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Earthed PSU question

I`ve been struggling with this for quite some time and hope someone may be able to point out the pros and cons:

Imaging a situation where I`m designing a new product and I need to select an external AC=E0DC switch mode PSU with a very standard barrel connection output. The product is a metal case, with chassis connected to system wide 0v.

Examples below:

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pters

I note some of these power supplies have the output 0V output connected to the input Earth, but most do not=85 (IE: the zero volt outside of the DC Barrel connector is connected to the AC connector Earth pin internally to the PSU). I see more and more laptop power supplies using this earthed negative system...

What are the relative EMC advantages / disadvantages of having this 0V =E0 Ground connection?

I think earth loops may be a potential downside to this but the earthed 0v seems like a great way of sinking some noise currents in the system?

Many thanks to all, anna

Reply to
anna
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EMC can be a can of worms, but the presence of a ground wire in the supply cord determines the shape of the can.

Common-mode suppression in a unit without the ground wire can be be misleadingly assessed - various apps show compliance expected from small converters with minimal filtering, simply because the output is not grounded in testing. This is impractical in many real situations.

The ground wire in the supply cord allows EMC control over a wider range of applications, as sheild currents can be returned locally and safely. It's not easier, or cheaper, but it's more repeatable and dependable.

RL

Reply to
legg

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