- you'll learn about hacking the LM2596 DC-DC converter (eBay module) to make a dimmer for LED strips
- you'll see how to use a salvaged power supply from a car cooler box to power up the LED strip.
I found 2 modules for this project in the dumpster: an LM2596 module and a car cooler box to get the power supply from. Let's start with hacking the eBay module.
The DC-DC converter
The solution is to limit the voltage on the regulator using the feedback pin AND use a shorter (about 1m) LED strip to limit the current. As long as you don't exceed the 10W output restriction you can use these DC-DC modules. This would be enough for about 1m of these standard cheap LED strips depending on the current requirements, probably you can stretch it to 1.5m. Mine is limited to 0.75A and the length of the strip is about 1.3m.
Calculating the output voltage on the LM2596 is a piece of cake based on the datasheet. Here's the diagram from the datasheet:
These eBay LM2596 boards do not use the 1k recommended R1, instead they use a 330 ohm resistor as R1 and the trimmer (acting as R2) is a Bourns 3296 W-1-103 10k clone. Otherwise the circuit is straight from the datasheet.
This trimmer is no good if you want to drive LEDs, just plug the numbers in the formula: Vout = 1.23 x ( 1 + 10k/0.33k ) = 38.5 V maximum output voltage. That's by the way more than the 35V rating on the cap they're using on the output. So I wouldn't use these near 35V for sure. However, at 12V there shouldn't be a problem.
The mechanical stability of that little trimmer is another issue. That's not designed for constant adjustment.
The task is simple: to find an appropriate potentiometer that will change the voltage between 8 and 12V on your LED strip.
The 0.33 k resistor is a given on the board so just use the formula:
R2min = 0.33( 8 / 1.23 - 1) = 1.8k
R2max = 0.33( 12 / 1.23 - 1) = 2.9k
So the solution is to replace the 10k trimmer on the board with a 1k potentiometer and a 1.8k resistor in series with this acting as R2. That'd give a range of 1.8k - 2.8k, let's not push that LED strip too much... The maximum output is fine just below 12V. Just to confirm this with two quick measurements I used an 820 ohm resistor in series with a 1k first acting as R2:
I found the matching Molex connectors for the power supply connectors, too, after making a few measurements with a calliper. I had the contacts for these Molexes already, because of my Muonhunter project, so hooray, I'll have 230V connectors, too.
Putting it all together
Few words on safety
The higher the resistance of R2 the higher the voltage on the DC-DC converter as you can see from the datasheet. In my setup the voltage can go up to 13V in case the pot fails, provided the power supply doesn't fail (the supply has its own protection circuitry) and that's not good for the LED strip. So should this happen, the PTC would kick in. However, if you use let's say an old laptop power supply as your mains power supply, then the situation can get more serious in this fault case, since those operate around 18V, so the output voltage on the LM2596 can get up to 18V-ish.