How to build LED dimmer for room illumination - PWM, low pass filter?

Question

I want to illuminate a room. This will be done by two LED strips 60led/m, each 5m long, located along the tops of opposite walls.

In order to get full light power, they together consume 4A@12V. There's also additional 4m of cable needed to power each of these strips. In total, I get 18m of cable around the room.

Then, I want to have a dimmer. The simple way is to have uC switching a transistor with PWM. But having such a large antenna pulsing with 48W seems to be unwise.

What I learned, is that I could use an LC low-pass filter, to flatten the PWM on +12V line to a lower voltage.

EDIT: This is the latest diagram of the circuit. The only things left are uC and dimming potentiometer. C2 and L1 are calculated for 5kHz PWM.

Answer 1

This is a good idea. Theoretically you could use the pwm as is to dim the lights since an LED shines brighter the longer the on-periods are. Your concern about sending waves might be negligible as long you stay in the lower kHz-domain, yet no blinking will be seen for the human eye. So first I suggest you to make a roughly 5kHz pwm signal and apply it to your LEDs.

If you want to use a LowPass the calculation is easy: f=1/(2*pisqrt(CL)) This describes the -3dB frequency. So if you apply any signal, frequencies above f will be attenuated by 3dB at f, and falling with 40dB/decade. Example: choose C and L for a frequency of 100Hz, then apply a signal, of 1000 Hz and it will be 40dB less than the original signal, means devided by thousand. A 10kHz signal will be have -80dB, means there is nothing left of the signal.

So in your special case my suggestion is: make a 5kHz square signal from a controller and make an LC-LowPass for f=100 Hz (choose C e.g. 10 microF) and calculate L accordingly. It will produce perfect DC proportional to the pulsewidth.

Only your cicuit layout is wrong and won't work.

^{simulate this circuit – Schematic created using CircuitLab}
Added: Diode D1 added. Inductor current cannot be interrupted. Current flows as per top arrow when Q2 is on and via D1 when Q2 is off.

I would try this, but I drew it quickly, so someone please look over it again! Hope this helps