Shield of a device is connected to earth or ground - Page 1
Forum section: electronics / beginners

author: madhu.wesly01
author_rank: Regular Contributor
timestamp: April 05, 2018, 12:34:27 pm
content: Hi,I have a query regarding connection of shield in AC-DC converter designIn an AC-DC converter, Is the shield connected to earth or DC ground ?Please note that I am having some part of the circuit which operates on DC supply generated from AC-DC conversionThanks in advance

author: madires
author_rank: Super Contributor
timestamp: April 05, 2018, 12:50:55 pm
content: Could you please more specific about what you mean by shield?

author: Nitrousoxide
author_rank: Regular Contributor
timestamp: April 05, 2018, 01:34:26 pm
content: For AC-DC power supplies the earth is usually connected to the chassis (unless the device is double insulated), and by extension, shielding is mains earth reference.For shielding against radiated EMI in small signal applications, you can route and anchor pads to the closest (most appropriate) ground net.You would have to be more specific about what is being shielded.

author: David Hess
author_rank: Super Contributor
timestamp: April 05, 2018, 02:44:12 pm
content: If you are talking about something like an off-line AC to DC power supply, then ground, neutral, and hot are all bypassed at the input so for RF purposes, it does not matter which the shield is connected to. For safety reasons however, ground is used.For low voltage AC to DC or DC to DC conversion, the shield is usually connected to a low potential which keeps circulating currents out of sensitive paths which may include the power ground. Usually however this just means the single point ground at the output of the converter but it could mean a separate chassis ground if that is considered sensitive.

author: madhu.wesly01
author_rank: Regular Contributor
timestamp: April 05, 2018, 02:49:59 pm
content: Metal/plastic Shield for enclosing the whole circuit

author: madires
author_rank: Super Contributor
timestamp: April 05, 2018, 03:49:01 pm
content: A metal enclosure/box would be connected to PE/earth in case of a class I device. Seehttps://en.wikipedia.org/wiki/Appliance_classesfor explanation of the different classes.

author: madhu.wesly01
author_rank: Regular Contributor
timestamp: April 07, 2018, 02:42:03 pm
content: Quote from: madires on April 05, 2018, 03:49:01 pmThat gives good protection to the equipment. I have a small doubt here, won't the DC supply be floating If DC ground is not connected to Earth(from 3 pin AC supply)

author: madires
author_rank: Super Contributor
timestamp: April 07, 2018, 05:29:56 pm
content: Yes, usually the DC output is isolated from the primary side of the SMPSU. But there's an EMI suppression cap between primary and secondary side. Some SMPSUs connect the secondary's ground to PE/earth, e.g. an ATX PSU for PCs.

author: madhu.wesly01
author_rank: Regular Contributor
timestamp: April 08, 2018, 11:29:48 am
content: Thanks all for your repliesQuote from: David Hess on April 05, 2018, 02:44:12 pmAs far as I know, offline and online power supply id used in case of UPS. But here, I am having only AC input and AC-DC converter. In this case, how do I decide if my supply is offline or notQuote from: David Hess on April 05, 2018, 02:44:12 pmIn my case, do you want me to connect the shield to DC ground instead of earth(from 3 pin AC supply)Thanks in advance

author: madhu.wesly01
author_rank: Regular Contributor
timestamp: April 08, 2018, 11:38:49 am
content: Quote from: Nitrousoxide on April 05, 2018, 01:34:26 pmThanks for the good point, I didn't think which to be shielded in specific. As I think, shielding both AC-DC converter part and the rest of the circuitry would give me best performance.If this is the case, AC-DC converter part should be shielded by earth connection and rest of the circuitry with DC ground. Please acknowledge my understandingAlso, If I would like to shield the whole circuitry To what should the shield be connected to ?Thanks in advance