VOGONS

ATX to AT adapter

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First post, by soupapes

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I was trying an ATX to AT adapter that I am planning to use on an AT motherboard.

The adapter has 2 wires (green/black) to connect to the power switch, but my power switch that come with the AT power supply has 4 connection points to be wired.

I did some experiments and it seems the old AT switch can be used to turn on the ATX power supply if I connect the 2 wires from the adapter into two side connection points (it seems to work both on left or right side).

My question is why the AT switch has 4 connection points? And is it OK for me to use this AT switch wired to the ATX to AT adapter?

- IBM Aptiva 2176/P200MMX overdrive/Voodoo 4mb
- IBM PS/2 model 60
- IBM PS/2 55 SX
- IBM PS/1 2133
- IBM PC 330 - P90
- 486 DX2 66

Reply 1 of 2, by mkarcher

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soupapes wrote on 2021-12-28, 13:40:

My question is why the AT switch has 4 connection points? And is it OK for me to use this AT switch wired to the ATX to AT adapter?

It is OK to use the switch in the way you are currently using it: The AT switch has four two connection because it is a double pole single throw switch. This means it makes or breaks two connections at the same time. With your ATX to AT adapter, you only need to make a single connection to turn the supply on or off, so using either of the two poles is perfectly fine.

The reason for the AT switch being dual pole is that a common european wiring system does not enforce plugging a mains socket in a certain way, so there no way the device designers can assume which mains wire is live and which mains wire is neutral. On the other hand, it is highly recommended that turning off the power switch interrupts the live wire. So the designers of AT-compatible supplies decided to switch both poles at the same time, so it doesn't matter which of the two poles is mains and needs to be switched, and which one is neutral and would be fine if it weren't switched.

EDIT: fixed the number of connections (including change markup, as the wrong text is already quoted)

Last edited by mkarcher on 2021-12-28, 14:52. Edited 1 time in total.

Reply 2 of 2, by soupapes

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mkarcher wrote on 2021-12-28, 13:50:
soupapes wrote on 2021-12-28, 13:40:

My question is why the AT switch has 4 connection points? And is it OK for me to use this AT switch wired to the ATX to AT adapter?

It is OK to use the switch in the way you are currently using it: The AT switch has two connection because it is a double pole single throw switch. This means it makes or breaks two connections at the same time. With your ATX to AT adapter, you only need to make a single connection to turn the supply on or off, so using either of the two poles is perfectly fine.

The reason for the AT switch being dual pole is that a common european wiring system does not enforce plugging a mains socket in a certain way, so there no way the device designers can assume which mains wire is live and which mains wire is neutral. On the other hand, it is highly recommended that turning off the power switch interrupts the live wire. So the designers of AT-compatible supplies decided to switch both poles at the same time, so it doesn't matter which of the two poles is mains and needs to be switched, and which one is neutral and would be fine if it weren't switched.

Great, thank you for the explanation. I learn something new today 😀

- IBM Aptiva 2176/P200MMX overdrive/Voodoo 4mb
- IBM PS/2 model 60
- IBM PS/2 55 SX
- IBM PS/1 2133
- IBM PC 330 - P90
- 486 DX2 66

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