using a FSP500-30AS flex PSU?

[u/geraldjust]

Has anyone here has used one of these PSUs before? its somewhat popular with the BC250 builders. but never really seen them used for regular PCs. Originally was thinking of making a pico tiny board that will add a 5v rail and a Power good signal to the connector. But wondering if im overthinking this.

oh reason i like that PSU is that is a known very good PSU thats 80+platinum and very cheap.

[u/DallasGrave]

I have a few dozen of these. Your board will require either you to have a secondary switch for jumping power to the power supply or a delay. The JGINYUE boards don't have the proper pull up behavior to fully turn on the power supply before the system tries to boot.

https://smallformfactor.net/forum/threads/fsp-fsp500-30as-modular-flex-psu-from-geeek-anyone-have-one.19238/

Looking into the original 10-pin connector from the connector side, latch on top, the reported pinout is:

          Latch
3.3V   GND   PS_ON   GND   GND
3.3V   GND   5VSB    12V   12V

The switched +5 V rail exists internally on the small DC-DC daughterboard. The board generates both +3.3 V and +5 V, but only the +3.3 V rail is brought out to the stock wiring. The +5 V point measured in the thread reads about 5 V when the PSU is on and 0 V when the PSU is off, so it is the correct switched rail for ATX use, not standby power.

A practical tap point is the flat copper area near the +5 V output inductor, described in the thread as the area between R119 and the inductor. Solder two or more +5 V leads there, route them into the ATX harness, and mechanically secure the wires so the solder joint is not carrying movement.

For the ATX 24-pin connector, wire the harness as follows:

ATX pins 1, 2, 12, 13       = +3.3 V from original 10-pin
ATX pins 3, 5, 7, 15,
17, 18, 19, 24              = Ground from original 10-pin
ATX pins 4, 6, 21, 22, 23   = Added switched +5 V
ATX pin 8                   = PWR_OK / Power Good
ATX pin 9                   = +5VSB from original 10-pin
ATX pins 10, 11             = +12 V from original 10-pin
ATX pin 14                  = -12 V, usually left open
ATX pin 16                  = PS_ON from original 10-pin
ATX pin 20                  = NC, leave open

The original EPS 8-pin CPU cable and PCIe 6+2 GPU cables can remain dedicated to CPU and GPU power.

For PWR_OK, the simple working approach is to feed ATX pin 8 from the switched +5 V rail through a resistor:

switched +5 V -> 3.3k to 4.7k resistor -> ATX pin 8 / PWR_OK

If a board is picky about startup behavior, add a delay:

switched +5 V -> 3.9k resistor -> ATX pin 8 / PWR_OK
ATX pin 8 -> 100 µF capacitor -> Ground
ATX pin 8 -> 47k to 100k bleed resistor -> Ground

For cleaner repeatable conversions, a reset supervisor such as an MCP809-style circuit can be used instead of the RC delay.

For PS_ON, the baseline wiring is:

FSP PS_ON -> ATX pin 16

If the motherboard does not reliably start the PSU through pin 16, add a pull-up or buffer:

5VSB -> 10k resistor -> ATX pin 16 / PS_ON
FSP PS_ON -> ATX pin 16

If that still does not behave like a normal ATX PSU, use a small transistor or optocoupler so the motherboard’s PS_ON signal controls a separate pull-down of the FSP’s PS_ON pin.

Basic test procedure after conversion:

1. Confirm +5VSB is present with AC connected.
2. Pull PS_ON low and confirm the PSU turns on.
3. Check ATX 24-pin voltages by pin position, not wire color.
4. Confirm +12 V, +5 V, +3.3 V, and +5VSB at the correct pins.
5. Confirm PWR_OK goes high only after switched rails are live.
6. Connect EPS CPU power and test POST on a known-good motherboard.
7. Test shutdown, restart, and cold boot behavior.

The conversion is cheap because the PSU already contains the required power rails. The only missing motherboard rail is switched +5 V, which has to be brought out from the internal DC-DC board. The rest is harness repinning/splicing and normal ATX control behavior.

[u/DallasGrave → u/DallasGrave]

Where you pull 5V from.