LED Brightness Control

[lakmalp]

Hi,

I would be much thankful to you if anyone of you could please suggest a way to control brightness of LED [with/without PWM] as described below.

when you,
* switch ON, LED brightness goes from 0 --> 100% within "X" seconds and stays there,
* switch OFF, LED brightness goes from 100 --> 0% in "Y" seconds,

Your help is highly appreciated!

Regards,
/Lakmal

[Tomi Engdahl]

Here are some ideas for LED dimming circuits
http://www.epanorama.net/circuits/led_dimmer.html

You could make that on/off dimming by having the control voltage generated by large capacitor charged (on power up) and discharged (on power down) though a large resistor. You might need a buffer (for example opamp) between capacitor on circuit voltage input.


Other way to do that could be a microcontroller using PWM generated with software.. microcontroller turns the transistor on/off as needed.
Here is one application note related to that topic
http://www.microchip.com/stellent/idcpl ... e=en524189

[lakmalp]

Hi Tomi,

As I have heard, we have to control current through the LED, but not the voltage. If we take a look at I-V characteristics of LED we can understand that we have to control current. Please see the below graphic:



If we do control voltage, it should be in between 1.5 -> 1.9 V which is quite difficult.

Regards,
/Lakmal

[Tomi Engdahl]

It is true that for sensible and reliable brightness control to control current through the LED, but not the voltage.
The LED output is pretty linearly dependent on the current through it. With a current controlling you get good control range and reliable operation no matter if LED parameters change (for example LED temperature affects it's voltage drop noticeably). By using a current source to drive a LED you get reliable operation no matter which is the LED voltage. This means that the same control circuit can work o wide variation of LEDs (different voltage drops).

Methods for current controlling:
- traditional adjustable linear current source/sink circuit controlled with voltage
- adjustable switch mode current source
- fixed current source and turning led current on/off with PWM control