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Topic: Common ground and why you need one (Read 1 time) previous topic - next topic

PerryBebbington

Dec 15, 2019, 02:32 pm Last Edit: Dec 22, 2019, 04:39 pm by PerryBebbington
A common mistake for people new to electronics is to have 2 circuits fed from different power supplies with signals passing between the circuits, but with the grounds not connected. This leads to questions asking why the project doesn't work. You don't have to spend long reading these fora to see this is asked often. Here I attempt to explain the problem and why it matters to connect the grounds. Note that in all of the schematics that follow I have used a battery as a power supply; this is not important, the power supply could by anything that can supply the appropriate DC voltage. What is important is how it is connected.

Electrical circuits are called 'circuits' because for them to work there has to be a complete, uninterrupted circuit all the way around and back to the power supply. Not just any power supply, the power supply that is supplying that bit of the circuit.

Consider these two circuits with a power supply and LED:

In the top circuit the LED does not light as the circuit is not complete and the current cannot flow all the way round. The bottom circuit is complete and the current can flow, lighting the LED. This is an important principal and applies to all circuits, however complex, including micro-controllers, which are very complex inside.

This schematic shows 2 devices connected to separate power supplies and with a signal between them.

Device 1 has an output connected to an input on device 2. In this schematic the ground of device 1 is not connected to device 2, which means the current representing the signal from device 1 can get to device 2 but it cannot get back to device 1's power supply, so the signal connection between the two devices does not work as the circuit is not complete.

In this schematic the grounds are connected:

Now the signal from device 1 to device 2 has a complete circuit from the power supply of device 1, through the output of device 1 to the input of device 2 and then back to device 1's power supply. The circuit is complete and will work.

Credits
Thank you Robin 2 and Ballscrewbob for your help with this tutorial

ballscrewbob

#1
Dec 21, 2019, 08:37 pm Last Edit: Dec 22, 2019, 07:43 pm by ballscrewbob
Two common methods of grounding.

Common ground can be broken out as two types. (simplified)

Both are known as GROUND.
In most cases you will probably use something similar to the post from Perry

However if you are planning a CNC, multiple motors, servos, actuators etc. type of project there is another method to consider.

STAR GROUNDING

In its most simplistic it is a central point where all grounds meet using a "star" formation.
Often also used in audio, musical instrument situations to name just two.

The star ground is a single reference that connects both analog and digital ground planes.
This is to help eliminate ground loops.
A ground loop (simplified) is a dual route to ground that can cause signals to interfere with each other.
They can often be awkward to diagnose and in some cases fix.



Electrically there can be multiple differences in the way both schemes work even though it may on the surface look like the grounds are all just connected to each other.

Simple GROUND tips.

  • Try to keep ground and signal leads as short as you can.
  • The current return paths must also be kept short (aka ground).
  • Long wires can pick up noise and should be kept as short as possible or shielded.
  • Any grounds that present high frequency signals should not be shared with other more sensitive grounds.
  • Try to avoid introducing inductance with coiled wires or similar although twisted pais are a basic loop
  • Better to avoid ground loops that can be a source of interference into your ground or transmission of your signals to other parts of the circuit.

Other aspects to be considered but not dealt with here are :-
  • Ground Planes.
  • Grounding Rods.
  • Chassis Ground.
  • Ground Bonding
  • Earth Ground.

And a few more but for the purposes of keeping it simple I only added the above to compliment Perry's post.

Also my thanks to Robin and Perry for the insights and assistance.

It may not be the answer you were looking for but its the one I am giving based on either experience, educated guess, google or the fact that you gave nothing to go with in the first place so I used my wonky crystal ball.

PerryBebbington

#2
Jan 01, 2020, 02:03 pm Last Edit: Jan 01, 2020, 02:09 pm by PerryBebbington
Sometimes you don't want to connect the ground of one circuit to the ground of another because you want to isolate the power supply of one from the other. In this case you need something that will pass the signal from one part to the other without any electrical connection. Common devices that can do this are relays and opto-isolators. Relays use magnetically operated mechanical contacts to transfer the signal from their input to output, opto-isolators use light. Both provide electrical isolation between 2 separate circuits.

The example below shows a relay used to isolate a low voltage circuit from a mains powered circuit. The common reason for doing this is safety; you are not at risk from electric shock if you touch the low voltage circuit even though it is controlling mains.


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