OC & Modding DIY LED Dimmer control - Need suggestions


PhOeNiX

Well-Known Member
Adept
Well, i got my new build iup and running, and i will be adding a WC kit with a lot of bling soon :p So this means loads of LED fans and Cathodes etc etc. Now i wanted to control the intensity of the LED's (of the Fans only at the moment,to start with) and i was thinking - why not use a fan controller knob setup to act as a dimmer switch.

Here is my idea -

Disconnect all LED's on all the fans, pass them all through a single channel and into a single 3-pin fan connector which will then connect to the fan controller. Now will this setup work ? Also what else do i need to put in the circuit, like resistors etc ? Also, any tips on designing the circuit ? Trying to make it as mess free as possible.
 

NinByChoice

Well-Known Member
Adept
Most fan controllers are PWM based. If yours is, then you should be ok. Actually, I dont see any issue with a resistor based dimmer. Its inefficient, but at a few Watts of load, it hardly makes a difference.

Btw, do be careful wiring the LEDs. At full speed, the fan controller will provide 12V to the LEDs. Depending on how you have wired them, might overheat/burn out.
 

PhOeNiX

Well-Known Member
Adept
NinByChoice said:
Most fan controllers are PWM based. If yours is, then you should be ok. Actually, I dont see any issue with a resistor based dimmer. Its inefficient, but at a few Watts of load, it hardly makes a difference.

Btw, do be careful wiring the LEDs. At full speed, the fan controller will provide 12V to the LEDs. Depending on how you have wired them, might overheat/burn out.

I was thinking along the same lines. And as per what i read in the article greenie posted, the PWM circuit is used mainly when different LED's are being used. Here i am connecting nearly identical LED's from identical fans.

Also, what would be the best way to wire the LED's so that they are not fried ? Also won't a resistor help by controlling the current and providing a Voltage drop ?

http://www.mikesflightdeck.com/led_dimmer.htm

How does this sound ?

Also i think a good fan controller has the ability to easily power 3-4 fans per channel, so i think it can easily handle 4x5 = 20 LED's per knob ? Also is it possible to put all LED's in a series with one resistor, or should i put them in groupings of 4 LED's in series with one resistor sets parallel to one another ? And which resistors would i need here ? I mean the specs.
 

cranky

Well-Known Member
Veteran
LED brightness is a function of current and not voltage, so essentially a resistor will help only marginally to dim the LED. This is what Greenie meant.

If you use a resistor in series, you will have a very narrow control range within a small change in the resistor value (or position of the potentiometer) and a very weak change in brightness over a large change in the value (or potentiometer rotation).

A PWM controller or a variable current source will be a much better way to control the LED. A current source can be built with a fixed LM7805 type of regulator or a single transistor, and a couple of resistors and the LED/s. A single LM7805 running off a 12V rail should be enough for all the LEDs in a single computer.

Edit: Here's a basic concept to get you started: http://radiolocation.tripod.com/LEDdimmer/LEDlampDimmer.html
 

PhOeNiX

Well-Known Member
Adept
Ohh. But if there is a voltage drop , does it not correspond to a current drop too ? As the resistance is constant in the circuit, won't Current be linearly proportional to voltage ? Do correct me if i am wrong, my electrical knowledge consists pretty much of my school and first semester physics and BEEE classes :p

Since i think the voltage part is being adjusted inside the Fan Controller, will i need an additional PWM controller between the fan controller and the LED's ?
 

cranky

Well-Known Member
Veteran
LED is not a resistor, it is a semiconductor. Ohm's law does not apply - which is how you're thinking of it.

The energy passing through an LED is used for emission of light and heat. This only depends on the current in the gap, not the voltage across it. This is because all LEDs have a fixed operating voltage and are sometimes used as (very stable and quiet) voltage references.

A series resistor is used to drop the incoming voltage to a safe level for the LED to operate correctly, but the effect on brightness is actually not very much except if you drop it so much the LED is starved for current and emits very little light. Until that threshold, not much change in visible brightness is seen.

