Simple LED PSU Fade in mod (Works with any regulated PSU)

[Berni]

Recently installed a bunch of 12V LED lightning in the living room and along with the LED bulbs picked up a genuine WonHunLow switching PSU. The thing is pretty decently built inside considering how cheep it was(Fused, MOVs, clearances etc) but those chinese caps are probably not going to live long (Eh il replace em when it happens). I soon discovered that it had a almost 1 second startup time upon getting mains input. I fixed that by changing some caps, but then i got another idea. What if i gave the LEDs more style by making them fade in slowly like a halogen. And so this simple afternoon project was born.

The mod works by tapping in to the feedback circuit of the PSU in order to make it slowly ramp the output voltage. This is done by adding a large capacitor in series with the top divider resistor of the PSU. This cap slowly charges up and makes the feedback network see less voltage, making the PSU compensate by boosting the output up. That is until the diode across the capacitor starts conducting and stops the process.(I used a blue LED because i couldn't find a 3V zenner in my part bins)



That worked great alone, however i found out that the capacitor discharges very slowly. This meant that the PSU needed to be off for more than 30 seconds to make it ramp properly on next switch on. To solve that i added the transistor Q1 that shorts the capacitor when the output falls below about 7V. This is done by Q2 turning off when the divider on its input can't forward bias the base anymore and so lets R2 turn on the base of Q1. R3 is there just so that the base of Q1 does not see too much negative voltage and breakdown.



The circuit was built and frankensteined in to our victim PSU and it actually worked!



This is a scope trace of the output voltage, showing a nice quick shoot up to the point where the LEDs can turn on and then slowly ramping up to full brightens. On turn off later there is a slow output capacitor discharge that gives the mod circuit time to discharge the timing capacitor.

I re soldered the wires in to more permanent locations, put the mod cirucit in some heatshrink and buttoned the puppy up, ready to be installed in the living room. Since this mod just taps in to the feedback circuit i thought id share it since it can be done to any regulated PSU out there, but the resistor values might need to be adjusted for anything else than 12V.