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Why AT PSUs have 4 wires connecting to the on-off switch

P

Phase

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I need to replace the regular AT PSU switch with a round 16mm latching switch, preferably illuminated (hole already there). From what I have seen those are single pole single throw. Why does the AT PSU has 4 terminals going to the switch? Will I have to concoct something with a relay?

Well the other solution is to use an ATX PSU. Since the cables that convert one to the other take a SPST switch, but I would prefer to use an AT PSU. Thoughts?

Thanks!!
 
Phase, your location is unknown, so we can't draw any conclusions about what your power distribution is. However, if you're in North America and have 120VAC power, then you need only switch the "hot" side of the line. Electrical codes in other countries (240V ones) may insist that both line connections be switched.
 
Awesome. I’m int the US. So, if I understood you correctly, I need only to switch the “line” side of the wire. Correct?
 
That's correct. There's no connect (other than via the line filter caps) between the neutral side and power/chassis ground in these switching PSUs, so they're effectively isolated.
 
I'll add that on my 240VAC shop machinery, both sides of the line are switched, since both sides are "hot" with respect to neutral/ground in North American power distribution. On 3 phase equipment, all three phase lines are switched, since all are "hot" with respect to neutral.
 
Off topic perhaps but I owned a Thomson DVR at one point that was fused (internally) on the neutral. I thought that was a tremendously bad idea!
 
pearce_jj said:
Off topic perhaps but I owned a Thomson DVR at one point that was fused (internally) on the neutral. I thought that was a tremendously bad idea!
Click to expand...
Why? The fuse is triggered by overcurrent from a short. It doesn't matter where it sits. A fuse it not there to protect people from getting a shock!

In Germany and many other European countries, you can't even tell what neutral and what live is from the equipment side, as mains plugs are not keyed and also go in when rotated 180°.
 
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See, for example, the Schuko (CEE 7) receptacle and plug family. That one always baffled me--usually German-origin stuff is pretty decent with respect to safety.

Hence, my answer regarding non-North American systems. Although it's possible that a receptacle will be wired incorrectly (a code violation), it's less likely nowadays. As usual, a short story...

When we moved into our current home more than 30 years ago, I couldn't understand why a Toshiba microwave oven refused to power on in the kitchen. Plugged into other outlets, it was fine, but not in its intended spot. A quick check with a lamp indicated that power was indeed present at the subject outlet, but still the oven refused to operate. Eventually, I got out the multimeter and checked the outlet--backwards. Apparently the oven was smart enough to figure that one out (and no, the ground line wasn't being used as a power return). Checking around the house I found about 8 outlets that were similarly wired.

Back then, there was still a fair proportion of "hot chassis" equipment around. That could lead to unpleasant surprises down the line...

But Timo's response is why DPST power switches are common on most electronic gear today.
 
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Chuck(G) said:
See, for example, the Schuko (CEE 7) receptacle and plug family. That one always baffled me--usually German-origin stuff is pretty decent with respect to safety.
Click to expand...

I believe it was because originally the German supply was centre tapped so it really didn't matter which way round you plugged it in :)

It must be said, the UK 13A plug is the most dangerous of all.

Not when in use, but when left on the floor. The pins inevitably point upwards and really really hurt when stood on with bare feet.
 
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