Author Topic: High voltage/Low voltage/AC voltage grounding  (Read 6134 times)

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Offline VEGETA

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High voltage/Low voltage/AC voltage grounding
« on: February 13, 2014, 02:43:49 pm »
If I have a circuit which has for example:

1- AC voltage via mains (or via step-down transformer).
2- Mains rectified voltage. ~250v DC.
3- DC low voltage source like 5v, 3.3v for powering digital stuff like MCU. these come from the rectified voltage through voltage divider and then regulator. Or from a separate AC-to-DC adapter (9v) and then regulator.

Question is: How can I connect the ground for all of these?

In the circuit, you will find many pins connected to "ground" using one symbol so are all of them (high and low) are connected to the same pin?

I saw some PCB design videos making 2 ground planes which are not connected together (is that correct?), I guess one for high voltage (and thus high current) and the other one is for low voltage ALTHOUGH the schematic itself has only 1 symbol and thus 1 node/point.

And there is "earth" terminal too... what is the difference between it and "ground"?

thanks
 

Offline Simon

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Re: High voltage/Low voltage/AC voltage grounding
« Reply #1 on: February 13, 2014, 02:47:33 pm »
earth and ground usually mean the same thing, earth is generally used to define mains power earth where as ground will be the negative in a low voltage circuit.

If all of your voltage sources are sperate and floating you can gound them all together, just check you don't cause any inadvertant shorts.
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Offline VEGETA

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Re: High voltage/Low voltage/AC voltage grounding
« Reply #2 on: February 13, 2014, 03:06:46 pm »
Well, connecting 220v negative pin from a full-wave rectifier at the same pin as say PIC MCU gnd is safe?

In my question, all of these supplies basically comes from the mains... then gets rectified/regulated or just used as AC voltage.
 

Offline madworm

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Re: High voltage/Low voltage/AC voltage grounding
« Reply #3 on: February 13, 2014, 05:31:45 pm »
Why do you want to connect 220V to a low-voltage circuit anyway?

If you can, use an opto-isolator to keep things separate.
 

Offline VEGETA

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Re: High voltage/Low voltage/AC voltage grounding
« Reply #4 on: February 13, 2014, 09:29:11 pm »
Why do you want to connect 220V to a low-voltage circuit anyway?

If you can, use an opto-isolator to keep things separate.

I said previously, a rectified mains will be about 250v DC... and putting a divider making 5v output from it to power digital circuits or so. The 2 sources (250 and 5) are from the same source actually. so the negative pin of the rectifier and the gnd pin of the 5v regulator, should the be connected to the same node/point (Ground)? if not then how?

 

Offline madworm

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Re: High voltage/Low voltage/AC voltage grounding
« Reply #5 on: February 13, 2014, 10:01:14 pm »
Ah, well if you use a "divider", you already have a common point. I won't call it ground though, as that might make it sound safe somehow.
 

Offline Neverther

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Re: High voltage/Low voltage/AC voltage grounding
« Reply #6 on: February 13, 2014, 10:40:25 pm »
Divider?

If you got AC mains and don't wish to use common switchmode setup to stepdown the DC rail, why not just use PCB transformer with linear regulator to generate stable voltage? It wont be absurdly big if you don't need much power for control.

Separate groundplanes/star grounding is for signal integrity, think about current loops.

AC mains doesnt have real ground, just live and neutral.
The ground is usually included for safety reasons. The product may have good enough isolation (like TVs and such that have passed the spec) so separate groundlead is not needed.

Rectified mains (just full/half wave rectified), grounding that is asking RCD to trip. Who can surely say what neutral/live really is across different systems when referenced to earth.

AC transformers isolate the output -> safe to ground whatever, usually after rectifying so real DC negative is ground referenced. But it doesnt need to be for it to work.
Even switchmode powerbricks are isolated to some degree (don't trust the chinese blindly) -> safe to ground the output.
 

Offline 440roadrunner

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Re: High voltage/Low voltage/AC voltage grounding
« Reply #7 on: February 14, 2014, 03:58:25 am »
I  can't answer  your question  directly  because  I don't know where you are.    But in the  U.S.    you can not  (by  national electrical code)  ground   some things  to some other  things.

Most notably,  you cannot  ground  a  chassis   to a neutral,  nor can you use  a  grounded  conductor  to carry  current

This can  get confusing,  because in reality,    "ground" ground  and  the  "mains" neutral  are the same electrical  point  at one place........where  the   mains  comes into the house  at the main  power  / meter panel.    again,  I'm  speaking  "American"  LOL

So an example..................

You have  a typical  U.S.  electric  kitchen range which  (in this case)  uses  no  120V  circuits,  and therefore  needs no neutral  for  actual operation.    These used to be wired up with a  three wire plug...........a  ground,  and  2  240V  hot wires.    The   ground was purely for safety, and connects to the range  housing / case.