Discrete control (mostly on/mostly off) is therefore possible if you use a large enough resistor, but not dimming.
 

PhOeNiX

Well-Known Member
Adept
What i meant was i use a circuit similar to what is shown below.

[FONT=Century Gothic, Arial, Helvetica]


[/FONT]The incoming voltage is from the FAN Controller, and is controlled by using the knob, something similar to this.



So wouldn't varying the voltage across the channel change the current accordingly too ? And the resistor i meant was the one as showed in the circuit in series with the LED's. Because with varying voltages being applied, the resistor in the circuit will change the amount of current flowing, won't it ? And won't that change the brightness ? [FONT=Century Gothic, Arial, Helvetica]

[/FONT]BTW, can you have a look at the following ? This is where i am basing my knowledge on :p[FONT=Century Gothic, Arial, Helvetica]

[/FONT]
[FONT=Century Gothic, Arial, Helvetica]Back to dimming LEDs [/FONT]

[FONT=Century Gothic, Arial, Helvetica]If you simply reduce the voltage to a resistor-LED series combination, you will dim the LED. The slight hiccup is that the LEDs will not begin to light up until there’s about 1.5 volts across each LED. If you have four LEDs in series, the power supply has to be cranked up to 6 volts to get those red photons started. [/FONT]

[FONT=Century Gothic, Arial, Helvetica]So if you’ve got a 0 to 12 volt variable power supply dimming your 4-LED chain, you have to turn the knob half way around before you get some action. A subtle point, to be sure, but an important one to the true dimming aficionado. [/FONT]

[FONT=Century Gothic, Arial, Helvetica]Here’s an inexpensive dimmer that starts at about 5.6 volts and goes up to 2 volts below the supply voltage. It uses readily available parts and can supply up to an amp of lighting current if the 7805 has an adequate heat sink. You could have up to about 50 groups of four LEDs, although the 7805 may start to get a bit toasty.[/FONT]
[FONT=Century Gothic, Arial, Helvetica]
[/FONT]​

[FONT=Century Gothic, Arial, Helvetica]Not all LEDs have the same forward voltage drop. The 2.0~2.2 volts common for red LEDs is not shared by all LEDs. Blue and white LEDs may have up to 5 volts across them when operating normally, although 3.5 volts is more common. If you want to use this dimmer circuit with 3.5 volt blue or white LEDs, put just two LEDs in series and use 150 ohm resistors in place of the 82 ohm units.[/FONT]

Isn't the left part of the circuit basically a fan controller for varying the voltage ?
 

greenhorn

Well-Known Member
Veteran
if you've got a fan controller which does PWM, why bother running the LED's seperately ?

^^ above circuit is a rather inelegant solution. I wouldn't do it :ashamed:
 

PhOeNiX

Well-Known Member
Adept
Well i don't want the Fan speeds to go down with the LED intensity. I want the two to be mutually exclusive. BTW how does one know if the controller controls via PWM or not :p
 

PhOeNiX

Well-Known Member
Adept
Hehe :p

Well my plan is this. To use my fan controller (the one posted in the thread or found here) to control the intensity of the LED's.

I was planning to disconnect the LED's and join all of them to a single channel with a appropriate resistor in the channel in series. This setup i would then connect to one controlling knob of the FC, and then use the knob to control the voltage/current in the circuit and control the LED brightness.
 

greenhorn

Well-Known Member
Veteran
that thing looks like its already running pwm. If you're using that to dim the LED's , you dont need a seperate DIY controller. all you need to do is make sure you put the appropriate resistor in series.
 

axeman

Ye Olde Systems Breaker
Veteran
LED's are very finicky animals. It may not be possible to DIM them by much, they will instead quickly shutoff at a point. So the range between bright light & working dim light may not be much, so even a knob may not be easy to use.
 

