LED Dimming and Brightening

[Lordbob75]

Alright everyone, I can find no good reason for why this is occurring for me.

As some of you know, I am trying to make a fan speed controller. So far, I am successful, save for the LEDs.
I am using a 25Ohm potentiometer to control the fan speed, and that part works beautifully. The issue is that I want to use an LED indicator that glows bright at max speed, and is very dim at low speed.

I have the set up like this:
12V from PSU connected to the 25Ohm potentiometer (prongs 1 and 2 connected), then the Fan and LED (with 500Ohm resistor) in parallel.

The fan slows and speeds up as intended, but the LED does not change at all...

Can anyone help me out here?

~Lordbob

 

[The Howling Wolves]

Alright everyone, I can find no good reason for why this is occurring for me.

As some of you know, I am trying to make a fan speed controller. So far, I am successful, save for the LEDs.
I am using a 25Ohm potentiometer to control the fan speed, and that part works beautifully. The issue is that I want to use an LED indicator that glows bright at max speed, and is very dim at low speed.

I have the set up like this:
12V from PSU connected to the 25Ohm potentiometer (prongs 1 and 2 connected), then the Fan and LED (with 500Ohm resistor) in parallel.

The fan slows and speeds up as intended, but the LED does not change at all...

Can anyone help me out here?

~Lordbob

Here's the man that can do it.
View attachment 75525

I couldn't help myself.

 

[kodi]

Try to connect the LED's direct and not through the potentiometer

 

[mpcrsc562]

Can you post a schematic?

 

[WindowsStar]

I don't think you are going to get the result you want. LEDs typically are on or off they are diodes. Some LEDs can be driven harder to be brighter but making that kind of change would be a huge change for a fan. You will need to use an incandescent bulb to get the effect you are looking for. There are some very cool bulbs now-a-days that even look like a RED LED.

 

[A Guy]

What is the voltage rating of the LED? What are the high and low range voltages when changing the fan speed pot? It may be they are indeed dimming, but it is a relatively small change. It may also be that the range is already near the brightest setting of the LED and therefore they are already almost fully bright even when the fan is turned all the way down. Just a thought. A Guy

 

[A Guy]

I don't think you are going to get the result you want. LEDs typically are on or off they are diodes. Some LEDs can be driven harder to be brighter but making that kind of change would be a huge change for a fan. You will need to use an incandescent bulb to get the effect you are looking for. There are some very cool bulbs now-a-days that even look like a RED LED.

LEDS can be dimmed, but I'm not sure this is true of ALL LEDS. The LED's intensity can be determined by the level of voltage being supplied. Or the LED's intensity could be controlled using pulse-width modulation (PWM). That is beyond me though.

A Guy

P.S. I'm a TV Tech BTW, mostly boards these days, lol. Newer LED lit LCD TV's have local dimming, very complex though.

 

[WindowsStar]

If you are talking about LED back lighting on LCD TVs they don't dim them. They turn some on and some off, the more that are on the brighter the less that are on the dimmer. They look like they are dimming because the light reflects off the backing and other LEDs.

O = on and o = off

OOOOOOO
OOOOOOO
OOOOOOO
OOOOOOO = all on bright

OoOoOoO
oOoOoOo
OoOoOoO
oOoOoOo = half on half off dimmer

ooOooOo
OooOooO
ooOooOo
OooOooO = very dim

 

[Lordbob75]

It is a red LED, so between 1.8V and 2.2V. I cannot draw a diagram right now, but here is the best I can do:

12V in from PSU to
25Ohm potentiometer to
(500Ohm resistor to red LED) and fan

Does that help? The fan and (resistor and led) are in parallel.

~Lordbob

 

[kodi]

One wire from PSU to Pot to fan
Another wire from PSU to Resistor to led

 

[Lordbob75]

One wire from PSU to Pot to fan
Another wire from PSU to Resistor to led

That would not dim it at all though...

~Lordbob

 

[kodi]

Sorry must have been another "Seniors moment" I will have to read the original post more carefully before i try to help.
I thought you wanted the LED's to stay fully on and not dimmed.

 

[Lordbob75]

Sorry must have been another "Seniors moment" I will have to read the original post more carefully before i try to help.
I thought you wanted the LED's to stay fully on and not dimmed.

It's alright.

I want the LED to dim when I turn the fan down, and brighten when I turn it up.

~Lordbob

 

[WindowsStar]

Again the simplest way to do this is to buy a small red or blue incandescent bulb to get the effect you are looking for. Very simple the bulb will work just like the fan does using the POT. Unless someone takes it apart you will never know it is not an LED.

 

[kodi]

I have just done a quick Google search and have found out that red LED's require a lot less voltage than other colors you may not be getting it to dim because there is still enough power to light it fully.
If blue is an option i would try it

 

[archie123]

you do have the resistor connected in series dont you? it wont work otherwise , well it kind of will but you wont notice.

 

[WindowsStar]

Looks like if you use Highly Efficient LEDs and a circuit board you can dim some LED(s).

Check this out: Highly Efficient 0-100% LED Dimmer

or

 

[Lordbob75]

Thanks Windowstar. Not sure I can figure that out on my own, or what. I will talk to my dad about it tomorrow and see what he says. I may just leave it with a regular LED (no dimming) because the knob has an arrow on it.

~Lordbob

 

[A Guy]

If you are talking about LED back lighting on LCD TVs they don't dim them.

No, was more an offhand remark. But LED's can be dimmed.

I do believe it has to do with getting the voltage down far enough to allow the led to dim. Perhaps experiment with more resistance until you find the correct amount to allow a dim LED at lowest fan setting (or lowest you will actually turn it down to, lest higher than complete lowest again gives you a bright LED), and brightning at the full brightness voltage. It may well be that the voltage difference between dim, and full bright may be too small a difference for you. A Guy

Did some googling:

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

Starts getting into building a circuit, and finding the right parts.

 

[archie123]

ill ask again just incase.........do you have the led conneted in series with the variable resistor? if you dont it wont work because of the way leds handle voltage and current

just incase your wondering why im spouting all this nonsence ive been an electrical engineer for 20 years and have dabbled quite alot in electronics