Application Note 31 - Relays and Switches
- Why do relay and switch contacts become intermittent?
- The most common cause of relay and switch contact failure or intermittence is caused by the deposition of contaminant materials which either, by themselves, increase contact resistance, or which cause corrosion of the contact. In the former case we find such contaminants as industrial oils, wood-based resins, tar and nicotine from smokers, and even plasticizers or breakdown products from the plastics used in the fabrication of and/or physical shielding of the relay or switch. (i.e. such things as a clear plastic case). In the latter class would be such things as corrosive by-products of the industrial process in the plant served by the switch or relay, or even in adjacent industrial locations.
- Can certain operating conditions aggravate this problem?
- Where switches or relays are used at very low currents, such as in TrL or, worse still MOSFET circuits, there may not be enough current present
to keep the contact electrically clean; the very low current levels can lead to microphonics, making the contacts vibration sensitive.
Contacts which stay in one condition (i.e. Normally-Closed or Normally-Open) for long periods of time have their own peculiar set of problems. Often normally-closed contacts will exhibit capillary action, drawing-in contaminants. This is more often found in contact pairs where the contacts have a large radius. Normally-open contacts on the other hand can more easily accumulate contaminants on the surface of the contact.
As noted, sometimes conditions exist where the relay or switch construction and/o materials can contribute to a problem in low-current applications. Contact covers molded from hygroscopic plastic such as polycarbonates, can, under conditions o long off-periods(coils unenergized)pick up moisture from the environment. When this is followed by a prolonged coil-energized state, the heat from the coil can drive the moisture out of the plastic increasing the potential for contact corrosion or degradation.
Excessive use of plasticizers in either the protective cover or the relay or switch body can, under certain circumstances, can cause plasticizer re-deposition on the contact surfaces. Occasionally, the use of solder-fluxes in internally wire-leads on relays can cause problems as most flux removal methods function essentially by massive dilution, and may leave potential migratory flux in place near or on the contacts.
While relay covers are useful in excluding environmental contaminants, unless hermetically sealed, they can, under conditions of thermal cycling, exhibit a "breathing" phenomenon which can cause contaminants to accumulate within the protective cover.
Many times the moving contacts of a relay are treated to no avail, it having been overlooked that the relays may themselves be mounted in sockets which can be just as prone to electromechanical-contact problems as the actual moving contacts.
- Can these problems be prevented?
- Yes, most cases can be prevented. The usual treatment is a rigorous and consistent cleaning of the contacts. This often fails because of the
number of contacts and/or the inaccessibility of the contacts and/or the rate of contamination build-up and/or the lack of available maintenance
time makes the process impractical. Then too, the potential problems of solvent use and the environment must also be addressed.
Stabilants have demonstrated their ability to eliminate many of the contact problems in a cost-effective manner as they are a resident treatment. Not only will their presence exclude contamination, their detergency action will loosen existing contamination and/or reduce thin-film rectification and microphonic effects from the contact pair.
- What precautions must be taken when using Stabilants?
- One general precaution is not to use too much of the material. This is not a case of "if a little is good then a lot is better'. While some applications
involving heavy existing corrosion may justify the use of thicker films in order to hold removed-corrosion in suspension, this should be considered
a temporary measure, this thick film of Stabilants should be scheduled for removal (along with the corrosion by-products) at an early
moment, to be replaced with a thinner film of Stabilant. This also applies to contact pairs where there may be existing hardened
contamination in areas of the contact's surface adjacent to the actual point-of-contact.
Under these conditions, the initial use of the Stabilants will probably loosen-up the deposited contamination opening up the potential for it to migrate to the actual point-of-contact. If this condition can be presumed to exist, it is wise to schedule a second Stabilant treatment consisting of an isopropanol wash-down (to remove the loosened contamination) followed by a re-application of the Stabilant.
in cases where heavy contamination is initially present, more aggressive solvents may have to be used to remove it, alternately, two or more cleaning and re-applications of Stabilants could serve the same purpose.
