Hi,

I have a PCB which will have mains, 16VAC from a transformer, 12VDC and 5VDC supplies (from the 16VAC) and general logic (including a microcontroller). Mains comes onto the board and is then fed out to the transformer. The mains is switched to a heater using a triac (which is isolated using an opto-triac).

I'm not sure of the best way to ground the board. This is how I'm thinking of doing it at the moment:

So there would be two ground planes on the board, G1 for the mains circuitry (kept well separated from the rest of the circuitry) and G2 for the rest.

Is this the best way of doing it, or should G1 and G2 be tied together? The circuits, sensors, actuators, motors are all inside an insulated case (plastic, not grounded).

Thanks.

 

It is pretty much optional with the set up, but I would be inclined to earth ground both.
But the incoming service ground conductor should not connect to the neutral at the transformer (G1) , this is only done at the service entry.
Max.

 

hi ra,
Check out this PDF, may help with a solution.
E

 

It is pretty much optional with the set up, but I would be inclined to earth ground both.
But the incoming service ground conductor should not connect to the neutral at the transformer (G1) , this is only done at the service entry.
Max.

Thank you.
I haven't explained myself correctly: the grounds I'm showing (G1 and G2) are not earth grounds (there is no mains ground connection to the board ... at least not in my sketch). What I am showing is the Mains Neutral as being the ground for the mains circuitry, and one of the transformer lines as ground for the rest of the circuitry (so the circuit would be floating wrt ground). The transformer is just the 16V transformer (in my sketch it seems to be 2 transformers, but it's just the one).

So are you suggesting that I should connect G1 to G2 (with the mains neutral connected to ground at the service entry, as it should be)? Would there be a possible risk there (for example if mains live and neutral were swapped by mistake?).

 

hi ra,
Check out this PDF, may help with a solution.
E

Thanks ... at first glance this would seem to recommend using mains ground (green wire) as the ground for everything (so case ground and PCB ground plane): that would complicate the PCB layout quite a bit. But I can see that it might be better as the ground plane would then not be current carrying.

 

Thank you.
I haven't explained myself correctly: the grounds I'm showing (G1 and G2) are not earth grounds (there is no mains ground connection to the board ... at least not in my sketch). What I am showing is the Mains Neutral as being the ground for the mains circuitry, and one of the transformer lines as ground for the rest of the circuitry (so the circuit would be floating wrt ground). The transformer is just the 16V transformer (in my sketch it seems to be 2 transformers, but it's just the one).

So are you suggesting that I should connect G1 to G2 (with the mains neutral connected to ground at the service entry, as it should be)? Would there be a possible risk there (for example if mains live and neutral were swapped by mistake?).

This is the reason you do not connect a earth ground plane to anything but the earth service conductor, using the Mains Neutral as earth ground as you show is a NO-NO.
If you have no service ground in the feed to the mains transformer, then it becomes a moot point as there is no Earth available.
The P.S. commons can be connected as you wish as they are isolated from true earth GND.
IF you do have a service earth GND conductor available then you can optionally connect circuit P.S. commons to this.
So your explanation is a little confusing.
Max.

 

This is the reason you do not connect a earth ground plane to anything but the earth service conductor, using the Mains Neutral as earth ground as you show is a NO-NO.
If you have no service ground in the feed to the mains transformer, then it becomes a moot point as there is no Earth available.
The P.S. commons can be connected as you wish as they are isolated from true earth GND.
IF you do have a service earth GND conductor available then you can optionally connect circuit P.S. commons to this.
So your explanation is a little confusing.
Max.

Perhaps the problem is that my terminology is woolly (putting it kindly). G1 and G2 are not connecting to earth ground at all: they are the mains neutral and transformer 'neutral' respectively. They would be the copper ground planes on the PCB. I'm not sure how to put this using the correct terminology.

(There is no earth wire to/from the transformer, but the transformer case will be connected to earth).

I do of course have a mains earth available and it will be connected to the equipment chassis. I could bring it to the PCB and I could connect this to the PCB copper ground plane. This would seem to be the best way to do it and it would certainly be safer. However it would make the PCB layout more complicated because every component ground would then need an individual connection to neutral (this would also make ground loops more likely) instead of simply being connected to the PCB "ground" plane/s.