I frequently see technicians that know how important contact resistance is, but attempt to make the measurement using their meter's resistance scale rather than running some current through the relay and measuring the voltage drop. If you have a power supply with current limiting you can use the current limiting feature as a current source to measure the drop. Alternately use a voltage source with a known load resistor in series with the contacts.
Be careful when switching into highly capacitive loads (like circuits with regulators on the front). A discharged capacitor looks like a short circuit to a relay, and the contacts will arc. Inductive loads can be handled with snubber ciruits, but caps are a different story.
Choose your contact plating material with care. Some coatings are arc resistant, some are oxidation resistant, some are full of cadmium are not very RoHS. (Those are the best, wouldn't you guess).
Choose your relay supplier carefully. We have had experience with low quality DPDT relays which were not very symmetrical, one pole would mate before the other and sometimes only one pole would mate, the other side missing by 0.2mm. Of course, if you shook the card...
One of the most perplexing issue that came up with relays in our system was when the purchaser was using a power relay (dual form C 10A DC contacts with magnetic snubbers) to drive a TTL-level input in their system. It wasn't until I went to a customer site in Nebraska that I found out what they had done. Althoug we had 16 low-level relays with gold contacts for the TTL inputs, they found it easier to grab one of the others. A close examination of the power contacts revealed a heavy build up of corrosion on the contcats, even though the other relays that were switching 74 VDC were clean. I cleaned the contactrs of the offending relay and restored the function. Then, I pointed out in their own specs where those relays were for power only.
Relays must be applied with full understanding. It has been presented here that excess current or inrush currents can damage but a relay may fail under no load as well. A number of years ago the power windows on a rather expensive car ceased to work and I went into what was relay logic giving proper operation and safety for multiple windows and multiple window switch locations. Turned out there was a relay that never opened under load because when it opened another relay in series with it was already in its normally open condition and it never closed into a load because it closed after the window limit switch had already broken the circuit. One would believe it would last forever. It did however have to carry relay coil current in its closed condition to enable some of the control switches to work. The relay was the same as the others and was a type designed for the current and voltage suitable for a window regulator motor in a 12 volt automobile. The contact material simply oxidized over time and the contact read open even after a number of actuations. The relay was a closed case design so I just temporarily jumpered so it actually turned the window motor on and off a couple of times and it was still doing fine a few years later when the owner sold the car. Look into it and you will find that logic relays that are not intended to make and break under current are precious metal contacts (usually gold) and do not do well switching much current. Summary; be sure you use the right kind of relay.
Regarding the DMM method I wanted to share an article, I found some time ago: http://www.aeroelectric.com/articles/LowOhmsAdapter_3.pdf . I think it is quite helpful for doing some quick field checks, avoiding the common problems with standard DMMs. The resolution of common used DMMs is simply to less to securly check the contact resistance.
Maybe a good point to give a rough indication of the commonly used relay contact materials together with some properties:
Gold (Au) – highly corrosion resistant, most important material for reliable switching of low contact loads – due to high cost gold is often used in the form of a layer on the contact surface – a layer free of pores, useful for low loads, should have a thickness of at least 3 μm – gold flash (typ. 0.2 μm thickness) is mainly used for storage purpose only – due to a danger of contact welding, unfavourable at high loads.
Silver-Palladium (AgPd) – for medium loads; if gold plated, for low loads also – corrosion resistant especially against sulphur gases – often used in telecom applications.
Silver (Ag) in pure condition or with a low amount of additives (e.g. 0.15% Ni) – good for medium loads – less useful for high AC-loads and high inrush currents.
Hardsilver (AgCu) – good for medium loads – less useful for high AC-loads and high inrush currents – less contact erosion than Ag.
Silver-Nickel (AgNi) – for medium and higher switching loads – better resistance against contact erosion and welding than AgCu.
Silver Cadmium Oxide (AgCdO – for high switching loads, especially for mains applications – low tendency for contact welding, good resistance against contact erosion – less useful for lower loads.
Silver Tin Oxide (AgSnO2) – for high switching loads, especially for mains applications, also at high inrush currents – very low tendency for contact welding, good resistance against contact erosion – at resistive loads lower electrical endurance than AgCdO – less useful for lower loads.
Tungsten (W) – especially for high inrush currents – mainly used as pre-make contact
Very cool - this looks like an inexpensive alternative to the the 4 wire resistance measurement capability typically found on high end bench top meters. When I worked as a test engineer for a major semiconductor company we had that luxury - this looks like something I can build on my current "just me" budget. Thanks for the link!
By the way, Dario - nice concise article on the basics. I appreciate all of the additional comments that have added even greater value, but I think your article did a great job at pointing out common problems and things to consider regarding relay contact resistance for anyone. It also points out the importance of datasheets so that you are using the right relay for the right application instead of just grabbing one that worked in something else you have done, and expecting it to perform comparably.
Thanks Nancy. Another factor to relay life is orientation. Most customers use a horizontal orientation, but there are a few that use vertical orientation where the coil is at the top and the contacts are at the bottom. This orientation is not recommended as gravity will affect the pressure on the contacts and can sometimes result in a higher pick up voltage to drive the coil.
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