# DC ground

#### colum

Jul 25, 2013
175
I have 1 DC circuit at 9v and another at `12 to 24v DC that are powering different sections
Question
Can I use a common ground for these circuits or do I need to isolate each
Many thanks...Jeff

#### Minder

Apr 24, 2015
3,478
Not always, depends on the equipment in question and also if there would be any side effects, but what are the circuits functions?

#### colum

Jul 25, 2013
175
One is a 9v plug in transformer to a 5v dc rectifier for a pulse generator.
The other 12 to 24 v dc plug in transformer powers a backwards transformer
this generates an AC current and works great with a 9v dc power on BOTH circuits
But can I use a common ground for a higher AC output and still keep the 9v circuit
Thanks...Jeff

#### Delta Prime

Jul 29, 2020
1,956
The other 12 to 24 v dc plug in transformer powers a backwards transformer
this generates an AC current and works great with a 9v dc power on BOTH circuits
Applying DC voltage to the primary or secondary of a transformer doesn't matter the only output you'll see if hooked up to oscilloscope is a spike whose amplitude would be equal to the DC voltage input the current in Rush and you will have effectively short-circuited the DC applied, so further information is required please a photo a drawing or schematic of your setup because I do not understand your question

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,716
Whether you need a common ground or not is a question of current flow between the different circut sections (see e.g. this tutorial).
If there is a current path from one section (1) into the other (2), there also needs to a path back from 2 to 1 because current always requires a closed loop. This is usually a common ground.
It here is no current path, both sections do not need a common ground. But a common ground can be used e.g. for technical reasons like PCB-layout, EMC etc.

A case where definitely separate grounds are required is when both low voltage circuits (ELV, SELV) and high voltage circuits with voltages or power levels above the limits for ELV or SELV circuits are part of one system. In hat case the two circuits need to be isolated from each other (e.g. by photocoupler, transformers, protective impedance etc.) so any defect in the high voltage section does not pose a danger to someone being in contact with the supposedly safe low voltage section. There's abunch of interbational standards covering this kind of application and the necessayr measures.

Generally it is recommended not to mix grounds as currents from one section going through the ground of another section can have negative impact on circuit performance (cross coupling due to non-zero impedance of the ground path, be it a trace or a plane). A compromise oftne used is a star connection where different circuit sections have their own ground plane and these grounds are connected at one single point called star point (see e.g. this discussion).
However, in recent years I have experienced another school of thought that recommends a single ground pane to absolutely minimize the impedance. In that case skilled placement of circuit elements can minimize cross coupling).

But I digress...

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