Effect of displacement and humidity on contact resistance... - Page 1
Forum section: electronics / general

author: Rick Law
author_rank: Super Contributor
timestamp: April 02, 2017, 09:03:34 pm
content: Ok, here is something fun. Often, we read theories and studies and don't see how it could occur in our daily life. But here is one odd one that showed up.My 1997 BMW frequently give me early warning aboutimpendingrain.When rain is coming soon, and when I start my car, I frequently would have "brake light failure" warning, or "parking light failure" warning, so on. Checking my lights, of course, they were fine. It was so "reliable" I could go back into the house to get my umbrella.So, for almost a decade, those warnings did nothing but wasted my time doing actual checks. About 4 years ago, I stumbled into the solution. BMW uses a bulb socket for their bulb - bulb into socket then socket into a slotted position in the car. I had to deal with a light that actually failed: the bulb-to-socket was fine, but the socket-to-car-electrical was bad. I had to bend the socket contacts to increase the contact pressure for the light to work. A while later, it hits me that since I improved the contacts, the "xxx light failure" warning had not occurred. I took mental notes and kept watching.I thought, ok, humidity is what causes the contact to be bad causing my various "light failure" warnings. So, that is that, but why? The "why" puzzled me every now and again but not enough for me to really think about why. Since that contact improvement, it had not warned me about rains since (~ 4 years).My car just resumed its weather station role and warned me about impending rain.This time, "license plate light failure" warning. This time, I decided to look for an answer why humidity should have such effect and I found it. In the book referred to below, "Moisture reduced wear debris pileup, producing a smooth contact area with more electrical junctions." In theory, humidity should make for a better contact. But in my case, the contact was so loose the debris pileup was what completed the circuit! Well, at least that is what my explanation is now."Effect of displacement and humidity on contact resistance of copper electrical contacts" (I.H. Sung, J.W. Kim, H.J. Noh, H. Jang).Sciencedirect.com highlights from the book:• The effect of humidity and displacement on fretting induced electrical contact failure was studied.• Humidity effect was pronounced at small displacements due to effective removal of debris.• Premature contact failure was found at low humidity and with large displacements.• Moisture reduced wear debris pileup, producing a smooth contact area with more electrical junctions.• Our results suggested a high propensity of electrical contact failure at dry conditions.

author: rrinker
author_rank: Super Contributor
timestamp: April 03, 2017, 02:36:06 pm
content: My 99 did the same thing, although that was largely alleviated when I just replaced ALL the bulbs in the taillights - some where tin plated bulbs and some were copper (or plated) and there is an issue using some types because of dissimilar metal action. Once I had all the right kinds (and you can find them in any auto parts store - there will be both types right next to each other with the same bulb type, ie 1156), I stopped getting random light failure warnings when in fact, the light worked. I didn't drive that car a lot so I never correlated it to the weather.

author: Rick Law
author_rank: Super Contributor
timestamp: April 03, 2017, 06:34:47 pm
content: Quote from: rrinker on April 03, 2017, 02:36:06 pmI went through the bulb changing routine also. That held off the weather station syndrome for a while but it returns soon enough. It took me a few years to notice the correlation. Somewhere between my check cycles, I confirmed that it was indeed the contact quality causing the weather station syndrome, and increase contact pressure is a fix.Last week's License Light Failure and again followed by rain.I knew pre-rain was the issue, humidity was a guess but I was really puzzled by how humidity can affected the contact quality. I was really pleased to have found a study verifying that humidity affects electrical contact quality and and explained the "how and why". "Removal of debris" is one I would not have come up with myself by just thinking about it.Side note: After dealing with that for years. I found the best way to check exterior lights. I took mental notes on where I found glass doors near and facing parking spots. So, when needed, I drive up (or back up) to the glass door and check the reflections. Such an obvious solution, but for a while, I was dumb enough to need to ask someone to stand behind the car and tell me if light X comes on.

author: rrinker
author_rank: Super Contributor
timestamp: April 03, 2017, 10:37:26 pm
content: Haha yeah, that was me. The driver's side I could see if I leaned out of the car and reached in to hit the brake. Turn signals are easy enough, but checking that passenger side brake light... Once I moved it wasn't too hard, just hit the brakes with the garage door down, it's pretty obvious if both or just one are coming on.So far so good on my 2012, no spurious fault indications. But it does live in the garage, never outside over night.