contact resistence vs use vs time
contact resistence vs use vs time
(OP)
A fellow here has witnessed a drop in resistance of a crimp type joint after the first application of current, and figures it is a contact resistance effect fromn welding.
Is there a name for this phenomenon?
-----------
Most all sources publish a resistance increase for bolted or crimped connection joint over time, or with temperature or service cycles. This would suggest the electrical "sweetening" is at best a one time event.
Is there a name for this phenomenon?
-----------
Most all sources publish a resistance increase for bolted or crimped connection joint over time, or with temperature or service cycles. This would suggest the electrical "sweetening" is at best a one time event.





RE: contact resistence vs use vs time
RE: contact resistence vs use vs time
The eye witness describes the effect as nearly instantaneous and visible on an oscilloscope, and permanent, at least short term in the lab. He was pretty certain it was an improvement in contact resistance. I'll try to find out more.
RE: contact resistence vs use vs time
RE: contact resistence vs use vs time
A crimp is a mechanical compression of two metals together. Whatever was on the surfaces is now between two metals. Sufficient current could potential burn off volatiles and break down other contaminants and allow better metal to metal contact.
Addtionally, since most surfaces only contact at the tips of asperities, the current density at those points might be high enough to cause deformation and broadening of the contact points, thereby lowering the net resistance.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: contact resistence vs use vs time
How much current are we talking about that is being used?
RE: contact resistence vs use vs time
Ordinarily the changes to one single connection as described would be so small that they would go quite unnoticed even using very sensitive measuring equipment.
Just how dirty does a crimp tag (and there are some very tarnished ones waiting to be used on many shelves)need to be to have such a noticable effect? I suppose the same could be true of some cables having grotty surfaces on their wire cores.
In a correctly crimped connection the interface of each face effectivly welds to the mating face giving a gas tight and therefore somewhat stable and corrosion resistant joint.
Dirty contacting faces will prevent the joint from being properly formed and should have no place in anything which we would want to be reliable.
RE: contact resistence vs use vs time
RE: contact resistence vs use vs time
I am a little suspicious of the meausurement method and would like to know exactly what was measured (voltage, current, resistance), using what meter and what test setup.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: contact resistence vs use vs time
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: contact resistence vs use vs time
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: contact resistence vs use vs time
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: contact resistence vs use vs time
Four leads?! Do you mean measure voltage, current and use
ohms law? Or is there a procedure I need to learn about?
Please explain.
Thanks
RE: contact resistence vs use vs time
Basically the current is applied using one set of wires and the resultant voltage is measured with another set of wires. This makes for a much more accurate (and thus more sensitive as well) measurement of R.
The instrument drives an accurate current down the stimulus leads where voltage drops don't matter anyway. And there is virtually no current in the voltage leads (Hi-Z input) so there isn't any voltage drop to speak of.
In practice, instead of using probes, you use two clips. Each side of the clip is a separate connection to the instrument.
End result is a lovely stable R measurement with lots of digits to the right of the decimal point.
RE: contact resistence vs use vs time
Thank you!
RE: contact resistence vs use vs time
http://en.wikipedia.org/wiki/Four-terminal_sensing. The approach reduces the impact of the probe resistance from the measurement of the voltage drop.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: contact resistence vs use vs time
Dan - Owner

http://www.Hi-TecDesigns.com