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In the top answer to Why were animated screensavers used instead of a black screen on CRT monitors? the main reason was said to be that it avoids users accidentally turning off the machine, thinking it is turned off and trying to turn it on.

Well, that's indeed not a good thing to happen, especially back when it posed a real risk to hard drives. But... who had the idea of using power buttons that allow this issue to happen in the first place? It seems utterly trivial and obvious to avoid it by using flip power switches with a built-in light, like those found in many devices (guitar amps, fridges, power strips...).
In more modern computers there are of course real reasons for having push buttons (which anyways aren't directly connected to the power), but back when switches really were just mechanical switches this doesn't apply.

So: why use push power buttons?


Clarification: I'm specifically talking about the early mechanical push switches used in the 80s, which actually did power the machine hard-off with just a simple press, not only send a signal to the OS to shut down gracefully. (I thought this was obvious from the original questions, but seemingly not.)

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    I wonder whether "real estate" was a factor. For a toggle switch, you don't want the switch to protrude from the case, as it'd be too easy to bump. So it has to be recessed, as in the PC Big Red Switch that Raffzahn's answer shows. But that means the switch and its housing now occupy several cm of depth in the case. Moreover, the recess, and therefore the housing, needs to be large enough for the operator's finger as well as the switch itself. A pushbutton switch can easily be flush with its housing, or even slightly recessed, and take up quite a bit less space. Apr 23 at 17:39
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    @BrianH: The sound of a fan is not so "obvious" in a noisy office or industrial environment. Apr 23 at 17:42
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    All of the answers so far assume IBM products (including PC). But there were other unrelated computers with push-on-push-off switches. I'd like to see an answer address the TRS-80 Color Computer, which had such a switch.
    – DrSheldon
    Apr 24 at 3:48
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    With the current title, my first impression on seeing the title in HNQ was that it was going to be about soft power switches (ATX style). Perhaps since current machines have a momentary soft power switch at the front, and a rocker / flip hard power switch built-in to the PSU if at all. The bold clarification is still necessary, but perhaps we can clarify the title without making it excessively long, perhaps using the word "hard"? Apr 24 at 4:33
  • Maybe "When/why did early computers have a push-button hard power switch instead of flip or rocker to mechanically cut main power"? More technical phrasing so I'm not making this edit myself. "Push-button" is apparently the technical term for a non-momentary switch that works like a ball-point pen where you can click it in / out, and talking about "cut main power" reinforces the point that it's not a soft-off. But your phrasing reinforces the actual problem it creates of off being the same action. Apr 24 at 4:35

5 Answers 5

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TL;DR:

So: why use push power buttons?

The same reason that it is used in other appliances: it's convenient.


The Works:

Just think, early (IBMish) PC had their power switches at the right side all the way at the back (*2). One had to to feel one's way all around to flip it - of course with no chance to look at its state. The switch was part of the power supply. This made sure that all high voltage switching only happens within the PSU.

enter image description here

_(Picture taken from Wikipedia)

Putting it up front wasn't really a solution as this would break the encapsulation of high voltage within the PSU. Keep in mind, the PC was a device intended to be opened by any average user, thus exposure to high voltage was a complete no - usually prohibited by most electric codes worldwide. The same reason goes for why it had to switch mains, as only that would guarantee that the PC is completely powered off if switched off but not unplugged - which, depending on workspace setup may not always be possible.

Moving the PSU upfront was not really a great idea, as that would mean having the power cord coming out on the side. Only a PSU reaching all across the case would avoid that.

Using a push switch is about the only (*1) solution, implemented by extending the visible push button by a long 'stick', going all the way back to the PSU, pushing the 'real' toggling power switch. Now the user could operate the power switch from the front of the computer while still having a code-legal encapsulated PSU at the back.

The original AT-class (286) HP Vectra of 1985 was one of the first IBM(ish) PCs to offer this.

HP Vectra

(Picture taken from Wikipedia; marking mine)

Long story short: It's all about convenience.


Equally important, adding a certain type of switch is a design decision not necessary based on technical issues, but what look/feel/operation habit a designer wanted to achieve.

