What is the purpose of grounding the live wire in this circuit?????

ronsimpson

Joined Oct 7, 2019
3,200
I think the "ground" in this case is not earth ground but a ground for the BL0937. This is the ground for the MCU. It is not green wire ground.

The power meters I have built must work if N and L are reversed.
 

AnalogKid

Joined Aug 1, 2013
11,194
Based on the level of the question, this needs to be stated:

What you have is an extremely dangerous circuit. Even if the AC/DC converter is fully isolated and safety-certified, the direct connections to the mains through other components makes the entire circuit lethal. Whatever the MCU is doing, its components, switches, indicators, and communications ports all are potentially lethal.

The data sheet says the two outputs can source and sink 5 mA. That is enough to drive an optocoupler. And of course, the MCU circuits can drive an optocoupler for the SEL input. Please give us more details about what you intend to do with this circuit, so we can advise.

ak
 

panic mode

Joined Oct 10, 2011
2,865
agreed...

this is a very dangerous circuit. this screams murder...

also the "ground" symbol does not connect to Earth or Mains GND, this is just local reference point. that IC is not isolated from the mains. the connection to L is needed to measure voltage on VP. my guess is that they chose to connect it this way is to keep low common mode voltage across current sense inputs.

a much better and safer way is to use isolated devices like ACS7xx.
 

panic mode

Joined Oct 10, 2011
2,865
Whatever dude. There are building guidelines/practices that are performed. But even your TV can have a hot + non-isolated ground.
true...


the difference is that TVs are designed and built by companies that know the symbols/standards/practices and come with a warning. datasheet for that chip came with none and clearly someone inexperienced is trying to use it.
and even TVs have electrocuted or even killed many which is why we are ringing warning bells and trying to educate.

problem here is not the circuit in operation (circuit can be put in a box). the problem is troubleshooting/debugging. that MCU will be connected to live mains and user computer/programming device will be connecting to it.
 
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MisterBill2

Joined Jan 23, 2018
19,366
REALLY, the circuit should have used an arrow with a tag, and not a ground symbol. OR, better yet, arranged the drawing so that the connection of those three points could be a solid line.
BUT, given that there is no way to fix stupid, it was done like we see.
Sorry to be so very harsh about it, but after seeing two people die from shocks because what they touched was not ground, it made me rather intolerant of some things.
 

LadySpark

Joined Feb 7, 2024
194
REALLY, the circuit should have used an arrow with a tag, and not a ground symbol. OR, better yet, arranged the drawing so that the connection of those three points could be a solid line.
BUT, given that there is no way to fix stupid, it was done like we see.
Sorry to be so very harsh about it, but after seeing two people die from shocks because what they touched was not ground, it made me rather intolerant of some things.
Its fine the way it is.
Its probably a power meter that you couldn't access the circuit unless its totally disconnected anyways.
Its not like a hot ground in a radio or TV that back then didn't give anyone warning that you were working with a hot chassis ground.
 

MaxHeadRoom

Joined Jul 18, 2013
28,820
Earth ground is a newer term. It used to be called DC ground.
Also Depends where you originate from!!
In all my original training, you would run supply conductors and Earth,
Term Ground was not in use for this purpose.
e.g. Instructional material, "Art of Electronics, et-al, use the earth GND symbol throughout the book, indiscriminately for circuit examples. :confused:
 

Thread Starter

aakasps

Joined May 22, 2024
9
Based on the level of the question, this needs to be stated:

What you have is an extremely dangerous circuit. Even if the AC/DC converter is fully isolated and safety-certified, the direct connections to the mains through other components makes the entire circuit lethal. Whatever the MCU is doing, its components, switches, indicators, and communications ports all are potentially lethal.

The data sheet says the two outputs can source and sink 5 mA. That is enough to drive an optocoupler. And of course, the MCU circuits can drive an optocoupler for the SEL input. Please give us more details about what you intend to do with this circuit, so we can advise.

ak
Well I am trying to implement a energy meter circuit with bl0937 IC , so I don,t know why they are given ground at the live wire. Ln this circuit I have to implement buck converter also , that's why I am in a confusion?
 

LadySpark

Joined Feb 7, 2024
194
Also Depends where you originate from!!
In all my original training, you would run supply conductors and Earth,
Term Ground was not in use for this purpose.
e.g. Instructional material, "Art of Electronics, et-al, use the earth GND symbol throughout the book, indiscriminately for circuit examples. :confused:
Technically, AC power and signal is line, neutral, and earth and DC is power and ground.

The term people associate incorrectly in a circuit is "signal ground" because there is no ground to the signal. Because the signal is referenced equally from DC power and DC ground.
 

ronsimpson

Joined Oct 7, 2019
3,200
Build the PCB like this. Use any name you want for what the BL0937 thinks is GND. Call it (L) if you want. Call it BLgnd.
Next, all those parts ridding on (L) need to be double isolated from humans. No hand can touch the ICs. If there are buttons they need to be isolated by two layers of insulation.
Sometimes I use opto isolators from BL0937 to MCU and sometimes I don't. It adds cost to have the MCU isolated and have a second power supply for it.
1716467798562.png
 

nsaspook

Joined Aug 27, 2009
13,538
Technically, AC power and signal is line, neutral, and earth and DC is power and ground.

The term people associate incorrectly in a circuit is "signal ground" because there is no ground to the signal. Because the signal is referenced equally from DC power and DC ground.
This is the standard that I and IMO most people in industrial engineering use.
1716481401028.png
1716481939959.png
https://www.ni.com/en/support/docum...ignal-grounding--terminology-and-symbols.html

Here we have: Earth ground , Chassis ground and Signal ground all connected to the same ground 'wire', bus, plane or whatever. The reason there are different symbols and names for connections to the same ground wire are related to the energy paths for various electrical conditions that exist or could exist in the circuit.
 
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