hacking the KIS-3R33S power supply to drive 10W LEDs

I've been looking for a way to drive power LEDs from a computer for my terrarium lighting. Newer LEDs are giving 160lm/W, which is on par with CFLs and is 'easily' computer dimmable. But to get that performance requires a fairly efficient power supply.

I noticed the KIS-3R33S 3.3V supply on special for just $0.5 each on a drop shipping/distressed stock internet shop, so I ordered a box to see whether I could hack them into independent supplies for driving LEDs:

Out of the box they happily drive a white LED at pretty much the perfect current (fluke I think) and thus I can fill my terrarium with 16000lm, an astonishingly large amount of light, and plenty for the plants I want to grow. However, I also wanted to use this light as an alarm click to replace the 150W par38 spotlight. This meant dimming the light (or really, undimming it in the morning). Now the KIS has an enable pin which could allow me to simply turn them on and off with PWM. an oscope confirmed the switching frequency was around 340kHz, so turning them on and off at 1kHz should be fine. I tried it out and unfortunately the LEDs tended to flicker, especially below 50% brightness.

Looking at the chip used (MP2307) I realised that the problem is the soft start feature, which is designed to bring the load up gently, avoiding transients and whatnot. But for leds this is exactly not what I want (LEDs actually prefer to come straight up to full current).

It turns out that the circuit is close enough to the data sheet that we can just remove the soft start cap with a side cutter.

A little bit messy because I was trying to video the process at the same time, and it just ended up being a bit of a botch. Luckily I have a box more to practice on :)

Sure enough, now I can get a smooth dim over 1024 steps without any flicker. And the colour is better when dimmed, too.