Burnt Connectors - What process causes them? - Page 1
Forum section: electronics / beginners

author: Szewczykm
author_rank: Regular Contributor
timestamp: May 14, 2014, 02:55:56 pm
content: I work on pinball machines a lot and we see a lot of burnt connectors. I'm curious to know what process is at play here? What is the cause?http://www.aaarpinball.com/STTNG/DSC_1910.jpgNew, clean connectors work fine. What causes them to get so hot they discolor and become defective? I've always heard that oxidation/corrosion will raise the contact resistance which causes the connection to heat up. This is from a Wikipedia article about contact resistance:"A sufficiently high contact resistance can cause substantial heating in a high current device."Is contact resistance different than regular resistance? I would imagine that as the resistance increases the current flow decreases which would mean that the heat would go down, not up. Clearly I'm missing something.Is there an easier way to explain this?

author: Szewczykm
author_rank: Regular Contributor
timestamp: May 14, 2014, 03:03:56 pm
content: Sorry to pile on but is it similar to wire size or trace width? Meaning, as the contacts corrode, less of the surface is conductive. Which means high current is going to pass through a smaller and smaller area of that contact. The contact area heats up because the contact area becomes too small to dissipate the heat?

author: PedroDaGr8
author_rank: Super Contributor
timestamp: May 14, 2014, 03:12:59 pm
content: Quote from: Szewczykm on May 14, 2014, 02:55:56 pmIt is the inverse of thermal run away in LEDs. In LEDs, the hotter the LED gets, the lower the resistance so the more current it draws. Which makes it get even hotter, which lowers the resistance even more. This repeats ad nauseum until the LED dies a smoky death. In this case, as the contact corrodes the resistance between the two parts increase, which means that for a given current more heat is generated in the connector. As the connector gets hotter, it oxidizes more. Which increases the resistance, increasing the heat generated. At high currents, it doesn't take much of a change in resistance to dramatically change the amount of heat produced. Remember, these are typically low ohm or milliohm resistances we are dealing with so the resistance of the circuit will still dominate the equation. These are not designed for heat dissipation so its easy to see how heat can build up and eventually leadto failure. This is a much slower process than thermal run away but the end result is the same.

author: grumpydoc
author_rank: Super Contributor
timestamp: May 14, 2014, 03:44:44 pm
content: Also pinball tables have pretty high-current supplies - I fixed one for a friend last year. One of the things he'd done was to miss-plug the "general lightling" (i.e the lamps which are lit continuously) connector shorting the transformer.The supply was rated at 20A+ - you can imagine what sort of current a 20A transformer can put through a short circuit so there was a lot of melted cable. However even when not shorted the lamps drew about 18A which can cause a lot of heating if there is any increased resistance.The connectors are usually just nickel or tin plated and not gas-tight or rated for many insertions so oxidation can easily build up.

author: abaxas
author_rank: Regular Contributor
timestamp: May 14, 2014, 03:47:23 pm
content: Williams WPC games are known for eating GI connectors. Mostly due to running running them at max spec 24/7.Switch out some GI bulbs for LEDS and all should be right.

author: grumpydoc
author_rank: Super Contributor
timestamp: May 14, 2014, 03:52:05 pm
content: QuoteUnsurprisingly it was, indeed, a Williams gameWe just went for a lower wattage bulb, keeps the right look and feel and the difference in brightness was minimal (esp as we were swapping old and gunked up lamps for new ones) but dropped the current by 25-30%

author: olsenn
author_rank: Frequent Contributor
timestamp: May 14, 2014, 04:26:12 pm
content: Heat!

author: electr_peter
author_rank: Unknown
timestamp: May 14, 2014, 05:35:21 pm
content: I had similar problems with car backlights - one connector become "black", "hot", developed bad connection (very similar to picture in the first post). Failed partly due to corrosion, high current, wet environment, vibration and poor connector choice. Solution - bent some pins to get it to work again.Same lesson learned again and again - connectors are very difficult mechanical stuff, it is very hard to make them reliable.

