Contact resistance

[Yzordderrex]

Greetings,

I am using a gold pogo pin to make contact in a 2amp application. The
metal it is in contact with is stainless steel. I am measuring 0.12
ohms across the interface. Does this seem correct? I would like to
know if anyone has a link to a paper which would discuss this. I
might have to take a look at changing the stainless to something else.

regards,
Bob
N9NEO

 

[[email protected]]

Greetings,

I am using a gold pogo pin to make contact in a 2amp application. The
metal it is in contact with is stainless steel. I am measuring 0.12
ohms across the interface. Does this seem correct? I would like to
know if anyone has a link to a paper which would discuss this. I
might have to take a look at changing the stainless to something else.

regards,
Bob
N9NEO

What method are you using to measure the resistance? Hopefully not an
ohm meter alone. I can't say 0.12 ohm is correct but at 2-amps that's
a 0.24v drop. How about getting 4-clip leads, a constant current
source and a voltmeter and make a 4-point measurement. Tie the current
source to the pogo pin and the steel plate, connect the voltmeter to
the pogo and steel plate near the point of pogo contact then touch
the pogo to the steel plate and measure the voltage. Apply ohms law
and determine the resistance. If the measurement point is away from
the point of contact you may be measuring the resistance of the steel
plate. Perhaps you could apply multiple pogo's and/or a sensing scheme
to minimize the losses.

good luck!
al

 

[[email protected]]

Greetings,

I am using a gold pogo pin to make contact in a 2amp application.  The
metal it is in contact with is stainless steel.  I am measuring 0.12
ohms across the interface.  Does this seem correct?  I would like to
know if anyone has a link to a paper which would discuss this.  I
might have to take a look at changing the stainless to something else.

Stainless steel is stainless because it is covered with a coherent
layer of not-all-that-conductive chromium oxide - 0.12 ohms sounds
very like the sort of resistance levels that caused me to dump
stainless steel electrodes in a conductivity meter and go for
platinum.

For you application gold would be traditional. Printed circuit
manufacturers used to routinely selectively plate a micron or so of
hard gold onto the contact fingers on plug in printed circuit boards.
Even that couldn't take enough pressure to force metal-to-metal
contact through the gold sulphide that builds up - someone had to
polish the connector pins every six months or so to keep the contacts
reliable - and two part connectors eventually took over.

Pogo pins do provide enough force but they will probably wreck the
target surface if you make contact more than a few hundred times,
which is plenty for production testing (where they are normally used).

 

[Archimedes' Lever]

Stainless steel is stainless because it is covered with a coherent
layer of not-all-that-conductive chromium oxide

Stainless gets abrasively polished. Any coating placed on the raw stock
before polishing gets abraded off. Everything that stainless is, is
throughout the alloy matrix that makes up the medium.

- 0.12 ohms sounds
very like the sort of resistance levels that caused me to dump
stainless steel electrodes in a conductivity meter and go for
platinum.

He should go by a set of lawn mower engine points. Easy to find and
nice-n-cheap. Both contacts are platinum plated, which NEVER oxidizes,
and is the proper point contact medium if longevity is a concern. All
that would be needed then is an actuation mechanism.

 

[Archimedes' Lever]

Pogo pins do provide enough force but they will probably wreck the
target surface if you make contact more than a few hundred times,
which is plenty for production testing (where they are normally used).

Another solution is to place a rheostat style variable resistor inline
with the contact being used, and dial up the resistance during placement
and removal of the contact pin.

Hot swapping heavy amperages always leads to degradation of the "points"
between which the circuit is "make" or "break". If there is any inductive
loading, there may be a reversion spike on "make" and "break" as well. So
accounting for that additional amperage or "pulsed flow" is wise as point
degradation is fast and furious in most hot switched applications.

 

[Al]

That does sound a little high, but the shape of the contact will matter
a lot. Of you use a crown or point pogo, against that hard flat surface,
the actual contact area may be very small. If it matters, try some other
shapes, like a moderate-radius ball end.

John

Contacts should be like metal to like metal. The gold to stainless will
be a problem. There are many papers out there on the subject.

Al