Using a Plug-In AC/DC adapter Negative as Earth GND

Thread Starter

morzh

Joined Jan 18, 2017
38
Hi everyone,

Now, this is not a general question about any plug-in 12V DC supply with a barrel connector output. They are all different. There are those without EGND wire in the AC power input, those with it but without it being connected to the output negative, and then there are those with that connection.

My specific power supply is the TRH70A120-02E01-Level-VI (yes this is their exact P/N) by Cincon.
The part is here
https://www.mouser.com/ProductDetail/Cincon/TRH70A120-02E01-Level-VI?qs=P8zB4ONU6fxl97c0YpS2Bw==
The datasheet is here
https://www.mouser.com/datasheet/2/75/Datasheet_TRH70A_series-2997934.pdf

It has a plain barrel connector and the Negative is connected to Earth. It is not shown in the datasheet (or I have not been able to find it there). How do I know? By measuring the Negative to the Earth pin in the AC input receptacle. I have seen similar units inside, and they simply have a wire, sometimes going through a ferrite bead, going from that PE to that 12V-.

I have used it successfully on one product without having connected the case to the 12V-. Successfully meaning, it passed compliance, both the immunity and compatibility.
However I'd prefer my case to be under Earth potential if possible, and for that reason I would like to know if anyone has considerations against it.

One way is to connect the 12V- directly to on of the mounting posts, holding the board to the case, the one right where the power enters the board. That is the case will be electrically held to the 12V- which is the Mains Earth at the same time.
The other is to connect it the 12V- to the same mounting post via a ceramic cap, say 0.1uF, and a 1M resistor in parallel. I also like to add an ESD clamp in parallel to the two.

PS. I am redesigning a product we have that is using a power supply that has Earth connected separately to the power connector's (4-pin DIN) shell. This is preferred, but this power supply is $65 vs $20 for the other. And the board barrel jack is $0.75 vs $12 for the Kycon DIN-4 they used before.
 

BobTPH

Joined Jun 5, 2013
8,967
According ti the darasheet it is isolated, good to 3000V.

A low voltage device does not need its case connected to earth.

To determine if it is grounded, do not try to measure a voltage. Measure resstance.
 

MaxHeadRoom

Joined Jul 18, 2013
28,688
The output DC negative is allowed to be referenced to earth GND, as long as galvanic isolation exists between supply power and output (DC) power.
Applies to most jurisdictions.
 

MisterBill2

Joined Jan 23, 2018
18,518
Here is a word of cautio: Connecting the outer metal frame of an electrical device to the supply negative is running a risk of damage or destruction. The reason is that the external frame of a battery powered device may possibly be tied to any point in the internal circuit.
In addition, tying the case of a battery device to "Safety Ground" gains you no safety at all.
 

Thread Starter

morzh

Joined Jan 18, 2017
38
I asked that question not really from the safety perspective. It is safe if only because it is an isolated SMPS, and touching it's battel, plus or minus, is safe. An SMPS is as safe as it's pulse transformer or bridge capacitor.
What I am trying to gage, I giuess, is its compatibility performance (mostly Radiated) and Immunity (mostly ESD and Radiated).
I have always been able to pass Class A. With not much additional effort other than some sensible connection and a large electrolytic in parallel with a film across the 12V input. Plus a Transorb.
But I want to try (though it is not a requirement...yet) to get Class B in compatibility.
 
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