author: darko31
author_rank: Regular Contributor
timestamp: May 14, 2014, 06:45:28 pm
content: Simply, pulling too much current through a connector generates heat. Heating leads to faster oxidation, and increased resistance. The higher the resistance, the more heat is generated at the connector (P=I^2*R). Basically you have a resistor in series.Solutions are, as already mentioned, lower current, higher quality/amperage connectors, or parallel more pins.These things also often happen in older PCs which used 5V and 3.3V rails, because of the higher current.Also happens in newer PCs, specifically mining rigs if they pull too much current on 12V rail through 24 pin connector if system has several graphics cards.

author: SeanB
author_rank: Super Contributor
timestamp: May 14, 2014, 07:08:33 pm
content: A thin coat of grease, not silicone unless you want to make grinding paste, on new contacts to keep them from corroding helps a lot. I would love to get a bucket of the grease used in 3M scotchloks to use, it works very well.

author: electronics man
author_rank: Frequent Contributor
timestamp: May 14, 2014, 08:11:06 pm
content: A good connector will have very low resistance (let's say 0hm) so p=i^2R if R is zero then p will be zero if R = a value you will have power disipated.

author: scientist
author_rank: Frequent Contributor
timestamp: May 14, 2014, 08:15:25 pm
content: Pinball machine manufacturers often used cheap and non-derated connectors. Cheap connectors = failure.

author: AG6QR
author_rank: Frequent Contributor
timestamp: May 14, 2014, 08:45:58 pm
content: Quote from: Szewczykm on May 14, 2014, 02:55:56 pmYes, contact resistance is just plain old resistance. It's resistance of a mating pair of contacts, maybe due to slight tarnishing or something like that.The reason increased resistance leads to increased heating has to do with the context of the rest of the circuit. Imagine a power source feeding a light bulb, with a set of contacts on the wire between them. To make up some pretend numbers, let's say the power source is 12.00 Volts, and the bulb draws 2.00 amps at 12.00V, for 24 watts. The bulb must have a resistance of V/I, or 6 ohms.If the contact resistance is zero, then the full voltage of the power source is delivered to the light bulb. There is zero voltage drop across the contacts. Zero voltage drop means zero power dissipated at the contacts, even though they're carrying 2 amps.Now, let's put an ohm of resistance on the contacts.The voltage is still 12V, and if we assume the bulb's resistance is constant (not quite right, but a reasonable approximation) the total resistance of the circuit has gone from the original 6 ohms to 7 ohms. Current has dropped to 1.7 amps. But power dissipated at the contacts has gone from 0 watts to 3 watts (I squared R). Power dissipated at the bulb has gone from 24 watts to around 17.6 watts.The total current went down, and the total power went down, but the power dissipated at the contacts themselves went up.

author: turbo!
author_rank: Regular Contributor
timestamp: May 15, 2014, 01:12:29 am
content: Also overloading.Twice the current is four times the heat. You need to use RMS current over a typical functional cycle rather than average.If something runs with PWM using 4A pulse, a 50% duty cycle is an average current of 2A, but the RMS current is about 2.8A, not 2. The ohmic loss is twice that of 2A, so if you design with 2A in mind, overheating will occur.

author: electr_peter
author_rank: Unknown
timestamp: May 15, 2014, 08:40:20 am
content: I would add some metal fatigue as well - if connector is in a vibrating environment, heavy cables and connectors may losen the connector (that increases resistance->IR drop->heat->corrosion->more resistance->failure).

author: abaxas
author_rank: Regular Contributor
timestamp: May 15, 2014, 09:27:45 am
content: Quote from: grumpydoc on May 14, 2014, 03:52:05 pmI see you are in the UK toohttp://www.pinballinfo.com/community/is, by far, the best UK based pinball forum.NOTE : I have no affiliation with them, other than being a member.

author: G7PSK
author_rank: Super Contributor
timestamp: May 15, 2014, 11:03:13 am
content: As Seanb says grease the connections, I use a high melting point lithium grease on terminals and connection whenever there may be corrosion problems, you can get that from almost any automotive parts seller in various sizes from 100 gram tubes to 200 litre barrels. I like 25 litre tubs myself big enough to get your hands in and light enough to carry.

author: Szewczykm
author_rank: Regular Contributor
timestamp: May 16, 2014, 02:28:15 pm
content: Quote from: AG6QR on May 14, 2014, 08:45:58 pmI like this practical example. This makes a lot of sense. Thanks for taking the time to explain.