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Power Supply
The power supply of the PS1 is an all-in-one provider for the following:
- Power outlet hookup through a non-standard C7 power cord.
- Rectifies, stabilizes and transforms the AC domestic voltage levels to roughly 8v and 3.3v.
- Handles power on and off by a dual pole switch on the 8v and 3.3v lines.
- Handles reset through a momentary switch.
- Delays and monitors the reset logic through a supervisor chip.
- Provides power and reset logic to the motherboard through a 5-pin cable.
The final pinout of the power plug ends to be always the same on all versions of the console:
| 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|
| 8V | GND | 3.3V | GND | !RESET |
The motherboard then breakouts the power plug into the following subrails:
- "MOT_+8V", with a 0.8A fuse, a 1uH inductance, and a 47uF 16V polarized capacitor.
- "EXT_+8V", with a 0.8A fuse
- "DIG_+3.3V", with a 2A fuse, a 1k pull up resistor to !RESET, and a 220uF 4V polarized capacitor.
- "CTR_3.3V", with a 0.6A fuse.
The datasheets are claiming that the second power rail is 3.5v, but it magically becomes 3.3v on the motherboard itself, without any component in between. Doing some investigation shows that using a plain 3.3v power line will work fine.
The 8v power rail has two main consumers. The first one is the cd-rom and its motors. The second one is a bit more peculiar: there are two LDO regulators that are chained to each other, behind a 0.8A fuse.
The first regulator is a 5v regulator, that brings power to the audio amplifier and video d/a converter, both behind their individual inductance and capacitors, and provides a "DIG_+5V" power rail, also on its individual inductance, used by the audio dram chip.
Then the second regulator is a 3.5v regulator, breaking out to its own power rails of "AUD_+3.5V", with an inductance and capacitor and "SER_+3.5V", with only a capacitor, both feeding into the analog power rails of the SPU.
As for the 3.3v power rail from the PSU, the CTR_3.3V rail is used exclusively to power on the controllers, while the DIG+3.3V rail powers on every chip of the motherboard. In fact, the SPU uses a total of 3 power lines: DIG_+3.3V, used on many different pins, and also the AUD_3.5V and SER_3.5V rails, used on fewer, dedicated pins.
It seems fairly clear that the 8v power rail from the PSU is used mainly to handle the higher voltage motors, and anything that's analog signal related - with the exception of providing 5v to the SPU DRAM chip - while the 3.3v power rail from the PSU is used exclusively for anything digital and logic-related.
The reset circuitry is mostly handled through the PSU itself, active low as typical for chips, with the exception of a single 1k pull up resistor on the motherboard itself. This means that the motherboard can be powered on with simply the first 4 pins of its power plug, and the reset signal will be naturally asserted high, meaning all of the logic will boot and hopefully work.
The reset controller chip on the PSU will do 3 things:
- Monitor the power levels of the 8v and 3.3v power rails
- Listen for the user button for issuing a reset on the console
- Debounce and ensure a minimum reset delay on power up and user reset
During testing, not providing a proper reset signal will sometime result in bootup glitches, up to complete system freeze.
Preliminary tests shows that the console is consuming about one amp with no accessories connected. Further analysis is required.
There are several reasons why one would want to replace the original PSU with something different.
- The power on switch is AFTER is transformer, meaning that the AC domestic voltage section of the PSU is always energized and flowing through the transformer's coil. This has two consequences:
- The PS1 will always consume electricity, even its main button is powered off, which is a waste of power.
- When working on the system with the plastic shell removed means there is an exposed AC domestic voltage lingering around, even when the console is turned off, which can prove dangerous.
- One may want to remotely turn the console on or off, or reset it remotely, for automation purposes. Note that asserting the reset line low means that the various memory chips aren't erased, which can be useful in some debugging scenarios. Also, the PIO port won't lose power during a reset.
- One may want to replace a defective power supply board.
- Using DC 12v as a power input can sometimes be more accessible than AC domestic.
While the console shouldn't require too high of a power supply, a strong 5A 12v power supply is considered, such as https://www.amazon.com/gp/product/B01461MOGQ/
In order to build a compact, versatile and flexible replacement PSU, it is best to simply use two holes on the PCB to provide power. Then, one can simply use the power connection of their choice. Still, a typical 5.5x2.1mm connector, as a pigtail wire is probably a wise choice: https://www.amazon.com/dp/B0768V9V5Q
The soft power on sequence will be handled through a solid state relay, such as the KF0602D. This provides the added feature of having several built-in diodes, protecting against backward currents, in case the power supply is wired wrong.
The LM7808 in TO-220 package is considered, but any 8v LDO regulator should do the trick nicely. The typical pinout is going to be
| 1 | 2 | 3 |
|---|---|---|
| Input | GND | Output |
There are several competing standards when it comes to 3.3v regulation, with regards to pinouts. Some are available with the same pinout as the LM7800 series as described above, such as the AZ2940-3.3. But the most common, the LM1117-3.3 (or its cousin, the AMS1117-3.3) is unfortunately different from other sorts of regulators:
| 1 | 2 | 3 |
|---|---|---|
| GND | Output | Input |
Since the 3.3v regulator will soak 8.7v worth of power, it may emit a non-trivial amount of heat, and also consume more power than needed. It might be worth investigating buck regulators for the 3.3v power rail. The TIDA-00947 demonstration board from Texas Instruments, centered around a TPS54202 buck regulator could be worth such investigation, given its low EMI, and TO-220-style package.
Heat sinks should be considered, such as https://www.amazon.com/gp/product/B07G5ZV2J7/
It is worth to note that if the LM1117-style pinout is used, the central pad is going to be energized with 3.3v, instead of being grounded like with the other pinout. Care needs to be taken.
While the original PSU monitors both the 8v and 3.3v circuits, it might be worth simply monitoring the 3.3v power rail. This results in easier logic to put in place. The MCP100-315 is a good candidate.