Electromechanical relay contact closed resistance - Page 1
Forum section: electronics / projects

author: RoGeorge
author_rank: Super Contributor
timestamp: October 04, 2025, 12:08:05 pm
content: Bought a few mechanical relays, and checked the contact resistance. They measure anywhere between 0.1 and 5 ohmsfor a closed contact, and at each new close, the resistance is different, can be any value in that range. If tapped with the nails, the contact resistance changes a lot.On their body is written SONGLE, SRD-6VDC-SL-C, and 10A at either 250Vac, or 30Vdc.https://www.alldatasheet.com/datasheet-pdf/pdf/1132029/SONGLERELAY/SRD-06VDC-SL-C.htmlWill the oxide go away under load (from the sparks normally happening between any contacts), or are these relays a fire hazard?

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: October 04, 2025, 12:43:47 pm
content: there's aminimumload current spec for power relays that helps to keep the contacts clean. hence, you have to do a "wet" contact resistance check (i.e., contact must carry a minimum current while testing) instead of doing a "dry" contact check (what you probably did by using a multimeter with too low testing current). if testing current is too low, measuring results can be all over the place.

author: WattsThat
author_rank: Frequent Contributor
timestamp: October 04, 2025, 01:15:52 pm
content: Well, they are Chinesium quality. A very common name seen often on low cost board level products from China.Are they are a hazard? That’s dependent upon what you switch with them. I certainly would not use them for anything of a critical nature. Turn a light bulb on and off? Sure. Will they get better with use? Doubtful. But, as mentioned, any power handling relay contact does need a minimum current through them. Reed relays and other sealed contact devices, no, they should have stable resistance with no current.You do get what you pay for…

author: 2N3055
author_rank: Super Contributor
timestamp: October 04, 2025, 01:37:21 pm
content: Quote from: Le_Bassiste on October 04, 2025, 12:43:47 pmI will second this.Push 1A from lab PSU through it and measure voltage drop over contacts.Then you will know. With 1A you will have direct scaling 1mV ⇒ 1mΩ

author: Terry Bites
author_rank: Super Contributor
timestamp: October 04, 2025, 01:59:47 pm
content: Deoxit contact cleaner or similar?But I think you might find you're guilding some turds.

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 04, 2025, 03:07:32 pm
content: Quote from: WattsThat on October 04, 2025, 01:15:52 pmThis sort of behavior is typical of many types of relays, and can be observed in parts made from the "good" vendors such as Omron, Fujitsu, TE, etc. I think you have a lot to learn about relays.If you want a relay for a "cold" switching application (i.e. no current/voltage is present when operating the relay), then you want gold plating on the contacts to prevent oxidation (also bifurcated contacts help). However, gold plating will not stand up to hot switching, it will be ablated away very quickly by the resulting arcs.For hot-switching applications (the vast majority of power relays, like the one in the OP), you want contacts with heavy plating of materials like AgCdO or AgSnO2. These will withstand arcing, but they also will develop an insulating oxide while not in use. Thus their "dry" contact resistance can be very high, as Le_Bassiste alluded to. Sometimes just running a larger current is all that's necessary to break the oxide and "wet" the contacts. Sometimes they really need hot switching to be properly "wetted".

author: TimFox
author_rank: Super Contributor
timestamp: October 04, 2025, 03:59:33 pm
content: Look for “dry contact” or similar dry term when choosing a relay for signal-level circuits (e.g., audio). Consider mtwieg’s comments above about power contacts.Mercury-wetted contacts are no longer politically correct, and may be seriously orientation-dependent (i.e. vertical mount with one end up) due to the mercury reservoir, but the can work for either case (see individual data sheet).

author: MarkL
author_rank: Unknown
timestamp: October 04, 2025, 07:24:46 pm
content: Quote from: 2N3055 on October 04, 2025, 01:37:21 pmI also agree this a good way to test the contacts. Often accompanying the wetting current in the datasheet is a minimum contact voltage, which is also important.Perhaps it goes without saying, but I would add a resistor in series with the power supply to limit the current spike when closing the contacts while testing (perhaps 5 ohms or so). Most lab and other power supplies have a large capacitor on the output. A 10A relay might not be harmed, but dead-shorting a capacitor with a relay is a good way to weld the contacts.

author: nightfire
author_rank: Frequent Contributor
timestamp: October 04, 2025, 09:38:30 pm
content: In appliance testing, when you check appliances for safety, in germany regulations state that you have to poout at least 0.2A through the PE conductor to determine its resistance. Appliance testers often have a 10A setting for more difficult things like electric kettles that can be separated from their base, where the contacts after some time are high-resisitance due to oxidation etc.- the 10A will burn away oxidation and dust and dirt and such.So I second the idea of putting some current through the load side of the relays, after that you should be able to measure a voltage drop and from that determine the resistance of the contacts of the relay.