Inrush current making relay contacts stick - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: October 12, 2015, 02:30:49 am
content: I have this automotive project that I am redesigning, and one of the problems to be overcome is +12V battery is fed via 16 amp N/O relay contacts to a 470uF capacitor, then to the rest of the circuit. The inrush current is enough to occasionally weld the relay contacts together. tap it with a screwdriver and they come unstuck again. It is not practical to use a smaller capacitor or a series current limiting resistor.Different relays have different contact material. Are there some contact materials that are less prone to sticking like this?

author: singapol
author_rank: Frequent Contributor
timestamp: October 12, 2015, 03:18:30 am
content: Such is the nature of relays. If you study the construction of these relay contacts better types use rare earth metals like palladium, rhodium,silver etc. Relays do have a working life and need to be replace after certain number of hours. So take it as regular maintenance but buy good quality or "automotive grade" relays if there are really such around. On the other hand you can try solidstate relays ( no relay contacts) but you have to add protection around them and watch the environment they are used in ( water proof?).You can add a high power resistor and timer bypass relay to take the surge and reconnect after the surge but would that be too much trouble?

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 12, 2015, 03:46:59 am
content: maybe you can add big inductor to limit the arching..

author: calzap
author_rank: Frequent Contributor
timestamp: October 12, 2015, 03:49:51 am
content: Any reason you can't use a NTC thermistor in series with the contacts? I've had a similar problem with electric vehicles, and a NTC thermistor solved it. In the particular application, I was able to use a smaller relay to bypass the NTC after the primary relay activated.Mike in California

author: Xenon Photon
author_rank: Unknown
timestamp: October 12, 2015, 04:08:14 am
content: You can get acar starter relayfrom automotive parts shops. They are rated for high currents (40A).If that didn't work look at a suitableContactorfrom the Electrical power parts shops.

author: exmadscientist
author_rank: Frequent Contributor
timestamp: October 12, 2015, 04:48:06 am
content: A hot swap circuit is probably the cleanest solution to this. On the simple end that can be an NTC thermistor. On the more complicated end you have solutions like the LTC4231, which I've employed quite successfully to solve similar problems with strict efficiency requirements.

author: Wirehead
author_rank: Regular Contributor
timestamp: October 12, 2015, 04:55:19 am
content: RC snubber?

author: Circlotron
author_rank: Super Contributor
timestamp: October 12, 2015, 05:09:02 am
content: Forgot to mention - the relay is on the board next to the cap so 20mm cube is about max allowable; Maybe a little more.Considering a P-channel mosfets right now. They have come a long way since I last looked.

author: Psi
author_rank: Super Contributor
timestamp: October 12, 2015, 05:57:00 am
content: I would use an automotive high side driver chip rather than a pfet.They have built in protection for shorts/overcurrent/temp etc. and are designed for automotive environments.Price difference is negligible.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 12, 2015, 06:02:43 am
content: Quote from: Circlotron on October 12, 2015, 05:09:02 amI was going to suggest a bigger relay, but you seem to have been painted into the corner.

author: Psi
author_rank: Super Contributor
timestamp: October 12, 2015, 06:05:03 am
content: 20mm cube is large enough for a to220 high side driver and heatsink.The high side driver i use in a product can do 10A continuous with just pcb heatsinking. Its 85A internally current limited too. So inrush will be limited to that. It's a VN5E010AH but i dont think you can get those in to220 package, so you'll need to go with another brand/version. They are all pretty much the same though. Should be dead simple to find something in a convenient to220 package for easy heatsinking.Input is logic level on pretty much all of them.Just be careful to check what it does in the event of a short/overtemp. Some will just switch the driver fet off until they cooldown and auto-restart, others will switch off until you cycle the power. So you need to decide which approach suits your application the best.

author: Phoenix
author_rank: Frequent Contributor
timestamp: October 12, 2015, 06:14:52 am
content: Maybe a 2 stage soft charge circuit. In parallel with the big relay have a small relay and series resistor. Close this in first and let the capacitor charge then close in the big relay to short it out (and then open the small relay).NTC as others have mentioned is also a common solution to inrush.

author: fcb
author_rank: Super Contributor
timestamp: October 12, 2015, 08:02:46 am
content: Use a precharge circuit, even if you use a MOSFET as the main pass element (&a second one for the precharge circuit).

author: aargee
author_rank: Frequent Contributor
timestamp: October 12, 2015, 11:08:22 am
content: Back in the day we used to use mercury whetted relays to prevent arcing/welding. I'm sure ROHS will have killed that off by now?Side note: Best I ever saw was a 40A three phase contactor welded shut when a car hit a power pole and 11kV lines fell onto 415 distribution mains. Also welded several TO3 devices to heatsinks.

