setting the isolation gap between traces and ground plane

Hi,

What are the rules for setting isolation gaps between the ground plane and your traces?

I plan on having AC mains touch my PCB only to get through the bridge rectifier, through the smoothing caps, and into the regulator as quickly as possible. On another part of the board I will have 3amp 10VAC touching down under the same circumstances. In these situations, how far should my isolation gap be in between my high power traces and my ground plane for best results.

Thanks,
-JP

My recollection for spacing between mains voltage and safety ground or non-isolated circuitry is  0.250".

The low voltage isolated stuff is less critical. In reality spark plug air-gaps are tens of thousandths of an inch and 10kV. but spacing for agency safety is anticipating dust and other contamination.

JR

JohnRoberts said:

My recollection for spacing between mains voltage and safety ground or non-isolated circuitry is  0.250".

The low voltage isolated stuff is less critical. In reality spark plug air-gaps are tens of thousandths of an inch and 10kV. but spacing for agency safety is anticipating dust and other contamination.

JR

Click to expand...

Gotcha. I increased my gaps to 75 mils which I think should be fine then. 0.012" was not a good size.
I'm also wondering if it makes sense to have a ground plane on both sides as opposed to just one, since this design only needs one layer and I have the option I figure why not shield it even more. Right?

buildafriend said:

JohnRoberts said:

My recollection for spacing between mains voltage and safety ground or non-isolated circuitry is  0.250".

The low voltage isolated stuff is less critical. In reality spark plug air-gaps are tens of thousandths of an inch and 10kV. but spacing for agency safety is anticipating dust and other contamination.

JR

Click to expand...

Gotcha. I increased my gaps to 75 mils which I think should be fine then. 0.012" was not a good size.
I'm also wondering if it makes sense to have a ground plane on both sides as opposed to just one, since this design only needs one layer and I have the option I figure why not shield it even more. Right?

Click to expand...

Just to be clear 0.250" is 250 mils,,,

JR

JohnRoberts said:

buildafriend said:

JohnRoberts said:

My recollection for spacing between mains voltage and safety ground or non-isolated circuitry is  0.250".

The low voltage isolated stuff is less critical. In reality spark plug air-gaps are tens of thousandths of an inch and 10kV. but spacing for agency safety is anticipating dust and other contamination.

JR

Click to expand...

Gotcha. I increased my gaps to 75 mils which I think should be fine then. 0.012" was not a good size.
I'm also wondering if it makes sense to have a ground plane on both sides as opposed to just one, since this design only needs one layer and I have the option I figure why not shield it even more. Right?

Click to expand...

Just to be clear 0.250" is 250 mils,,,

JR

Click to expand...

Thanks for keeping things clear. I'm aware that mils are much smaller than inches, but now I know by exactly how much.

Thanks JR,
-JP

I don't know where the use of mils came from (it's one "mili"-inch, I know) but get's confusing with mili-meter, why not to use mili-inch, and get some symbol for that...

Good to know the number, wired 5 pins relays have only ~200 mils between the pole and the winding, and are used for 250VAC the clearance will be quite lower than that, since you usually need to get through both winding pins with the pole of the switch. I'm making one board right now with those but I'm using a second insulation with optocouplers so not a problem, I have enough clearance between mains and the micro controller, but still, I don't know why the standard footprint is like that, they could take that pin to the other side of the package inside with proper insulation...

I'm talking about this guys:


JS

joaquins said:

I don't know where the use of mils came from (it's one "mili"-inch, I know) but get's confusing with mili-meter, why not to use mili-inch, and get some symbol for that...

Click to expand...

I first heard the term "mils" used as a teenager working in a machine shop back in the '60s. Mils was easier to say than thousandths of an inch, when working a milling machine or lathe.

Good to know the number, wired 5 pins relays have only ~200 mils between the pole and the winding, and are used for 250VAC the clearance will be quite lower than that, since you usually need to get through both winding pins with the pole of the switch. I'm making one board right now with those but I'm using a second insulation with optocouplers so not a problem, I have enough clearance between mains and the micro controller, but still, I don't know why the standard footprint is like that, they could take that pin to the other side of the package inside with proper insulation...

I'm talking about this guys:

JS

Click to expand...

My recollection is just that and as I recall was mostly in the context of wiring (like point to point inside the chassis). PCB traces can clearly be much closer. Just look at the lead spacing on sundry isolation devices like opto-isolators. I've used some SMD parts with thousands of volts protection between input and output that are smaller than 1/4"

for a more authoritative answer see http://www.smps.us/pcbtracespacing.html

JR

JohnRoberts said:

joaquins said:

I don't know where the use of mils came from (it's one "mili"-inch, I know) but get's confusing with mili-meter, why not to use mili-inch, and get some symbol for that...

Click to expand...

I first heard the term "mils" used as a teenager working in a machine shop back in the '60s. Mils was easier to say than thousandths of an inch, when working a milling machine or lathe.

Good to know the number, wired 5 pins relays have only ~200 mils between the pole and the winding, and are used for 250VAC the clearance will be quite lower than that, since you usually need to get through both winding pins with the pole of the switch. I'm making one board right now with those but I'm using a second insulation with optocouplers so not a problem, I have enough clearance between mains and the micro controller, but still, I don't know why the standard footprint is like that, they could take that pin to the other side of the package inside with proper insulation...

I'm talking about this guys:

JS

Click to expand...

My recollection is just that and as I recall was mostly in the context of wiring (like point to point inside the chassis). PCB traces can clearly be much closer. Just look at the lead spacing on sundry isolation devices like opto-isolators. I've used some SMD parts with thousands of volts protection between input and output that are smaller than 1/4"

for a more authoritative answer see http://www.smps.us/pcbtracespacing.html

JR

Click to expand...

I know, I'm using DIP6 4n35 optocouplers rated for 2.5kV but I'd just like this component to have a bit more of clearance since this boards are going to work in a really dusty environment, thanks for the link. Making a cut in the board can increase the clearance which will help a lot in a dirty environment but would be killing my project since I'm so short of time I can't even wait for proper fabricated boards to arrive so I have to etch them myself, so the only option I have is more space, which is ably limited by the components.

JS

joaquins said:

I don't know where the use of mils came from (it's one "mili"-inch, I know) but get's confusing with mili-meter, why not to use mili-inch, and get some symbol for that...

Click to expand...

In the UK we tend to say "thou" for a thousandth of an inch.

My recollection is that European standards demand a 3mm gap between 250VAC traces.

Can I ask why you think you need a ground plane anywhere near these tracks?

Cheers

Ian

ruffrecords said:

In the UK we tend to say "thou" for a thousandth of an inch.
[...]
Cheers

Ian

Click to expand...

Sold!

JS

ruffrecords said:

In the UK we tend to say "thou" for a thousandth of an inch.

Click to expand...

I figured that out a number of years ago. I then thought someone had a pulmonary disease...
For many of us non-english native, it's easier to use mils; I don't think there's much risks of confusion since ther is a 1:50 ratio between mils and millimeters.
Back to the subject, there is not a single answer to the question "what distance?". It depends on several things, and in particular mains connections can use much smaller distance than 1/4", as long as they are protected. Wiring must be fully shrouded and a piece of insulating material must cover the HV tracks (it can be hot-glue). Then the constraints are only electrical.