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where to connect the shield

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osmanpasali

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hi,
we designed an electronic card which is supplied by a dc adapter(i.e. no safety ground connection). the card is in a box and the box is shielded. i wonder where should i connect the shield? because there is no earth connection.
 

Old Nick

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I'm a bit confused by your question.

There must be a ground/earth connection on the DC supply as the current needs a return path. it may be that I'm not quite getting what you mean.
 

osmanpasali

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there is a return path(GND) of course but what i mean by saying earth is the third of the mains supply: Phase-Neutral-EARTH. DC adaptors dont have the earth connection. as far as i know shield is connected to earth.
 

laktronics

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Hi,
For two wire AC connections it is always a problem from the safety point of view. Since you are not bringing in direct high voltage AC into your equipment, this design will provide safety depending upon the isolation charcteristics of the adaptor transformer. These are mostly used for equipments with plastic enclosures. From a circuit point of view, the metalic enclosure will still shield the circuit from external interferences and connecting it to a circuit point like DC power supply return would provide it a reference potential, thus preventing it from capacitively injecting noise into the system, if kept floating.

Regards,
Laktronics
 
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