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I have assembled a project that runs on 12 volt DC power. I have currently grounded the green AC power input ground wire to the chassis on my project. Should I additionally run a wire from the V- negative terminal on the output of my power supply to the the chassis, effectively tying the DC and AC grounds together?

The power supply does come with built in anti-short protection

PINS 6-11 are power output and the ground is on the metal side of the metal power supply chassis. I grounded my AC power to this metal chassis and am considering grounding my DC circuits I am running off this power supply to this point as well.

enter image description here

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  • \$\begingroup\$ Show the PSU schematic and where you intend to ground. \$\endgroup\$
    – Transistor
    Oct 8, 2021 at 23:25
  • \$\begingroup\$ @Transistor edited \$\endgroup\$
    – Oscar
    Oct 9, 2021 at 0:20
  • \$\begingroup\$ Yes, "earth ground", the green wire, must be connected to the chassis. There are cases where complete isolation is needed, between Ground and DC circuitry, the reason usually is obvious. That case, you wouldn't rely on the isolation of the power brick. Otherwise, you must ground DC for various reasons, like EMI, static discharge. \$\endgroup\$
    – jay
    Oct 9, 2021 at 2:24

3 Answers 3

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To ground or not to ground - that is a perennial question. Seems everyone has a conflicting opinion. In many cases you can have conflicting requirements - EMC vs a potential ground loop and so on. Sometimes you have to comply with a specific standard and it can prescribe the solution. In short, there is not one simple answer to cover all situations!

If the 12V does not go outside of the enclosure, then probably not. If the 12V does extend out of the enclosure and extends a distance, then I would suggest you ground it. There's an IEC standard whose number escapes me at the moment that mandates that you ground it - this is for instrumentation systems. It should be grounded at the source.

I can tell you from bitter experience in large distributed systems that everything tends to find its way back to ground one way or another and in a way that is not good and when you least expect it.

jp314's answer covers some other situations.

Ultimately, it really comes down understanding where currents are flowing and where they might flow (fault situation).

Your psu is grounded so that should take care of the situation where the 'Y' capacitor in the psu may cause leakage currents as this is mainly an issue with ungrounded switching power supplies.

Another common grounding method is to use a capacitor between your 0V and chassis/electrical ground. This shunts AC currents whilst letting 0V 'float' DC wise. Your power supply might be already doing this for you.

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schematic

simulate this circuit – Schematic created using CircuitLab

There are a lot of things to consider when connecting DC 0V to AC Protective Earth, PE due to the parasitic effects of ESD, lightning and CM noise from separate DC supplied systems connected together creating a ground loop of DCDC noise. Some Pro's and Cons can be minimized or prevented with CM choke filters and other methods of understanding any interference.

Pros

  • more immunity from stray radiated noise ingress due to low CM impedance to AC earth for high impedance signal inputs.
  • floating outputs can be cascaded or polarity reversed.
  • lower possible egress or radiated noise on RF IO cables, with PE bond to 0V.

Cons

  • Increased stress during Hipot as the lowest pF part (XFMR isolation) is now AC bypassed to earth and is now inside AC primary gaps and insulators of DCDC converted which is only tested with unearthed DC for Hipot.
  • More likely to see higher impulse noise on DC due to lightning from non-isolated XFMR2.
  • more complex ground paths for ESD ingress into CMOS that can latchup.
  • more USB cable noise with earth ground noise from DCDC converter Y cap noise coupled to CM signals by gnd shield.

When analyzing lightning effects consider the BW as 100 kHz to > 1 MHz and caps can be almost a short circuit.

When analyzing ESD paths consider BW as DC before breakdown and >> 1 GHz after.

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If you do ground the DC-, then the PSU will see a short (and its output will become 0 -- you can't power a load) if its + output is accidentally shorted to and 'real' GND.

If you don't ground it, then (depending on your AC isolation), you may pick up some 120 VAC leakage across the transformer and if a person touches the PSU, they may detect some (safe) disturbance.

If this PSU is powering other objects, and they have their own ground, then there is a possibility of creating a ground loop -- 60 Hz magnetic fields will induce a current in this loop; if it is an audio system, the hum could be significant.

So, for maximum flexibility, I would not ground the DC- terminal. If touching it is a concern, perhaps connect DC- to AC GND with ~ 100 Ω (of sufficient power rating).

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