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Parallel resistance

john2k

Jun 13, 2012
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In the following drawing example I have a CANBUS that is terminated on both ends with a 120ohm resistor giving the final ohm reading of 60ohms across the two lines. I want to connect the instrument cluster that also has a 120ohm resistor inside it. This will essentially mean instead of there being two 120ohm resistors in parallel, technically there will now be 3 120 ohm resistors. What ohm value will that give across the two lines? I need the ohm reading across the two lines to remain at 60 ohms. Any ideas how I can do this?

Thanks.

Wncp4hN.png
 

Tha fios agaibh

Aug 11, 2014
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If you wanted to leave the internal 120Ω inside the cluster, you could change just the 2 resistors to 237Ω to achieve the desired 60Ω resistance.
 
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john2k

Jun 13, 2012
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Sorry i forgot to mention I dont have the ability to change any of the 3 resistors, i can only add extra resistors between the canbus and the instrument cluster
 

Tha fios agaibh

Aug 11, 2014
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If you put 3, 120Ω in parallel it equals 40Ω.
Paralleling more resistors will only make it even lower.
 

john2k

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Could I not add some in series on either the red or the black wire to increase the resistance?
 

Tha fios agaibh

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Could I not add some in series on either the red or the black wire to increase the resistance?
I'm not familiar with canbus but I believe it would degrade the data signal strength.

The purpose of a termination resistors are to reduce ac noise and interference imposed on the network.
 

davenn

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Sep 5, 2009
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Sorry i forgot to mention I dont have the ability to change any of the 3 resistors, i can only add extra resistors between the canbus and the instrument cluster

If you know the resistor is there, on the can bus. remove the one closest to where you will terminate the instrument cluster so that there is only 1
resistor at that end.
With the work I do on can bus gear, I find it hard to believe you cannot get at or see the resistors
 

Martaine2005

May 12, 2015
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I’m still doing the math. 1/120x3.
I can however count my empty cans!...

Martin
 

Kiwi

Jan 28, 2013
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The theory is that there should be a 120Ω resistor at each end of the CAN bus(backbone) giving a total resistance of 60Ω, but I have seen systems working with three resistors.
Suggest you just hook it up and see if it works.

Manufacturers that insist in putting resistors in their units are a real pain in the butt?
 
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