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PCB to cope with radiated EMI on PICs

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2007-2009 » Archive through 25 April, 2007 » PCB to cope with radiated EMI on PICs « Previous Next »

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sounded_simple
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Username: sounded_simple

Post Number: 243
Registered: 12-2005

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Posted on Sunday, 15 April, 2007 - 01:42 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi All,

I have a PIC16F873A reading inputs.
The inputs are connected with 10m of printer cable (sheilded)
The project is working fine except when I test for Electro-Magnetic Interference
(EMI)

To get this testing done properly costs thousands but I have a few rough tests that I use myself
- Holding mobile phone close to PCB
- Elecrtronic cooker lighter (little sparker thingy for lighting gas cookers) - make sparks close to the PCB
- A relay wired to "chatter" - hold close to PCB
- Switching on and off my variac.

Not scientific but better than nothing.

Anyhow when I do these tests without the cable attached there is no problem. When I attach the cable then I start to experience random resets to the PIC (both the PIC that the inputs are attached to and another that the inputs are not attached to)

I have added 100nF caps to the inputs but this has not helped.
All the PCB tracks are as short and thick can be.

Does anyone have any suggestions on how to solve this?
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epithumia
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Username: epithumia

Post Number: 376
Registered: 06-2006

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Posted on Sunday, 15 April, 2007 - 02:44 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Is the circuit earthed at both ends of the cable? Because if so then you could be getting unexpected power or noise current flowing through the cable.

Rob
If you need me, Neil and me will be hanging out with the Dream King. - Tori Amos
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sounded_simple
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Username: sounded_simple

Post Number: 244
Registered: 12-2005

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Posted on Sunday, 15 April, 2007 - 03:15 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

The sheilding to the cable is earthed at one end only Rob.
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miked
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Username: miked

Post Number: 39
Registered: 05-2005

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Posted on Sunday, 15 April, 2007 - 11:42 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

You might try putting ferrite beads on the individual wires of the shielded cable where they attach to the pc board.

Mike
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obiwan
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Username: obiwan

Post Number: 1650
Registered: 12-2005

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Posted on Monday, 16 April, 2007 - 03:43 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I can't add too much to that.

But, you'd be surprised to know, that many companies out there use the exact same methods you do. We had one test called the "clacker". God I hated that test. But it involved a particular type of contactor, and it just opened and closed, opened and closed.... Over and over.

But the thing weighed about 10 pounds, so you can imagine that thing jumping up and down on a plastic cart. Not to mention the Whack Blam! Whack Blam! It made every time it operated.

We used cell phones, walkie-talkies, high voltage DC sources, anything we could think of.

Ferrite beads, lots of 'em. They do have noise suppressors for such applications, it consists of a ferrite bead and a capacitor, and there's about 4 or 8 of them in a package.

You'd have to look in a catalog to find them. But they're used on such things as printer interfaces and such.

Maybe add some back to back diodes on there too to clamp the voltage down.
Do Not Hit The Fly That Lands On The Tigers Head.
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john_kent
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Username: john_kent

Post Number: 130
Registered: 12-2006

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Posted on Monday, 16 April, 2007 - 10:39 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Some PCB faults, such as open circuit or high resistance tracks to inputs can act as aerials and introduce some really wierd symptoms. The easy way to check whether it is the PCB is to put the PCB into an insulating polythene bag and then wrap it up in aluminium kitchen foil, connected to a suitable earth. Only the power lines should be connected, to eliminate external cables etc.. This should block out any EMI from the outside world. It works well with the Piezo electric gas lighter test.
First we make it work .... Then we make it simple.
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epithumia
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Username: epithumia

Post Number: 380
Registered: 06-2006

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Posted on Monday, 16 April, 2007 - 11:12 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

This document has a short section on terminations for EMC on the printer port.

http://www.bitechnologies.com/pdfs/rc-series-an.pdf
If you need me, Neil and me will be hanging out with the Dream King. - Tori Amos
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grab
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Username: grab

Post Number: 465
Registered: 05-2005


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Posted on Monday, 16 April, 2007 - 11:26 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

What are you doing with the reset pin on the PIC? Check all the posts on the other two currently-active threads about PICs resetting. You might need to set some time aside for this - they're long! :-)

Graham.

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