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Topic: What does this schematic symbol mean?? (Read 427 times) previous topic - next topic

TheAliw1

Oct 06, 2019, 01:32 am Last Edit: Oct 06, 2019, 01:33 am by TheAliw1
What is the symbol marked with the red square? I assume it has to do something with ground?
Thanks in advance!

Paul_KD7HB

It means the ground provided by the building mains power. Usually through the power supply power cable.

The device data sheet shows a regular ground symbol. Where did this schematic come from?

Paul

MarkT

They are using two different ground symbols because there are two different grounds, logic ground and power ground.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

falexandru

So the logical ground and the power ground can be different?
Interesting. What is the practical use of this separation?
Thank you!
I just love to learn, I apologise for popping up here.

Paul_KD7HB

So the logical ground and the power ground can be different?
Interesting. What is the practical use of this separation?
Thank you!
I just love to learn, I apologise for popping up here.
A separate power ground is necessary when large currents are involved in the power circuit. Especially when the power current fluctuates. ALL electrical connections have some resistance. A very large current through a ground connection will produce a voltage at that connection. If a signal ground is being shared, that little voltage will be reflected in the signal voltage.

That is the reason that modern vehicles turn off all unnecessary electronics while the starter circuit is running to start the engine.

Paul

JohnRob

I think here to designer is trying to communicate the physical layout of the grounds.  I would have used different symbols because the symbol with the 3 downward lines is (as stated above) used for mains or "earth" ground"  As opposed to a local common.


However I believe the designer is trying to say:


C1, C2 and PGND should be close together.

C3, R2 and GND should be close together

And the two grounds should be connected together at one point only.


I should note, the symbols alone do not say all that, but general knowledge of circuits plus the symbols leads to that understanding.

John
Please do not PM me with thread based messages.  If your thoughts are worth responding,  the group should benefit from your insight.

falexandru

@Paul

Thank you very much!
I imagine that if I use a shared ground for a 12 V powerful motor and a logic circuit, then at the engine start, the logic circuit could be damaged or at least experience a blackout?
I do not want to hack the topic, just cant resist curiosity.

Paul_KD7HB

@Paul

Thank you very much!
I imagine that if I use a shared ground for a 12 V powerful motor and a logic circuit, then at the engine start, the logic circuit could be damaged or at least experience a blackout?
I do not want to hack the topic, just cant resist curiosity.
Yes, if the logic circuit is directly powered from the car battery. If you power it from the car's accessory power, then it will automatically shut off when you turn the key to start the car and turn back on when you release the key. If your car has a 12 volt accessory plug like mine, you can plug the logic in there.

Paul

Paul__B

That is the reason that modern vehicles turn off all unnecessary electronics while the starter circuit is running to start the engine.
Well, sort of ...

Shutting off the positive supply voltage is not going to protect anything against ground loops, and the engine electronics which are most susceptible to ground loops, are obviously not shut off.

I would say that the reason for shutting off the accessories circuit is to minimise the load on the battery - trivial though that may be as most cars fail to disable the headlights while starting; you have to remember to do that manually - and to avoid subjecting the accessories to surges and repetitive oscillations in the voltage.  :smiley-roll-sweat:

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