How do i use Common Mode Choke Coils for Signal Line

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Guyzz

I was planning on some RS-485 on CAT5-cable , using 2 wires for RS-485 +/- , 2 wires for VCC 24v , 3 wires for GND.
And maybe 1 wire for some RS-485 ground (I think i saw that somewhere , w. a 100 ohm from GND to the RS-485 GND) , or else make it 3 wires for VCC.

I have been hearing about "Ground Bouncing" , if i run VCC+ GND along w. the RS-485 , but i use LT1785 , and was told that they were ok. The only thing was if i remember correctly , that they were slow in the enable , meaning you had to do a bit of waiting before starting to xmit.

http://www.linear.com/pc/downloa...

But i have a reel of : ZJYS51R5-4PT
http://www.tdk.co.jp/tefe02/e971...

As i read it is a 4x common mode 2Amp 200ohm/Imp inductor or transformer thingy :oops:
And was wondering if/what to use them for , could they be brought to good use used for something like the above ?

I am not sure what to use them for ... :oops: :oops:

But i have an idea about they can isolate some signal stuff .... (I got them cheap 8))

Any ideas to put them in use ??

/Bingo

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Your signal goes in one end and comes out the other! Common mode interference to both the signals get cancelled in the transformer.

Note they are not isolation transformers - I'd expect the type of core material is very lossy.

As for 'ground bouncing' - the thing to be more worried about is the voltage drop across your 0V conductor - so that 0V at the source end is not necessarily 0V at the other end.

The enable time of the LT1785 is slower than the other '485 drivers. Really only becomes a problem at the higher baud rates, nevertheless, it's something to consider. I just shove in a few NOPs and some other code before I send the first char out.

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@Kartman

So there is no need for those in a setup like the above ??

/Bingo

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Hey,

As a quick background on common mode chokes, check out http://www.murata.com/emc/knowho...

Essentially they are more used for reducing EMI radiated from a differential pair, or for cancelling noise induced on a differential pair.

Quote:
And maybe 1 wire for some RS-485 ground (I think i saw that somewhere , w. a 100 ohm from GND to the RS-485 GND)

If done carefully that could work. The idea there is to limit the ground current that will flow through the wire, killing your possible ground loop.

For this to work through you need to make sure there is no way you would get much ground current! As you could otherwise end up with more common-mode voltage than expected...

Another option is to power the 485 drivers from a small DC-DC isolated power supply, something like http://www.lightobject.com/1205S.... That way you will never have to worry about ground currents or high common mode voltages. It obviously depends a lot on your application!

-Colin

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@Collin

Hi Collin

Thankyou for the nice doc on Commonmode.

In the above setup i was hoping to do a RS-485 bus on PDS , around 30 mtr. long , and feed 5..8 small M168 sensor boxes w. RS-485 and power. I would feed 24v on the power wires , as i use LM2775 switchers and didn't get the HV version (i would have liked to use 48v).

These are the modules.
http://www.ere.co.th/(oxftfc55xjqq32v0t23m4qnq)/default.aspx?RedirectPage=Products&RedirectPage1=ProductsDetail&ProductID=18

So my biggest concern is apparently the GND , at the far end of the 30m PDS.

Well i do have some cute 220v 600ma 6.7v switchers , and a ton of mic2950-5 LDO's
http://www.micrel.com/_PDF/mic29...

Maybe i should abandon the power in the PDS , but that would get me in trouble w. the wifey. As they are sensors for the summerhouse. And she hates visible wires and especially if they are as thick as a 220v wire.

/Bingo