull the + lead before it went to the LED's run it through the rheostat and then it would return to the end supplying power to the series circuit. Essentially, I would be installing a variable resistor in the positive lead feeding the series circuit. When the resistance was turned up very high, the lights would go out. That would be simple also.

Have I got this right?
Thanks for your advice.
Lookingwest.

Psycogeec · Jul 27, 2011

yes,

wouldn't have to worry about adding in resistance since the resistance is on the positive leg.
but you must make sure that the resistance IS on the + side, there is no requirement for the resistance to be here or there, just somewhere in the curcuit, they could have put the resistance from the led to ground.
.

Lookingwest · Jul 28, 2011

Psycogeec said:
yes,

wouldn't have to worry about adding in resistance since the resistance is on the positive leg.
but you must make sure that the resistance IS on the + side, there is no requirement for the resistance to be here or there, just somewhere in the curcuit, they could have put the resistance from the led to ground.
.

....ah, your statement appears ambiguous. You say I must make sure the resistance IS on the + side. But, you also say that 'they could have put the resistance from the led to ground'. If the resistance is from LED to ground, wouldn't that mean the resistor is on the " - " side?

Psycogeec · Jul 28, 2011

yes, thst is still working in my brain

i was just saying not to assume that it is like this

+ |----{~~}---<|--------| -

because they can make it like this, and the leds will still work

+ ----<|-----{~~}-------- -

talk about ambiguous, I have the direction of the diodes wrong in the pictures, going to have to fix that.

How to mod LED fans to control LED's illumination

Lookingwest

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