Shielding Thickness?

Thread Starter

Chris65536

Joined Nov 11, 2019
224
I built a large digital circuit on multiple breadboards. It runs at 5V and 8MHz, and I imagine it puts out a lot of EMI. I was planning to enclose it in a box made of copper sheet, since it looks cool. My questions are related to this enclosure. What is the minimum thickness this copper should be? I figure the practical range is .016" - .032". Thinner sizes get really expensive. But would there be any difference electrically? Also, should I connect the shield to the circuit ground? The 5V power comes from a USB battery.
 

OBW0549

Joined Mar 2, 2015
3,546
I built a large digital circuit on multiple breadboards. It runs at 5V and 8MHz, and I imagine it puts out a lot of EMI.
Maybe. Maybe not; it all depends on what you mean by "a lot."

I was planning to enclose it in a box made of copper sheet, since it looks cool. My questions are related to this enclosure. What is the minimum thickness this copper should be? I figure the practical range is .016" - .032". Thinner sizes get really expensive.
I don't think there's any minimum (or maximum), unless your needs for EMI attenuation are truly extreme.

But would there be any difference electrically?
Not that you'll be able to notice.

Also, should I connect the shield to the circuit ground?
Yes.

The 5V power comes from a USB battery.
Watch out: those 5V USB power packs operate on a switching regulator, and some of them may not be well shielded. A really bad one may produce even more noise than your digital circuit.
 

Thread Starter

Chris65536

Joined Nov 11, 2019
224
Maybe. Maybe not; it all depends on what you mean by "a lot."
Well, I'd like to bring the thing to work, which is 1/4 mile from O'Hare airport. So I want it to be legal for sure.

I don't think there's any minimum (or maximum), unless your needs for EMI attenuation are truly extreme.
Thanks, this is the answer I was looking for. I ordered the .020" copper.

Watch out: those 5V USB power packs operate on a switching regulator, and some of them may not be well shielded. A really bad one may produce even more noise than your digital circuit.
I'm actually using a Lithium battery "power pack" with a USB-A socket, to make it portable. You are right about the AC-DC converters of course. I wonder if I could measure the EMI, before and after shielding? I have a couple of those software defined radio things, which go to 1.75 GHz I think?
 

OBW0549

Joined Mar 2, 2015
3,546
Well, I'd like to bring the thing to work, which is 1/4 mile from O'Hare airport. So I want it to be legal for sure.

Thanks, this is the answer I was looking for. I ordered the .020" copper.

I'm actually using a Lithium battery "power pack" with a USB-A socket, to make it portable. You are right about the AC-DC converters of course. I wonder if I could measure the EMI, before and after shielding? I have a couple of those software defined radio things, which go to 1.75 GHz I think?
I don't think you'll have any problem. If you go there and fire up your device and shortly an armored personnel carrier with a SWAT team from the FCC shows up at your door, you'll know you're putting out too much EMI.

Sort of that, I wouldn't worry about it.
 

MisterBill2

Joined Jan 23, 2018
7,004
For a low powered circuit working at medium frequencies the mechanical strength of the shield is what matters. Too thin and it flexes and can cause problems. Too thick and it is heavy and harder to keep in place. If it is copper then it will stop all of the electrical signals from passing through, and that is what you want.
 

Analog Ground

Joined Apr 24, 2019
416
I'm actually using a Lithium battery "power pack" with a USB-A socket, to make it portable. You are right about the AC-DC converters of course. I wonder if I could measure the EMI, before and after shielding? I have a couple of those software defined radio things, which go to 1.75 GHz I think?
I caution everyone I run across using these battery packs that they need a minimum load or they will shut off after 10 to 20 seconds. Typically this minimum is around 100 mA but can vary a lot.
 

OBW0549

Joined Mar 2, 2015
3,546
I caution everyone I run across using these battery packs that they need a minimum load or they will shut off after 10 to 20 seconds. Typically this minimum is around 100 mA but can vary a lot.
Good point. I've got one like that-- it won't power an Arduino because of insufficient current draw.
 

Thread Starter

Chris65536

Joined Nov 11, 2019
224
I caution everyone I run across using these battery packs that they need a minimum load or they will shut off after 10 to 20 seconds. Typically this minimum is around 100 mA but can vary a lot.
I did notice that problem with one particular battery, when running an ATMEGA chip. This circuit draws ~450mA though, so the batteries stay on.
 

Thread Starter

Chris65536

Joined Nov 11, 2019
224
I don't think you'll have any problem. If you go there and fire up your device and shortly an armored personnel carrier with a SWAT team from the FCC shows up at your door, you'll know you're putting out too much EMI.

Sort of that, I wouldn't worry about it.
I always do wonder, when helicopters fly over. :) Thanks for your advice.
 
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