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Routing thermocouple wires

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jmfpublic(at)comcast.net
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Posted: Sat Feb 11, 2006 5:59 pm Post subject: Routing thermocouple wires

Bob, or anyone:



Is it reasonable to route thermocouple wires along the spark plug wires?  I will be using a thermocouple amplifier with a + and - 15 volt supply.  The spec sheet for the AD596AH says to limit common mode input voltages to less than the power supply band.  One side of the thermocouple is connected to the power supply common, and I will be using insulated probes to avoid the problem of multiple ground paths and associated voltage offsets.  The probe in a ring terminal under the spark plug-see my prior post about this.  Neither the spark plug wires nor the thermocouple wire is shielded.  Since one side of the thermocouple is grounded to power common, the driving impedance of this source should only be the resistance of the wire itself, which has a resistance of less than 3 ohms.   On the Jabiru interest group on Yahoo, one person has run the TC wire with the spark without problem.  For design reasons which involve separate "ram air" ducts over each bank of cylinders, routing with the spark plugs would be the easiest.  Anyone see a problem with this?



Jim Foerster, J400, wiring

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ainut(at)hiwaay.net
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Posted: Sat Feb 11, 2006 7:30 pm Post subject: Routing thermocouple wires

James, let me strongly suggest you do not run tc wire alongside the 

spark plug wire.  With the large voltages carried in the plug wire, if 

there is ever the slightest short, you AD chips are toast, along with 

possibly whatever you're reading them with.

Secondly, you need to run a separate ground for each tc all the way to 

the AD chips, not use a common ground.  Talk to the AD engineers if this 

is questionable to you.  Yeah, I know I just doubled your firewall 

penetration wires, but it has to be done.  I didn't like it either .



David M.

James Foerster wrote:



 	  Quote: 	
		   

 

 Bob, or anyone:

 

 Is it reasonable to route thermocouple wires along the spark plug wires?  I will be using a thermocouple amplifier with a + and - 15 volt supply.  The spec sheet for the AD596AH says to limit common mode input voltages to less than the power supply band.  One side of the thermocouple is connected to the power supply common, and I will be using insulated probes to avoid the problem of multiple ground paths and associated voltage offsets.  The probe in a ring terminal under the spark plug-see my prior post about this.  Neither the spark plug wires nor the thermocouple wire is shielded.  Since one side of the thermocouple is grounded to power common, the driving impedance of this source should only be the resistance of the wire itself, which has a resistance of less than 3 ohms.   On the Jabiru interest group on Yahoo, one person has run the TC wire with the spark without problem.  For design reasons which involve separate "ram air" ducts over each bank of cylinders, routing 

wi!
	


 	  Quote: 	
		    th the spark plugs would be the easiest.  Anyone see a problem with this?

 

 Jim Foerster, J400, wiring

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