- What problems can be created by the capillary-effect?
- Where contacts are made with radiused surfaces to aid wiping-action-cleaning, the diminishing gap towards the point-of-contact can act as a capillary
drawing sufficiently mobile contaminants or even the Stabilant film itself into the area immediately adjacent to the actual point-of-contact.
This is especially true of contact pairs which, because of the design of the relay or switch, do not actually wipe when they come together. For obvious
reasons this is more prevalent in relays.
If too much Stabilant is used on a contact, there is a potential for the Stabilant to so-completely fill the gap around the point-of-contact that it may overly-cushion the closing contact. If there was excessive surface contamination present before the Stabilant was applied, this capillary action could, as noted, carry the suspended contaminant into the actual point-of-contact area.
The remedy is, as stated, the cleaning and re-application of the Stabilant film.
- Is Stabilant 22 just another contact cleaner?
- No, Stabilant 22 is a resident potentially electrically active material which through a synergistic combination of effects enhances conductivity within a contact without causing leakage between adjacent contacts. Thus large quantities of the material do not have to be "hosed" on, as is the case with cleaners.
- How does Stabilant 22 work?
- Contact failure is rarely caused by a single factor. Thus, treatments that solve only one problem don't
necessarily offer a reliable long term solution. For example, cleaners do not prevent the re-entry of
contaminants or the reformation of contaminant films; nor do they offer any lubrication. They must be used
each time a connector gets dirty. Lubricants in themselves are rarely cleaners. Corrosion inhibitors are
neither cleaners nor lubricants and are often specific to one type of metal or plating. Unsaturated oils
used as contact treatments can cross-link under the influence of elastomer or thermoset plastic curing
agents and accelerants.
While resident in the connector, Stabilant 22 performs several concurrent functions. Its very presence in the contact gap will prevent the entry of outside contaminants. It has sufficient surfactant action to lift surface contaminants and hold them in suspension. In cases where corrosion products are present, Stabilant 22 will penetrate them and prevent rectification effects. Due to its high dielectric constant it will act to form a capacitive layer which is in parallel with whatever residual resistance exists in the contact increasing the passage of AC signals. Given sufficient DC bias within the gaps of the contact the thin film of Stabilant will "switch", conducting by quantum tunnelling, and thus limit the resistance of the contact to a serviceable level.
- Does the action of Stabilants deteriorate with age or do they cause deterioration?
- Our first concern has always been that Stabilants should not cause any problem when used in a system. Not only did we do
lab-modelling and accelerated life tests we delayed the introduction of the material for several years until we satisfied
ourselves through field trials that real-life conditions did not show up any unexpected problems for the use of Stabilants.
Stabilants have been used in some applications for over fifteen years now without showing any sign of reduced effectiveness. The material has a high molecular weight and a very low vapor pressure, thus it is not prone to evaporation.
The Stabilants do not affect elastomers save for some slight swelling of some materials. The diluent employed (isopropyl alcohol) usually is responsible for this problem, although the potential for this is gone as soon as the isopropanol evaporates. Nor are plastics generally affected. There are a few restrictions, but they are very minor. For example we don't recommend the use of Stabilants on very low-cost resistive-paint-film type potentiometers. And we don't recommend that Stabilants be used on switches breaking inductive loads where sparking is present the decomposition temperature of Stabilant 22 is about 220° Celsius.
Once again let us emphasize the point that unlike some other contact treatment containing oils, Stabilant 22 will not cross-link when exposed to certain material such as high sulfur brass, in connectors having rubber or thermoset plastics containing accelerants or curing agents, or when used on contacts where cross-link promoting agents are present in the environment. This phenomenon of "Varnishing" does no occur with Stabilant 22.
Thus, besides their efficiency, the Stabilants are the safest long-term connection treatment available anywhere in the world!
Stabilants™ are a product of Dayton Wright research & development and are made in Canada
NATO Supply Code 38948
15 ml of S22A has NATO Part # 5999-21-900-6937