author: Gyro
author_rank: Super Contributor
timestamp: October 12, 2015, 11:23:01 am
content: QuoteMaybe not so good for automotive use though, they are rather shake and position sensitive... Not suggesting that the OP will be operating his vehicle invertedI still use them for small signal and audio at home though, can't be beaten in dry switching applications.OP, you haven't indicated how much current your project is using? You will probably find that a low - very low value series resistor would be sufficient to limit peak current to below contact welding point without significant dissipation problems. You could even mount it in-line externally if space is that tight.

author: Pjotr
author_rank: Frequent Contributor
timestamp: October 12, 2015, 11:35:33 am
content: If the inrush current is that high that an automotive power relay welds it contacts, then the capacitor will not survive that high inrush for long either. Strongly suggest a current limiter as said before.

author: Fungus
author_rank: Super Contributor
timestamp: October 12, 2015, 11:43:11 am
content: Quote from: Pjotr on October 12, 2015, 11:35:33 amYep.Keep your relay but add a resistor so the capacitor charges slowly at first. Add a MOSFET (or whatever) to bypass the resistor when the capacitor voltage passes a threshold value.

author: DanielS
author_rank: Frequent Contributor
timestamp: October 12, 2015, 02:38:45 pm
content: Quote from: Fungus on October 12, 2015, 11:43:11 amOr remove the relay, replace it by a FET and have the MOSFET also be the precharge resistor by giving its gate voltage a ~1ms rise time.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 12, 2015, 02:41:28 pm
content: To prevent this whole issue with stuck and faulty relays in car computer modules they (infineon) have developed automotive grade smart high side switches. Some even have (current) feedback.They are pricey compared to raw P/N channels, but compared to relays and returned units they are not. They do however generate small heat. But so do relays if you want some reliable switching.

author: AF6LJ
author_rank: Unknown
timestamp: October 12, 2015, 03:10:31 pm
content: Quote from: Mechatrommer on October 12, 2015, 03:46:59 amThat would be a bad idea since as soon as the relay opens there will be a large voltage spike and the relay contacts may arc.

author: AF6LJ
author_rank: Unknown
timestamp: October 12, 2015, 03:14:56 pm
content: I suppose I don't understand why the huge cap in the first place?What appears to be happening here is the relay make and break current rating is well below the inrush current the cap is demanding.I would do ether of the following.Replace the cap with a smaller caporfind a proper relay that will handle the inrush current, they do make them.

author: Niklas
author_rank: Frequent Contributor
timestamp: October 12, 2015, 04:59:51 pm
content: Just a speculation about the cap. It might have something to do with operation during cranking when the battery voltage can drop significantly.I have seen ECUs where they used 2 P-channel MOSFETs in parallel as wrong polarity protection instead of a diode to reduce power dissipation in high current applications. A resistor+zener to control the gate and maybe add a small timing capacitor to get a soft start.

author: AF6LJ
author_rank: Unknown
timestamp: October 12, 2015, 05:06:44 pm
content: Quote from: Niklas on October 12, 2015, 04:59:51 pmWe don't have near enough information to really tell what is going on.I would like to see the OP give more information.

author: DanielS
author_rank: Frequent Contributor
timestamp: October 12, 2015, 05:30:21 pm
content: Quote from: Niklas on October 12, 2015, 04:59:51 pmOr just board-level supply bypassing. If you are cranking, 470µF connected directly to the battery through a relay is not going to help much.

author: Niklas
author_rank: Frequent Contributor
timestamp: October 12, 2015, 06:29:16 pm
content: Quote from: DanielS on October 12, 2015, 05:30:21 pmI doubt that a polarized electrolytic capacitor is connected directly over the battery as it would not like the reverse jump start test. However, if it is located behind a diode it might be a useful energy storage.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 13, 2015, 02:27:00 am
content: Quote from: AF6LJ on October 12, 2015, 03:10:31 pmput diode protection then from gnd to relay output, the possible arc is during switch off, not switch on which is the op problem... i tempted to suggest NTC but i dont know any 16A NTC around...

author: AF6LJ
author_rank: Unknown
timestamp: October 13, 2015, 03:14:05 am
content: Quote from: Mechatrommer on October 13, 2015, 02:27:00 amI would go for that if we knew more about the nature of the device this relay is in.If this device draws large amounts of current for short peroids of time the inductor would effece input voltage regulation.