Computer History: From The Antikythera Mechanism To The Modern Era

Apple I And II: Switching PSU And The Lack Of Cooling Fans

In 1976, one year after the release of the Altair 8800 PC, Steve Wozniak and Steve Jobs finished building the Apple I, a microcomputer that was based on the MOS 6502 CPU. A month later, on April Fools' Day, they founded Apple, and a short time later, the Apple II was introduced. Apple II was an 8-bit home computer that enjoyed huge success among users. It was designed by Steve Wozniak, except for its power supply (PSU), which was designed by Rod Holt, an Atari engineer.

Since the Apple II used a plastic case, which doesn't conduct heat well, excess heat would build up inside the computer if a low efficiency, linear PSU was used, like in the other computers of this era (e.g. Commodore PET, TRS-80, Atari 400/800 etc.). This is why Holt designed a switching power supply, and the Apple II was among the first home computers to use a PSU of this kind. The efficient and passively cooled (fanless) PSU also allowed the Apple II to operate without needing cooling fans, despite its plastic case that actually trapped the heat inside the chassis.

For those with an interest in PSUs, we should mention that Holt's PSU delivered 38W and had four rails: 5V, 12V, -5V and -12V. It was based on a very simple design that used a flyback converter. Contrary to Job's claims, the Apple II wasn't the first computer to use a switching PSU, since many computers, like the PDP-11/20, used similar PSUs in the late 1960s. During the mid-1970s, switching PSUs were also used in color TVs, so they were quite popular. Initially, the Apple II's PSUs were made by contractors, however once the demand increased a company called Astec took over the manufacturing process. The same company also handled manufacturing the PSU of the first IBM PC (the 5150), which was introduced in 1981. In 1999 Astec was acquired by Emerson, which currently is among the largest PSU manufacturers.

The first Macintosh computer, which was introduced in 1984 and become a huge success, especially among graphic designers, used a switching PSU with 60W max power. In addition to supplying power to the computer, the PSU also fed the CRT display, which are extremely power hungry.

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Contributing Editor

Aris Mpitziopoulos is a Contributing Editor at Tom's Hardware US, covering PSUs.