[volt-nuts] Some questions to zeners (thermoelectric effects)

[David C. Partridge]

But if you have five solder joints on conenction to the +ve terminal and 7 on the connection to the -ve terminal at different locations on the PCB, then all bets are off ... 


Regards,
David Partridge
-----Original Message-----
From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com] On Behalf Of Mike S
Sent: 28 January 2013 17:11
To: volt-nuts at febo.com
Subject: Re: [volt-nuts] Some questions to zeners (thermoelectric effects)


On 1/28/2013 8:48 AM, Tony Holt wrote:
 > Could the sense wires be welded to
 > the ADC pins between the solder connection to the PCB and the package to  > avoid the thermal EMFs of a solder joint?

I don't think welding would make the difference, unless the wire is made of the same material as the pin.

I'm also not clear to me how low-EMF solder helps in most cases. Solder joints tend to be small and local, and it's the temperature difference between the terminals which brings the thermoelectric effect into play.

For example, two copper wires soldered together - you have a Cu-solder joint, followed by a solder-Cu joint, in very close proximity. As long as "close" is close, and/or there's good thermal mass/conductivity, don't the thermocouples simply offset each other?

More realistically, take a device with common tinned brass terminals on a PC board. You have brass/tin, tin/solder then solder/copper thermocouples in very close proximity, essentially resulting in a brass/copper thermocouple. It seems that the temperature difference between that connection and the similar thermocouples at the far end device connection would overwhelm the local effects due to solder, which require a temperature gradient across some small fraction of a mm.

Even with much larger, hand soldered terminals, something similar would seem to apply. Wouldn't thermally insulating the terminals (which by nature have pretty good thermal conductivity) to ensure a consistent temperature across them be as good or better than just low EMF solder?

---
Mike
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