I have a power supply to power an 8W LED board and a Raspberry Pi.
I have used a 24VAC (12-0-12V) 2A transformer to feed the bridge. I have also used a LM317 to provide a voltage of approximately 15.8V for the LEDs. The output is also used by the LM2576 to provide the 5V for the RPi. The V+ socket is connected to the LED PCB through a switch and the return to ground is an unbroken connection. The Rpi is always ON. The LEDs consume 500mA at 15.8V and the RPi consumes a maximum of less than 1A at 5V (measured using an ammeter.)
When I power ON, with the switch connecting power to the LEDs (full load,) the circuit works without a problem. However, when I switch off the LEDs after power ON, soon after a few seconds the RPi also goes down. The 5V output is now 2.3V and not sufficient for the RPi. The V+ reads 2.8V to 3.3V. Even if I power on without the switch connecting power to the LEDs, the RPi soon shutdowns due to low voltage.
This is really puzzling for me. How can a power supply work fine at full load but fail at lesser loads? Has anyone had such issues before and if so how did you solve it?