Simple Dimmable LED setup?

Hi everyone,

A friend of mine has a project which involves using a large amount (40+) standard 5mm LEDs to light a frame to a uniform brightness with the ability to dim the entire setup uniformly. Due to the shape of this frame its not economical or practical enough to use strip lighting.

Rather than try to explain the circuit design I came up with I have included it as an image.

Essentially I'm worried that I've overlooked something in this design and was wondering whether anyone with more experience would be able to take a look and point out any problems I've missed.

If anyone has the time I would be incredibly grateful.


Picture of Simple Dimmable LED setup?
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iceng2 years ago

Just use 13 parallel -max- strings for 39 total LEDs.

That
works out to about 290ma on the high end 14 in parallel using a -max- of 100 ohms and a
min current of 130ma with 200 ohms in series based on 3.3v fwd for a
white 5mm LED if I did my mental math correctly.

This is a DIY site and I would strongly recommend a 555 PWM oscillator which can handle 300ma for dimming your array of LEDs if you don't have the money or want make your own. Stay away from rheostats, they have to be matched to your LED load.

LEDimmer.gif
-max-2 years ago

It certainly will work, though I do not understand the purpose of R1, and R2. I think you are using them as voltage dividers, but that is not necessary. Just wire 40 of these in parallel:

(Vcc)---------l>|--------l>|--------l>|------/\/\/\/-----(ground)

(make R1 about 200 ohms if you plan to connect it directly to 12V, maybe less, like 100 ohms if you plan to have an LED driver or a rheostat to lower losses)

~~~~~

To change the brightness, the simplest solution would be to use a rheostat (variable resistor) but a better solution would be to use high efficiency LED driver rated properly to drive them LED's!

iceng -max-2 years ago

I don't think -max- meant to use 120 white LEDs.


verence2 years ago

Your circuit is not totally wrong, it might work, but it is ineffective and (in the LED part) too complicated. R1+R2 are not needed at all and the LEDs don't need to be stacked like you did. See this wizard to find a design.

For the dimming, using a variable resistor is wasteful. Did you calculate how much energy will be converted in VR1 when it is set to 10% LED brightness? Furthermore, the brightness of LEDs will not be linear with the voltage - far from that. Below its forward voltage, it will not be on at all. Around the forward voltage, little variations in voltage will have huge variations in brightness.

To dim LEDs, you give them always the designed voltage (like 12V) but switch that voltage on and off rapidly. As LEDs are super fast, the will also be full on or off. When the switching frequency is higher than our eyes can handle (>25Hz), the LEDs seem to be glowing continuously. The ratio of on and off time will define the brightness. See PWM (pulse width modulation) This instructable uses a NE555 to generate the signal.

What colour of LEDs are you using, and what current rating are they ? The scheme you have there isn't going to work well.