So  let's change this  some...........you are  building  a ham radio homebrew  amplifier  operating on  240V,  and it  DOES have  some 120V  components,  for controls,  etc

You can NOT  use    a 3 wire plug in this case,  and you can NOT use  the case  as the neutral  for the 120V  circuits.

If you are in the UK,  example,  I have no idea  how this affects

Here in the US  there used to exist  small cheap  kitchen  / table radios  called the  "all American five"  because they mostly all used the same  tubes  with series wired filaments  (heaters)  across the 120V  line.   These  were  usually in  plastic  cabinets,  and  "supposedly"  the  metal chassis  was  "isolated."

BUT THEY HAD   capacitors  to the metal chassis  from one side of the line,  and   the leakage current  in these caps  could be dangerous.   Depending on which way  the  (older style)  line plug was inserted,   the chassis  could be  "hot."
 

Offline VEGETA

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Re: High voltage/Low voltage/AC voltage grounding
« Reply #8 on: June 07, 2014, 11:08:44 pm »
Thanks for nice answers. Maybe I didn't mention the point correctly...

Think of it as a Transformerless power supply that is to be connected to microcontrollers and other high voltage stuff. Is it clearer now? here, the -ve side of the full bridge will have the ground sign right? but is it really safe to connect the GND of the microcontroller (or other low voltage chips) to that node while connecting other high voltage -ve (or GND) pins to that pin? < here, it will take in both high and low currents which can damage the low voltage chips.

This was my concern. Some time ago I wanted to read AC voltage of the mains into the PIC MCU by a transformerless method of rectifying and dividing... I wasn't sure that it will be safe or not, affect other ICs or not.... so I used some 220v-to-5v transformers. I want to know how to handle this without transformers.

Also, I had a similar circuit which drives 2 TRIACs using MOC chips... Again, here will have both high voltage and low voltage with a common negative node!

thanks always!
 

Offline Zero999

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Re: High voltage/Low voltage/AC voltage grounding
« Reply #9 on: June 08, 2014, 11:01:44 am »
We can't really answer with out a schematic.

There are two types of ground/earth. There's the common reference in a circuit (neutral for mains, 0V in a DC circuit, often refereed to as ground) which doesn't have to be physically connected to earth but often it is and then there's the protective earth conductor which is always connected to the earth.

Rectified mains is equal to the peak voltage,  minus some ripple. If your mains voltage is 220V, this is not 250VDC but 222*root(2) = 311V.
 

Offline rob77

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Re: High voltage/Low voltage/AC voltage grounding
« Reply #10 on: June 08, 2014, 11:51:23 am »
you can't use the main's neutral as "ground" - that apply in all countries (i hope, because if not then poor citizens of that country)
you have a ground in the mains plug (yellow-green wire) - that one is for grounding
the ground (yellow-green) wire MUST NOT carry current

in some countries on some older installations the main's neutral and ground are connected - those are 2 wire installations , but you still have 3wire wall plugs - and even in those cases you have to threat those as 3 wire , therefore use 3 wire plugs for your device and use the ground pin ONLY for grounding.

if your device is using 2 wire plug , then it have to have double insulation and no metal parts exposed.

if you're driving some switching elements (regardless of rectified or straight mains) from your MCU - use optocouplers for that, and power your MCU from a INSULATED power supply.
 

Offline Andy Watson

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Re: High voltage/Low voltage/AC voltage grounding
« Reply #11 on: June 08, 2014, 12:37:50 pm »
Think of it as a Transformerless power supply that is to be connected to microcontrollers and other high voltage stuff. Is it clearer now?
A schematic would make things a whole lot clearer.
Quote
here, the -ve side of the full bridge will have the ground sign right?
No. The -ve side of the bridge will follow the most negative of the mains - sometimes the neutral ~0V, sometimes the live ~ -330V.
Quote
but is it really safe
I feel compelled to say that nothing I've read in this thread would lead me to believe your scheme is safe. Take the advice given above and use a transformer or other suitably rated isolated supply.
 

Offline ejeffrey

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Re: High voltage/Low voltage/AC voltage grounding
« Reply #12 on: June 08, 2014, 06:02:51 pm »
If you are using a transformerless power supply you cannot connect any part of your circuit to the ground pin on your wall outlet.  You would use the 'neutral' line as your 0V point, and any chips with 'ground' pins would be connected to that.  When you are looking at a schematic, unless otherwise specified, "ground" just means "zero volts reference" which may or may not be connected to the physical ground/earth connection depending on how the circuit is used.

On the other hand, all externally touchable metal parts including the shield conductor of any cables must be connected to ground.

That means that if you have a transformerless design you basically can't have any external connections.

In general, transformerless designs should be avoided in hobby projects unless there is actually some reason you need to run on the line rather than just for cost savings, and even then you often want an isolated supply with as much circuitry as possible on the safe side, and then just communicate with optocouplers to the hot side.  You could do this for e.g., a power quality monitor or an electronically controlled power switch/dimmer.  Even then look for alternatives with the isolation built in like current sense transformers and opto-triacs/SCRs.
 


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