Contact resistance

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

What kind of pogo pins are you using? If they're needle points, then it's possible that they don't have enough pressure to dimple the stainless plate adequately to lower the resistance. Two amps is a bit much to push through a needle point contacting a hard surface. Possible to use a round tip pogo pin??... more contact area and higher pressure pins might help.

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).

-- Bill Sloman, Nijmegen

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.

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.

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.

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