Dissimilar metal voltage source issue

[bcdzt5]

Lets start things off with an admittance of ignorance. I am an undergraduate mechanical engineer just getting his feet wet in electronic design. I have searched the forums and Google, and found no satisfactory answer to the below question.


I noticed that in most circuits several dissimilar metal junctions are present. Lead, copper, steel and tin (and others) are all present, and connected.

Doesn't each junction create some small (mV) DC voltage, like a thermocouple? Is this negligible and safe to overlook, or is this a possible source of DC voltage output bias (in amplifiers) and other troubles?

[PigletsDad]

Thermocouples only produce a voltage when there is a temperature difference between the various junctions.

The voltages are too small to matter for audio, but are very important when building instrumentation, especially if you want to measure things at temperatures well away from ambient. I used to work on superconducting electronics, and the voltages we were interested in tended to be very small (microvolts to millivolts) while the device would be at say 40 or 50K, so the thermocouple effects in the wiring and fixtures were a major issue for us. Although we had to be careful, we could always work round it.

[Conrad Hoffman]

What usually happens is that the dissimilar metal junctions come in pairs. A resistor might have two steel end caps- they both generate a voltage, but it cancels out. As long as both dissimilar junctions are at the same temperature, you're ok. In precision DC instrumentation, you'd mount your resistors horizontally, rather than vertically, for this exact reason. In audio equipment you'd never see these offsets, and the only parts that get hot enough to generate much are in locations where the sensitivity is low.

[bcdzt5]

Alright, that answers it. Thanks PigletsDad and Conrad Hoffman!

[pooge]

I think the biggest issue is not the DC voltage that may be generated, but the issue of corrosion due to the dissimilar metals.

[unclejed613]

dissimilar metals are charted in what's called the "galvanic series".
a chart of the galvanic series can be found here:
http://www.corrosionsource.com/handbook/galv_series.htm

conversely, you can predict what voltage a battery made from two dissimilar metals would produce by summing the voltage difference between the metals (i.e. metal A=+.5V and metal B=-.5V, so a battery made from them would produce 1V)

i've seen dissimilar water pipes bypassed with a large ground braid cable to minimize corrosion at the joint