Signal/chassis ground

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lagger
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Posted by lagger: Mon. Aug 9, 2010 - 04:54 PM
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Hi,

I'm making a pcb with GPS and GSM modules. I've found a nice series of extruded aluminum casings that I have designed my pcb to fit in.

I need to have the two antenna connectors on the front bezel of this case, and i was planning to use MMCX to SMA bulkhead cables for this. Then I realized that this would short signal ground to chassis ground though the bulkhead connector.

I'd really rather avoid this if i could, but i haven't found a nice way to do this yet.

It's supposed to run on a 12V12Ah or similar battery, so one could accidentally burn the antenna cable by connecting plus to chassis and minus to signal gnd. :(

Any bright ideas?

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Learning and Information, General Electronics
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DocJC
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Posted by DocJC: Mon. Aug 9, 2010 - 05:18 PM
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How about a diode in series with the V+ power supply line?

JC

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lagger
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Posted by lagger: Mon. Aug 9, 2010 - 05:21 PM
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how would that disconnect signal ground and chassis ground?

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DocJC
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Posted by DocJC: Mon. Aug 9, 2010 - 08:40 PM
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My mistake, I mis-read the "by connecting plus to chassis..." and thought you were referring to high current flowing in the event of a reverse polarity connection.

You still need your signal and chassis grounds tie together, somewhere...

Perhaps a polarized battery connector so that the battery can only be connected to the correct cable, (and connected the correct way), would help.

A block diagram of the system would help to see the present layout, and current pathways, better.

JC

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alwelch
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Posted by alwelch: Mon. Aug 9, 2010 - 11:14 PM
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We used to use Teflon shoulder washers and flat washers also in Teflon to insulate our ADC input jacks from the brackets they were mounted in. We purchased them from

http://www.accuratescrew.com/CatalogPage.aspx?ProdCat=WSHSHLD1/4

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Kartman
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Posted by Kartman: Mon. Aug 9, 2010 - 11:39 PM
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what happens if the battery shorts to the signal ground? unless there is some form of isolation, the current will find its way back through your signal ground which might not be where you want it to go. Nevertheless, the battery should be suitably fused so that smoke won't occur.

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lagger
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Posted by lagger: Tue. Aug 10, 2010 - 03:58 PM
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Nice find alwelch, thanks a bunch :)

1/4 inch works for SMA
3/8 inch works for FME

Now, onto the chassis connected to ground issue.

Until now, I would rather avoid connecting signal ground to the chassis. So really I have these options:

- Connect chassis and gnd at a single point.
- Isolate chassis and gnd.

One of the environments i plan to install this on is a 4m metal pole which will happily work as a lightning rod.

I've seen lots of electronics where both choices have been taken.

I've also seen motherboards and harddrives not working whenever they were connected to chassis.

So what would you choose, and why?

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max_torque_2008
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Posted by max_torque_2008: Tue. Aug 10, 2010 - 05:41 PM
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Will the device sit on the end of the metal pole, or can it go some distance down the pole?? if it has to be on the end, i'd be a bit nervious about spec'ing the ESD protection for this one!!!

not sure how big a TVS you need for 400kV & 10kA ;-)

as for the other question, i'd go for single point contact for chassis and signal gnd.

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Kartman
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Posted by Kartman: Wed. Aug 11, 2010 - 12:51 AM
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For lightning protection you'll need special protection for your antennas. These suaully take the form of a metal block with rf connectors on both ends. The block has a flange that is bolted to earth in order to direct the lightning energy. You would most likely bolt your box to something metallic that is also earthed thus your signal gnd is connected to chassis gnd by means of the lightning protector.

By having the antenna connectors earth to the chassis by their mounting means that fault currents will flow through the chassis, not your signal gnd.

There's many reason for and against all of this, you need to evaluate likely faults (like lightning) and determine the best course of action.

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