Small USB PSU for STFs/STEs

[olivier.jan]

After getting shocked a few times while working on STs directly connecting to mains, I was looking for a safer solution and designed this small USB PSU.
It basically uses standard "AliExpress" USB-PD decoy which produces 15v from a USB-PD charger and convert it to 5v and 12v. The whole design is rated for 4A which should be OK for most STs.

IMG_2482.jpg

IMG_2480.jpg

IMG_2481.jpg

I just finished soldering the first one this morning and so far it looks OK, although I haven't plugged it to an actual ST yet.
I've tested it with the oscilloscope and voltages are good, with steady 5V and 12V, with 680mV noise for 12V rail and 120mV noise for 5V rail.

SDS00012.png

SDS00013.png

The only worry I have so far is the setup time, as it takes more than 7ms to reach proper voltages, could this be dangerous for the ST ?

SDS00011.png

As this is the first time I design a power circuit, I'm not too sure about what can go wrong. I already discovered that using 402 sized SMD components is a pain, but apart from that any feed back is welcome.

PS: The DiagCart on the picture is only there to give an idea of the actual size of the circuit.

[olivier.jan]

Forgot to mention: Overall costs from JLCPCB (PCB) and LCSC (Components), including shipping to France was 12€ per board.

[viking272]

Nice work Olivier, this sounds like a great project. Such a small footprint too.
It reminded me of Sporniket's work but that was a USB phone charger I believe.

[sporniket]

Yes, mine was just using a USB decoy to output 5V up to 3A directly, with a little 5V->12V to get the required 12V for video output.

My last attempts did use a 5V/4A power adapter with 5521 jack barrel. Works ok with my stock STE, but my contraption reusing the normal switch and a hacked main plug to 5521 jack adapter does not works well with the SmartBoard STe, as the mono/color detection result is inverted (workaround : leave the ST power switch on, and switch on/off the main powering the adaptor).

Back to @olivier.jan design, I feel that the noise is good enough for the 5V ? I remember that @exxos has looked into noise and in its psu page : https://exxosforum.co.uk/atari/last/psu/index.htm and there, a fixed psu (first model) sees its noise reduced to 400mV.

Also, if you wish to perform further testing without plugging it on the ST yet, you should also buy an electronic load (if you don't own one already) to see how much current you can actually sink without overheating, and also see how noise there is with the load increasing. For informative reference, I bought this one : https://fr.aliexpress.com/item/4001153199791.html as you see it's quite cheap.

[olivier.jan]

After chasing red herrings for a while, I finally tested this small board and it works perfectly. I was concerned about the rise time of the power rails (7ms) and started adding some ICs to reduce it. After double checking with original PSUs, this 7ms is actually an order of magnitude faster than the original PSUs, and fully covered by the reset circuit, so no problem here.

So, now, next step are:
- Add an USB-PD IC and connector to the board.
- Add a few LEDs to have a visual feedback of the PSU being on or off.
- Add a power switch to turn it on/off
- Redesign the PCB to fit vertically in the STF/E along the back panel (STE having audio outputs there requires the PCB to be shorter).
- Add an USB-A connector to power external devices (UltraSatan, ACSI2STM, etc…)
- Get a good 3D model to attach it in place of the original PSU and power switch holes in the STF/E case.

[viking272]

Nice work!
USB-A sockets sounds like a great feature.
Can they align with the power socket in the case to allow external use?
You talk of ST use but is Falcon possible?

[olivier.jan]

Does it align with the existing power socket hole: yes, that’s the plan.
Concerning Falcon, honestly I can’t tell: I have no Falcon to test it. Voltage will be OK, but I suspect the Falcon might require a bit more power than the ST. At the moment the circuit can handle around 4A on the 5V rail, I don’t know if this Ok for a Falcon. I guess someone with a Falcon will have to test and check for smoke…

[DoG]

Falcon have brackets for internal IDE. So that needs power. But everyone has probably upgraded that to CF card by now. And a fan.

This is taken from the service document about Falcon:

Output	Voltage	Voltage
Voltage:	+5 VDC	+12 VDC
Minimum Load Current:	1.5 Amp	0.0 Amp
Maximum Load Current:	7 Amp	0.2 Amp
Line Regulation:	5%	10%
Load Regulation:	5%	10%
Cross Regulation:n:	5%	10%
Ripple:	100 millivolts p-p	100 millivolts p-p
Noise:	100 millivolts p-p	100 millivolts p-p

It also says:

+5 VDC Set Point: 
The +5 VDC output must be
set to +5.05 0.1 VDC with a
+5 VDC output having a load
of 4.0 amperes.

[olivier.jan]

Just finished a new version, I ordered the parts and PCB this morning. This one includes the USB-PD circuit and is designed to fit vertically in the STF/STE along the back of the computer. Let’s see in a few weeks if it works correctly. So far only 3D renders.

3D_ST USB Full_2025-09-13.png

3D_ST USB Full_2025-09-132.png

[JezC]

Looks good - will await your next update with interest!