PhOeNiX

Well-Known Member
Adept
greenhorn said:
that thing looks like its already running pwm. If you're using that to dim the LED's , you dont need a seperate DIY controller. all you need to do is make sure you put the appropriate resistor in series.

Was asking that only :p So will the circuit i showed in my post above work ? And how do i decide on what capacitor to use ? And will a single knob be enough to power and control around say 20 LED's ? It can power 4 high speed fans, so i think 20 LED's will not be much of a problem ? Also which kind of wires should i use ? And anything to consider while selecting the resistor? As in quality etc

axeman said:
LED's are very finicky animals. It may not be possible to DIM them by much, they will instead quickly shutoff at a point. So the range between bright light & working dim light may not be much, so even a knob may not be easy to use.

Ohh. I'll try and use the setup with a couple of LED's first instead of the whole bunch :p Just to be on the safer side.
 

cranky

Well-Known Member
Veteran
If you try that schematic you've linked, here's what's going to happen.

As the voltage at the output of the 7805 increases, the LEDs will turn on when the current through them exceeds around 5mA. At this current you will barely be able to see them. Once it hits 6-6.5mA it will work at 80-85% of full brightness, and stay there all the way to current limit (around 30mA).

The current through the LEDs is calculated by:

Iled = (Vreg - Vf)/Rx

Where,

Vreg = voltage output of regulator

Vf = forward voltage of LEDs. This is constant no matter what, beyond the voltage threshold. 4 red LEDs will have a Vf of about 8-9 volts, for example.

Rx = resistor in series.

The other trick to use is to connect the ground reference of the regulator to the anode of the LEDs. The problem is you won't get a variable control over the brightness, but it is a constant current source.

PWM controller if you already have one, will work, but not with a resistor in series. The resistor damps out the control that the PWM needs to have over the load, so you will have to suitably modify the circuit, or use the exact number of LEDs after taking their Vf into consideration.

I think where you're missing the point is looking at voltage control using a resistor. If you had a fixed resistive load this would be fine, but LEDs being junction-based devices are not fixed loads. They strive to maintain a constant voltage across themselves (Vf) and will simply refuse to turn on at lower voltages.

If you have a series resistor, it will drop the difference between the input voltage and Vf, as the input voltage increases the current through the whole combination will increase for sure - but the resistor will still drop more voltage, increasing dissipation, changing its value, changing the voltage... you get the point.

The other problem is sizing the resistor appropriately, too small and it is dangerous for the LEDs and too large means not enough light.

The only way to tell is to try it out using some locally purchased LEDs first and see how it goes for you. Personally, I've never had good results using a series resistor without a small driver circuit for LEDs, unless it's a very cheap application. I would still look at a CCS of some kind. I've replied to your PM with my number, call me if you need to discuss it further.
 

greenhorn

Well-Known Member
Veteran
PhOeNiX said:
What i meant was i use a circuit similar to what is shown below.

[FONT=Century Gothic, Arial, Helvetica]


[/FONT]The incoming voltage is from the FAN Controller, and is controlled by using the knob, something similar to this.


this should work. try with some local LED's first

and do try to work out the polarity of the fans
 

PhOeNiX

Well-Known Member
Adept
cranky said:
If you try that schematic you've linked, here's what's going to happen.

As the voltage at the output of the 7805 increases, the LEDs will turn on when the current through them exceeds around 5mA. At this current you will barely be able to see them. Once it hits 6-6.5mA it will work at 80-85% of full brightness, and stay there all the way to current limit (around 30mA).

The current through the LEDs is calculated by:

Iled = (Vreg - Vf)/Rx

Where,

Vreg = voltage output of regulator

Vf = forward voltage of LEDs. This is constant no matter what, beyond the voltage threshold. 4 red LEDs will have a Vf of about 8-9 volts, for example.

Rx = resistor in series.

The other trick to use is to connect the ground reference of the regulator to the anode of the LEDs. The problem is you won't get a variable control over the brightness, but it is a constant current source.

