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I'm very confused about this design i've come across in the field. A data acquisition system is powered by a 12V ACDC power brick. But then inside of the chassis is a DC-DC converter, also isolated, where the output GND is tied to the chassis, and to earth...

Why? There are no high voltage systems here, so I can't imagine this can be for safety. Can earth grounding the common GND of a digital system prevent noise? Is it wise to do this?

Another question I have is whether grounding the output of an isolated converter breaks the isolation of that converter.. In other words, is the output of the converter no longer floating since it's now tied to earth? enter image description here

BobaJFET
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  • Just curious - how is the connection to earth made, if not via the power brick cable? Is there a separate, dedicated wire from chassis to earth? – Simon Fitch Jun 21 '23 at 21:36
  • That would be called a Class I device. – winny Jun 21 '23 at 22:11
  • @SimonFitch The earth ground actually comes from the AC-DC brick. Unlike barrel jacks, 5 and 8 pin DC connectors usually have earth ground. – BobaJFET Jun 22 '23 at 00:57

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If you're designing a piece of equipment with a metal chassis, unless there are other considerations at play it's a good idea tie your circuit ground to chassis ground in at least one spot. This assumes that the system you're working with was done wisely, that the designer assessed how the data acquisition system would work, and that they determined that grounding the boards to the chassis was the right thing to do.

For low-production equipment of a certain era, those DC-DC converters were often the cheapest way to get the voltage you wanted, even if you didn't want an isolated converter. That's probably why that converter was chosen.

TimWescott
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  • Why use an isolated converter on the inside of the chassis, then only earth ground the output side? If a wire came loose on the input side, would the protection still work? – BobaJFET Jun 22 '23 at 01:04
  • @BobaJFET Yes, because it's isolated. The equipment would just stop working. – Finbarr Jun 22 '23 at 10:45
  • @Finbarr That might be true but what if the metal chassis makes contact with both ends of the input side of the DC - DC converter? I don't have it shown in the diagram but there is a + and GND going into that converter. – BobaJFET Jun 22 '23 at 12:53
  • @BobaJFET Then it'll short out the power brick, regardless of whether the converter is isolated or the chassis is grounded. – Finbarr Jun 22 '23 at 14:30