An ATX to IBM PCjr Adapter Board that lets you use a standard ATX power supply to power an IBM PCjr.
The ATX to PCjr Adapter will allow you to use a modern ATX-style power supply on an IBM PCjr. Perhaps you are missing the original IBM external power supply, which is frequently separated from the original computer and can be difficult to replace? Or perhaps you want the convenience and efficiency of modern switch mode power supplies? Look no further, the ATX to PCjr Adapter is the product for you!
This project is based on the work of AkBKukU's ATX2PCjr adapter. It was forked in 2021. The original idea and execution was AkBKukU's, but several changes have been made:
- Several remixes of the back mounting plate to accomodate the C&K rocker style switches, of various mounting methods, depending on parts availability.
- Small improvements in switch manufacturing were made
- The User's Guide was added
The IBM PCjr originally came with a 33 or 45 watt power supply and was made up of two components: an external 18 volt AC transformer ‘brick’ and an internal AC-to-DC converter board which provided +5, +12, and -12 volt DC power. The ATX standard provides all three of these voltages, making an ATX power supply a convenient solution.
The ATX to PCjr Adapter allows the PCjr to be directly powered by any standard 20-pin ATX power supply, replacing both the external and internal power supply components.
The PicoPSU is what makes the ATX to PCjr a truly remarkable product. As its name implies, the PicoPSU is an extremely small DC to DC power supply capable of converting standard 12 volt DC power into the +3.3, +5, +12, and -12 volt DC power required by the ATX standard. The PicoPSU provides an impressive amount of power relative with its footprint. Its small size allows it to be directly integrated into the ATX to PCjr Adapter, being almost entirely contained within the PCjr. As the PicoPSU has no moving parts, and due to its efficient design, it does not require direct active cooling.
The Mini-Box PicoPSU-80 is the recommended variant of the PicoPSU family, and can be found for purchase on Mini-Box’s website here.
Mini-Box also offers a kit containing the PicoPSU-80 and a suitable AC-DC 12 volt, 5 amp switching power adapter here.
There are many clones of the PicoPSU available on Texelec, eBay, Amazon, AliExpress, etc. which can also work.
The IBM PCjr was originally designed to have enough power supply capacity for the system itself, as well as one or two sidecar expansion adapters. While the recommended PicoPSU-80 can provide up to 60 watts of power, the PCjr system board itself was not designed to transfer much beyond the original 33 to 45 watts. If your PCjr configuration requires multiple sidecar adapters to be powered, the special IBM “power” sidecar should still be used, as the ATX to PCjr Adapter does not interfere with its operation.
The IBM PCjr Technical Reference manual section System Power Supply specifies +5, +12, and -6 volts DC. Unfortunately the Technical Reference is incorrect and inconsistent with regards to the negative voltages. Pages 2-135 through 2-139 inaccurately indicate -6, when instead the correct voltage is -12. Also, the connector specifications on page 2-138 incorrectly labels pin A10 as +12, when it should be -12. The Power Supply Board schematic on page B-23 correctly documents the output pins and voltages. The ATX to PCjr Adapter, as well as the original IBM power supply, outputs -12 volts rather than -6 volts on the A10 position of the power connector. The only devices which use negative voltages are the 8250A serial port and the LM358 op-amp on the cassette port, and both devices are designed to accept a voltage range of up to -15 volts.
An example IBM PCjr consumes approximately 26 watts at idle and 4 watts when turned off. When converted to use the ATX to PCjr Adapter, the same PCjr consumes only 17 watts at idle and 0.3 watts when turned off. The original IBM internal power supply board used active linear regulators, which are very inefficient and give off a lot of heat. When converted to use the ATX to PCjr Adapter, there is a significant reduction in the heat generated by the PCjr while in operation.
The ATX to PCjr Adapter is only compatible with the IBM 4860 PCjr.