In this blog post, I created an LED lighting controller for common 12V LED tape which was installed outdoors. I wanted to install some more LED tape but indoors, and this also needed its own controller but with different requirements to the first installation: there was no longer any need for the daylight sensor or the timer, but the ability to continuously adjust the brightness was required.
The lighting was installed as shown in the photo below.
As before, a PWM-free implementation was desired, and the idea of adding components to the feedback circuit of an LM2576 switching regulator module was chosen as the solution again.
The potentiometer was wired into the feedback circuit of the switching regulator circuit. However, it cannot simply be used as a direct replacement for the feedback potential divider, because only a limited portion of the potentiometer wiper will correspond to a useful range of voltages, and it will be possible to turn the voltage up above 12V and damage the LEDs. A combination of series and parallel resistors are added to achieve the desired voltage range.
The LEDs were powered from a bench power supply and the voltage range from nearly off to full brightness was determined to be 7V to 12V. The surrounding resistors would therefore need to be calculated so that the output voltage of the regulator is 7V when the potentiometer is turned to the left or 12V when turned to the right. It was also assumed that the potentiometer would be part of the low side of the resistor divider; this ensures that the voltage cannot jump up above the designed maximum in case the potentiometer gets dirty and goes high impedance.
The final circuit with calculated resistor values is shown below.
The procedure for working out the correct resistors to add to the circuit is very much experimental but I made a spreadsheet to make this task much easier. The numbers for R2, R1-Par and R1-Ser were changed repeatedly until the desired output voltage range was achieved. Once the ideal value of 73k for the upper resistor was found, this number was simply stuck into one of the many E12/E24 resistor finders available online to get the 4.7k + 68k series combination which would be used on the real hardware.
A small project box was chosen to house the circuit. Holes were drilled for the potentiometer, DC jacks, switch and cable. It was swiftly discovered that the two large capacitors on the power supply module were too tall to fit in the case, so these were desoldered and new low profile capacitors were soldered to the back side of the module, where they fit well in the case either side of the potentiometer.
The whole assembly was mounted to a bookcase and a potentiometer knob added to finish it off.
