I have a lot of old computers. Many old computers have one thing in common: cheap, but annoyingly hard ot replace wall warts or power bricks with a bad habit of dying prematurely. I have a lot of computers, and many didn't come with a power brick. Many have dead power bricks, or power bricks made by Sinclair, and these days I have second thoughts about putting my multimeter probes on them, let alone plugging them into my Sinclair ZX80.
Realisation: I need some sort of power supply for my old computers. This description is still in its infancy, even though the PSU design seems is nearly done. Here are the features I'm going for:
- Maximise safety.
- Unnecessary complexity introduces the potential for failures. Go for simplicity.
- Don't leave things up to microcontroller firmware. Firmware goes wrong and can fry previous old hardware.
- Power supply design is an art form and it's well beyond me. Use ready-made modules for as many things as I can.
- Monitor voltage and current. Use a microcontroller for that.
- Maybe, maybe use a PC SFX PSU for some voltages. They're cheap and very reliable.
- Also power accessories.
- Allow space in the project case for more smarts, e.g. video input and conversion to HDMI.
Voltage
Many computers in the sweet spot of the Home Computer Boom used single-voltage power supplies, which is handy. Many (like the Commodore 64) used a second, higher voltage for analogue components like amplifiers, floppy drives, or video circuitry.
Computers that used 41164-kilobyte DRAM chips used three voltages, as the RAM chips needed +12V and ±5V DC supplies. Computers like the Newbrain and TRS-80 have horrendouslyweird requirements and came with truly byzantine power supplies by modern standards.
Check this out!
| Computer | VDC | VAC | Notes |
|---|---|---|---|
| Atari XE Game System | 5 | ||
| Atari 130XE | 5 | ||
| Commodore 64 | 5 | 9 | |
| Commodore VIC-20 | 5 | 9–11 | Some models may need 9 VAC, and no DC. |
| Commodore Plus/4 | 5 | 9 | |
| Atari 400 | 9 | No DC. | |
| Data General DG One | 5.8 or 6(?) | ||
| Commodore 16 | 9 | ||
| Didaktik M | 9? | ||
| Laser 110 | 9 | ||
| Oric Atmos | 9 | ||
| Oric Nova 64 | 9 | ||
| Sinclair ZX Spectrum 16K | 9 | ||
| Sinclair ZX Spectrum+ | 9 | ||
| Sinclair ZX-Spectrum 48k | 9 | ||
| Sinclair ZX80 | 9 | ||
| Sinclair ZX81 | 9 | ||
| NEC Starlet (PC8401) | 6 | ||
| Epson HX-20 | 6 | I'm unsure, but it's most likely. | |
| Cambridge Z88 | 6 | ||
| German Epson HX-20 | 6 | ||
| Acorn BBC Model B | 5, 12 | Not 100% sure. | |
| Acorn BBC Master 128k | 5, 12 | ||
| Amstrad CPC 6128+ | 5, 12 | ||
| Amstrad CPC-6128 | 5, 12 | ||
| Oric Telestrat | 5, 12 | ||
| Sinclair ZX-Spectrum +2A | 5, 12 | ||
| Sinclair ZX-Spectrum +3 | 5, 12 | ||
| Apple IIc | 15 | ||
| MGT Sam Coupé | 15 | ||
| OLPC XO-1 | 19 | ||
| TRS-80 Model I | 18–19 or 19–21 | 15–17 | |
| Acorn A4 | 21 | ||
| Mattel Aquarius | 8.8, 16, -19 | Weird PSU and 4116 DRAMs. Joy. | |
| Newbrain AD | 7.5, 13.5, -14.3 | Even weirder! | |
| TRS MC-10 | 8 or 10. | No DC. | |
| Dragon 32 | 9 (8.5), ±14.4 | Bizarre internal PSU design. | |
| Matra Hachette Alice | 10 | SECAM model, PSU may be different to the MC-10's. | |
| TI-99/4A | 16, -8 | Asymmetrically tapped transformer. | |
| Acorn Electron | 18 | 17.1–20V AC, no DC. |
So, the power requirements are as follows:
- Positive DC: 5, 5.8, 6, 7.5; 9, 12, 13.5; 15, 16, 19, 21V DC.
- Negative DC: -19, -14.3V DC.
- AC: 8, 9, 10, 11, 14.4, 16, 18V AC.
It's likely some of the voltages are superfluous, since many of these computers regulate the power internally and are happy with inaccurate input voltages.
Current
The requirements there are quite low, thankfully. Many computers used the 7805 regulator, and that only provides 1.5 A. Nothing like the insane currents put out by modern PC power supplies.
Computers with floppy drives and inductive loads needed more, but that was what the 12 V rail was for anyway.
Negative voltages were used mostly for bias voltages and the occasional op-amp, so low current is acceptable.
Protection
This is the point of the whole project. Every outout needs to have programmable overvoltage (and ideally) overcurrent protection and should be switchable. Every rail should have resettable fuses too.