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Contact Wetting Power Supply
Understanding basic terminology

I am having trouble with something that is probably so basic that you'll laugh. However, I'm going to ask anyway. What exactly is Contact Wetting Power Supply? Is it what I think it is? Voltage just make sure the contact doesn't corrode?

Any Help is appreciated?

By Curt Wuollet on 1 February, 2007 - 12:36 am

It is any supply supplying more than about 10ma. closed circuit. Or applying more than a few volts open circuit. Currents/voltages less than this are considered "dry switching" because there is insufficient energy to punch through the oxide films common to for example tin and silver contacts. Contacts for "dry switching" are often plated with gold or other noble metals that grow little or no oxides/sulfides, etc.

People tend to break these rules all the time switching small signal voltages with power contacts, etc. These same folks have a lot of "relay trouble". Specifying a contact wetting power supply is just a way to make sure there is enough energy available for reliable switching. Wetting the contacts, in this context, is simply breaking up the oxide films with a tiny bit of arcing.


For a variety of explanations of wet or dry contacts look at these threads:

By Jerry Miille on 2 February, 2007 - 12:49 am

Not so basic a question as you think. My understanding of a "wetting" voltage is exactly what you describe, that is: there is sufficient energy (voltage or spark) to "burn" through or "Wet" any corrosion or contamination on the switch contacts and thus there is a low ohm connection achieved.

I believe that this is a carry-on description from earlier days when contact closures could sometimes be made very delicately, such as by the physical movement of a bimetal strip where there is, very little physical force involved with the contact closure as temperature moves the bimetal strip. In this type of application, the lack of a physical force that forced switch contacts together caused problems because there was not enough energy to overcome the corrosion on the switch contacts.

The solution to this problem was to construct the "switch" so that the contact closure was actually a blob of mercury in a slim tube that was rotated by a bimetal strip. As the bimetal strip expands or contracts, the mercury blob moved either in contact with or away from contact between the switch contacts. If it was in contact with the contacts they were "wetted" (with the mercury) and if not they were "dry".

Another interpretation of a "dry" contact is that there is no voltage potential on either side of the contact.

Jerry Miille

By Michael Griffin on 4 February, 2007 - 2:14 pm

There were also mercury power contactors and mercury wetted reed relays. I've worked with both. The mercury power contactors were on an old heater system where the wear and tear of the constant on / off of the contacts was taken up by the mercury which "wetted" the lower contacts.

The mercury wetted reed relays were used for an instrumentation system where the reeds were mounted vertically in a small glass tube of mercury (and surrounded by the coil). The lower reed contact would wick a small amount of mercury up from the pool they were partially submerged in. The actual contact area would be above the level of the mercury pool. The initial contact and the final contact break would be by a small bead of mercury on the reeds, which would constantly be renewed by the small pool of mercury. As long as the current rating was not exceeded, there would be no detectable pitting or corrosion of the reed contacts. The only moving parts were the flexing of the reeds, so these relays would last virtually indefinitely.

Both of the above were orientation sensitive, as the mercury had to pool around the lower contacts to be effective.


In a past life I also remember mercury operated transmitters which used up to 30 pounds of the stuff (for power to operate the levers etc) to produce a differential signal which would move an arm on a rotating cam (honed to linearize the process to real world conversion) to activate a capsulated mercury wetted switch giving a time pulse signal output equivalent to flow, etc. with no chatter.

I've recommended to my clients (for the last 19 years) to replace this equipment since there will be a time when the disposal of mercury will be illegal.

Think RoHS.

A wetted contact to me means that there is a voltage and related current to activate a device reliably (leaving critical and safety related devices out of the loop for now).

In my realm a wetted contact can be 24 volts DC or 120 volts AC depending on my best guess as to the longevity of the contact being able to produce the correct result. When in doubt use the higher voltage.


By Michael Griffin on 8 February, 2007 - 1:13 am

In reply to Dennis: So far as I am aware, Mercury contactors are long obsolete and are replaced by solid state contactors in heating applications. Replacement of any in existing applications would be a good idea, as not only is normal end of life disposal becoming increasingly difficult, but if they leaked in service you may to treat this as a hazardous waste incident and suffer significant expense and plant downtime while the area is decontaminated. There may be additional consequences of a mercury leak, depending upon the country you live in.

Mercury wetted reed relays are also obsolete in all (or almost all) new applications where they have been replaced by solid state instrumentation relays. Many of the former manufacturers of them no longer offer them in their catalogues. There is at least one company in Europe that still sells them though, and they are also available on the surplus market.

Mercury wetted reed relays are often more difficult to replace in existing applications because they may be soldered into circuit boards and have a package form which does not match any modern replacement. In many cases the only practical means of replacing them is to replace the instrument. The smaller ones though often contained an amount of mercury which fell below the threshold which is considered significant (much less than a mercury thermometer).


Contactors (for heating) I do not know about but I have knowledge in leaks of Hg transmitters (in our water system) -- It is a waste hazard and significant time is required to record and mitigate the spill. I experienced a calibration manometer leak at a plant once where I spent several hours in safety meetings (you don't want to go there).

The normal end of disposal has three prerequisites -- failure, obsolescence and legislative.

This does not only apply to Hg but also NaCl and many others (have you ever noticed how the salt residue in the spring kills the grass along the road).

A long time ago Mesopotamia had a similar problem, for centuries they irrigating the land from the Tigris river (for local food production) resulting in a gradual increase in natural salt deposits (from the river) which reduced the yield of their crops. Eventually they moved out (to find greener pastures) and the area became almost a dessert.

I'm not a green person, but I am concerned where we are going and we don't have to accelerate it by adding more chemicals.

I end my rant.