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Using a PicoITX PSU to Replace AT-PSU
PSU is Power Supply Unit, and those large boxes inside old computers do the job of converting AC power into DC (typically either 12VDC or 5VDC). The PSU on many old systems were seemingly “over-spec” because, in those days, many people wanted a more “all in one” computer setup – meaning, they could also power on the monitor and main system in one step. So, you’ll often see two outlets on those power supplies, with the second one intended to go to a monitor.
Anyhow, long story short, those old computers didn’t really need all that much power. As you do start adding mechanical disk drives and hard drives, the power needs do increase. But I’d like to show how we can replace the old PSU with a modern day 160W PicoITX PSU using an adapter. This is also important because, inevitably, power supplies don’t last forever. I’ve built many computers over the years – I don’t recall yet a microprocessor or chipset failing on me (except in a case of accidentally dropping a screw onto a powered-up mainboard), but after 24/7 daily use I have had power supplies go out and need to be replaced (after about 5-8 years of use). So, while using an advanced PicoITX PSU won’t be period correct, it is preparing for an inevitable case of the original PSU likely being the first component to fail.
I do have some experience replacing a vintage PSU, such as in the 1978 IBM 5110 (here).
Converting AC to DC is a hot process that needs to regulate the incoming AC power. This is part of why PSUs often need a fan. These days there are far more efficient PSUs, and ultimately these digital computers just need DC power. Another appeal here is that the 286/386 generation were still both heatsink and fan-less CPUs. So, using a fan-less PSU means a more silent operation of the system.
BELOW: A typical “pico ITX” power supply. The name of these is not yet exactly standard, so you may find these under different names. They do depend on “something else” providing the main 12V DC input (which can typically be a “wal-wart” style AC adapter, or some DC-power source like a Lithium battery). The second connector (white end with yellow/red cable) is typically a Molex or SATA power adapter, or 4/6/8 pin GPU adapter.
The main thing to know is that 286/386 were pre-ATX standard. That standard became more widespread in the mid-1990s (1995). But prior to that, there was an “AT-style Power Connector” that many vendors complied with. There exist “ATX to AT” adapters that will help adapt PicoITX PSU over to power up an AT-style motherboard.
BELOW: A typical ATX to AT adapter. For AT-style systems, the “power switch” was straight on the PSU. So you would put some switch on the black/green cables shown below (something simple like these).
Wiring… The -5VDC was more common in the 20-pin ATX days, becoming N/C (not connected) and deprecated in the more modern 24-pin ATX standard.
TBD: how to wire up front panel connectors, and connecting up a power switch.