PWM controller if you already have one, will work, but not with a resistor in series. The resistor damps out the control that the PWM needs to have over the load, so you will have to suitably modify the circuit, or use the exact number of LEDs after taking their Vf into consideration.

I think where you're missing the point is looking at voltage control using a resistor. If you had a fixed resistive load this would be fine, but LEDs being junction-based devices are not fixed loads. They strive to maintain a constant voltage across themselves (Vf) and will simply refuse to turn on at lower voltages.

If you have a series resistor, it will drop the difference between the input voltage and Vf, as the input voltage increases the current through the whole combination will increase for sure - but the resistor will still drop more voltage, increasing dissipation, changing its value, changing the voltage... you get the point.

The other problem is sizing the resistor appropriately, too small and it is dangerous for the LEDs and too large means not enough light.

The only way to tell is to try it out using some locally purchased LEDs first and see how it goes for you. Personally, I've never had good results using a series resistor without a small driver circuit for LEDs, unless it's a very cheap application. I would still look at a CCS of some kind. I've replied to your PM with my number, call me if you need to discuss it further.

Ohh. I get it now (just remembered my semiconductor lectures :p). What you are saying is say below 2V per LED, the won't turn on, but beyond this, even a small change will lead them to 80-90% brightness. But isn't the Resistor used for widening the range ? Also i think what you mean by LED's not being constant load devices is because of their back-current phenomenon ?

Will definitely try it once :p Don't want to fry anything in the beginning.
 

NinByChoice

Well-Known Member
Adept
cranky said:
the LEDs will turn on when the current through them exceeds around 5mA. At this current you will barely be able to see them. Once it hits 6-6.5mA it will work at 80-85% of full brightness, and stay there all the way to current limit (around 30mA).

Wrong! LED brightness varies almost linearly with the current (all the way from ~0 to max. current). Check any LED datasheet.

PhOeNiX said:
What you are saying is say below 2V per LED, the won't turn on, but beyond this, even a small change will lead them to 80-90% brightness. But isn't the Resistor used for widening the range ?

Perfect!!

cranky said:
PWM controller if you already have one, will work, but not with a resistor in series. The resistor damps out the control that the PWM needs to have over the load

Wrong again!! Resistors dont damp out the control/voltage. This circuit will work very well indeed.

@cranky, I dont even want to point out the other issues with your post! But I like the way you mix some relevant facts with BS to make your posts look credible.

There are 3 easy methods to go about this.

1) Resistor in series - requires 1 fixed resistor and 1 variable resistor.

Extremely simple circuit, but non-linear brightness response (1/R response, will work it out for those interested). Should be ok for your application. But cant really turn off the LEDs, just make them really dim.

Design
Fixed, variable resistor and LED(s) are connected in series to the supply.
Peak current of LED = 30mA
LED Vf = 3V
Supply = 12V

Fixed resistor (sized for maximum brightness, when the variable resistor is at 0 Ohms) = 9V/30mA = 300 Ohms

Lets say 3mA makes the LED look dim enough. Then with the variable resistor at full value we need 3mA of current.

Variable resistor = 9V/3mA = 3 kOhms = ~3.3k pot should work well.

Linearity wont be perfect, but you will still have good control. Dynamic range is from 10% to 100% brightness.

If you have more than 1 LED in series (which I suggest), just add their Vf.

2) Voltage regulator (like the 7805 circuit you had earlier or as a CCS) - 1 IC, few resistors, capacitors.

Quite simple and easy to build. Easy to find ICs too. Works great, almost linear brightness response. Can be designed to work as a constant current source if you want. Will work out the designs if you need it.

3) PWM - 1 IC, few resistors, capacitors, maybe 1 MOSFET switch.

The most efficient circuit of the 3 designs. Works very well. Might be a little trouble finding a good IC which integrates the switch too. Otherwise, can always use a 555 timer and a MOSFET. Will post the design if you want.

LED current driver ICs are available, but prolly end up costing more or harder to get.
 

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