Besides, in real life, no user checks the state of a switch before flipping it, when he already assumes the computer being off - after all, he already ignores other easy to check signs like lit LED on keyboard, case or monitor.


*1 - which BTW was the same with the Apple II/III series, undoubtedly the major prototype for the IBM PC, with their enclosed PSU and a switch on the back.

*2 - Of course, like almost always there are other possible solutions. For example using some secondary low-voltage circuit, much like for the later power button, but operated by a switch. Just, this would complicate the PSU considerably, and thus increase the price - not anything compatible manufacturers, usually fighting on cost base, would want.

Equally, if not more important:

It would be a technical divergence from the example set by IBM. IBM had a fully concealed PSU. Having a different design, no matter whether better or worse compared to IBM, has always been used as a point by IBM sales force - and even more in the mind of decision making on buyer side. So it was extremely important for makers of compatibles to withstand that. A compatible had to be the same, or better, without changing anything. It took many years until the market was levered in a way to incorporate new ways.

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    But a) it's not more convenient: a flip switch is exactly as easy to use b) most appliances don't use push-push switches, but instead something with distinct on- and off-directions, even though accidental switch-off would typically have less problematic consequences than it does in computers. Apr 22 at 21:22
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    @Raffzahn I don't want to debate this, I want to hear the real reason for this design decision, which “it's convenient” clearly doesn't provide. And yes, people absolutely do check the state of a toggle switch. They certainly look at it (because the eye guides the finger much quicker to the switch than blind touch-searching), and when the switch is glowing that's a very obvious warning not to press it for turning the computer on. Even without a light, one would quickly get used to which orientation of a daily used switch is on and which is off, and pressing the other side has no effect. Apr 22 at 21:32
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    Now the answer makes complete sense, with all the context – it is in fact an interesting story. I still disagree with the tl;dr though, because the gist isn't at all that a push switch is per se more convenient than a flip/lever – but instead that this was the only feasible way of moving the switch to the front, and the advantage of having it on the front outweighed the inherent disadvantage of the push design. Apr 22 at 22:24
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    Many of IBM's early PS/2 machines had power supplies that reached all the way to the front, with a rocker switch for the power supply. (Examples I've personally encountered include the Model 50, 60, and 80)
    – Kaz
    Apr 23 at 8:16
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    Was encapsulation such a strict requirement in the 1980s? The 1990s AT-type power supplies I've seen tended to have the pushbutton power switch on a length of mains-voltage flex running back to the PSU, rather than a mechanical linkage to a button on the PSU itself.
    – john_e
    Apr 23 at 19:04
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AT power supplies often did have a line-voltage flip switch. Indeed many later machines also have such a switch around the back to totally switch off the system. However ATX and later systems use a "soft switch" which yields a number of capabilities.

First, and most obviously, it enables the computer to switch itself off. Once the software shutdown has completed the "power on" line is allowed to float up to +5V and the PSU drops the power good signal and shortly thereafter all power lines except +5V standby. This could not easily and cheaply be done with a flip switch.

Next, it allows the system to start itself up. For instance a wake-on-lan interface allows a remote "wake up" signal to be sent. The +5Vsb allows a minimal set of subsystems to be active such as PXE. On receipt of the message PXE grounds the power on line and the PSU powers up the main supplies, then asserts power good once things are stable.

It's helpful to regard the front panel switch not as THE power switch, but as A switch amongst a few possible switches. Indeed, it's not even a switch, it is merely a request to the PSU.

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    Ok. Yes, the automatic capabilities is where push switches make sense (as I already remarked in the question). However, push switches were standard before these features were added, weren't they? The oldest computer I ever used had a push switch on the front, but purely mechanical action (and when pushing, the travel was slightly longer when switching from off to on than the other way around – but you only really felt it when the button was already pushed in and it was too late to reconsider). What was the history behind all that? Apr 22 at 21:59
  • This is the correct answer. The power switch has to allow for multiple ways to turn on/off a machine. Apr 23 at 12:44
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    One important aspect of the ATX style soft power switch is that it allows graceful shutdown. This started to became relevant around the time of the Windows 95 that had significant amount of background processing and also implemented write caching in certain cases.
    – vhu
    Apr 24 at 8:05
  • Exactly, hence "Once the software shutdown has completed ..."
    – Martin
    Apr 24 at 9:27
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    Soft power-down doesn't prevent use of a flip switch as before long the state will match that of the switch. We're used to this, for example, with motors.When you turn off many appliances it takes them a while to "stop". Soft power on would be a good reason, but how common was it on early PCs?
    – Dannie
    Apr 24 at 11:20
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It's traditional :-)

Think back to when computers were large, lived in air conditioned rooms, and had a few dozen switches on various cabinets. Push buttons were common for "toggle" functions: online/offline, load/unload, power on/off.

