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

[Mike S]

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