Why do power supplies have dedicated Live and Neutral connections? - diyAudio
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Old 5th July 2014, 01:38 PM   #1
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Default Why do power supplies have dedicated Live and Neutral connections?

Probably a silly question but considering hardly anyone knows which side of the mains connection is live and which is neutral, why do power supplies have dedicated connections for these?

Is it still recommended to put the fuse in the line to the "live" connector? (I would anyway as there is no reason to put it in the other).
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Old 5th July 2014, 02:52 PM   #2
AndrewT is offline AndrewT  Scotland
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Many countries have Mains output sockets that do differentiate Live from Neutral.

It is always expected that the Live is fused. The minimum is fusing at the distribution board.
The general rule is that the fuse is rated to suit the lowest rated cable that is fed by that fuse.
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Old 5th July 2014, 03:41 PM   #3
nezbleu is offline nezbleu  Canada
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Think of it this way: If you fuse neutral, and the fuse blows, there is still "hot" live inside the box, which might find some other path to ground. If you fuse the live leg, the fuse will likely be located physically near the power entry, so there is no "hot" line floating around inside the chassis. The fuse should be the first thing the live leg sees inside the chassis, even ahead of the power switch.
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Old 5th July 2014, 04:04 PM   #4
kevinkr is offline kevinkr  United States
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Both preceding posts are spot on. Unpolarized power plugs/sockets are still found in parts of Europe which is IMO not a very good idea precisely for the reasons mentioned.

All wall sockets here in the U.S. are polarized, and with the exception of a some double insulated devices the plugs are also polarized so that hot and neutral are clearly defined - unless of course the wall socket is wired incorrectly. (It happens)
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Old 5th July 2014, 04:35 PM   #5
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In my plan for a class D amp with prefab board, I want to put an regulated SMPS. The live wire will go from connection to fuse to switch to connection on SMPS. The neutral wire will go straight to the connection on the SMPS.

Is this the correct way?

Should I take the inrush current of the power supply into account when selecting a fuse rating or should I take the "normal" max current?
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Old 5th July 2014, 05:08 PM   #6
kevinkr is offline kevinkr  United States
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Yes what you describe is correct.

The manufacturer of the switching supply should have specific recommendations for both type and rating of input fuse to use with the supply.
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Old 5th July 2014, 05:33 PM   #7
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Thanks for the valuable input!

I have my eye on a 15VDC 5A supply (on 230VAC mains). A TDK-LambdaLS75-15.

Manual
Datasheet

It in terms of current handling it has some head room for the class D amplifier (TPA3116) which I do not expect to draw more than 2 amps.

What should the fuse and switch be rated for?
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Old 5th July 2014, 05:42 PM   #8
kevinkr is offline kevinkr  United States
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From the manual:
Attached Images
File Type: png Capture.PNG (63.7 KB, 187 views)
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Old 5th July 2014, 05:46 PM   #9
kevinkr is offline kevinkr  United States
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Do read the manual and also observe the maximum rated external capacitance value for the model you intend to use.

All the information you require is there.

6A inductive load/tungsten lamp rated switch should be more than adequate considering inrush.
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Old 5th July 2014, 05:51 PM   #10
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Quote:
Originally Posted by kevinkr View Post
Do read the manual and also observe the maximum rated external capacitance value for the model you intend to use.

All the information you require is there.
Thanks, can't believe I missed that.
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