Square buttons with a light behind them were common. Check out this paper tape reader, and the other box behind it.

enter image description here

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  • Mainframe CPUs and peripherals had a real power switch, fully cutting off mains power, somewhere at the PSU, while (push) button switches at the front only served as secondary for convenience.
    – Raffzahn
    Apr 22 at 23:29
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    Of course; my point, somewhat tongue-in-cheek, was that computers and push buttons have an association going way back. Apr 23 at 1:31
  • Ok, this is a possible alternative explanation to Raffzahn's. But can you tell which was more important? Any references? Apr 23 at 20:43
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    I don't think this is 'really' an explanation; about the only thing it's good for is to suggest that maybe there was no expectation that people would press buttons without knowing what pressing the button would do. I think @Raffzahn's tale sounds plausible. Apr 23 at 22:05
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To answer the 'When' part of the question: At least as early as 1982, when the Sanyo MBC-550 was released with a push button power switch on the front. The MBC picks up its design cues from contemporary video recorders (compare the Sanyo VTC-5000) on which the power switch was also a front-mounted push button.

Internally, the power supply of the MBC occupies the right-hand edge of the case from front to back, so there is no need for a mains-voltage wire between the power supply and a separate switch.

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I know nothing about what was going on in the product planning here. But I do note one thing about the machines I bought that had a power switch. These machines had the power switch directly connected to the power supply. If you hit the switch, you cut the power. Makes sense. Except that it doesn't make sense. If you cut the power without an orderly shutdown of the operating system, you leave the disk in an internally inconsistent state. That means that, the next time you start up the operating system, you have to run chkdsk to clean up the data state of the disk before you can safely run appps. The first computer I bought with a pushbutton also had the following feature. The pushbutton didn't cut power. Instead, it generated an event for windows to field. Windows shut itself down taking several seconds to do so, or maybe even more. Once windows was shut down, windows signalled the power supply to cut the power.
All of a sudden the rule that only dummies hit the off switch was reversed. The only people who didn't hit the power button were old timers who remembered that cutting power would cause chkdsk to run at startup.

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    Getting power supplies to talk to the BIOS and getting the OS to listen to the BIOS took Intel and Microsoft pushing a standard (APM) on to PC makers. I think the bait was that you had to support APM to get to use the "designed for Windows 95" logo. Apr 23 at 12:10
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    As was already discussed both here and in the screensaver thread many of the early push-switch computers did not have any safeguards against turning off, so this answer is rather missing the point. And, even given that we have a safeguard, I'd argue that a flip switch would be preferrable: switch it “on”, and it always means on. Switch it “off”, and it means the OS tries to shut down in any way possible, and then stay off. Apr 23 at 20:33
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    @leftaroundabout While I agree this style of button predated this behavior, I disagree that a flip switch would be more useful for a soft shutdown. The way these buttons normally work nowadays is you tap the button to initiate a soft shutdown, and hold it to force a hard one. You can't make that distinction with a two-way switch. Apr 24 at 5:26
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    @KefSchecter true. (Though again – the long-press-kill is itself a bit of a hack to get a third feature out of the single button, and it's not particularly intuitive. No doubt a similar trick could have also been done with a soft flip-switch, e.g. that flipping on and off again rapidly three times in a row would cause hard shutdown.) Apr 24 at 8:51
  • It's possible that the pushbutton predates the function being to alert the OS. But they are certainly related. The early off swithes were simply in line with the power cord. The pushbutton always generated a signal, I think. Where that signal goes is another story. Apr 25 at 2:00

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