Why does a relay have a minimum applicable load?

Question

I am trying to find relays for my application and I read a data sheet which looks fine but specifies

Minimum applicable load: 10mV 10µA

In my circuit, it is expected that the relay closes but no voltage and current is applied. You can think of it as 2 relays in series where one is open and one is closed, so there is no current. This sounds like something I'd do since I was at school.

Why would a relay require a minimum voltage and current on the load side? Is it allowed to operate that relay under my conditions or not? What could possibly break in a relay if I don't respect this requirement? What does "minimum applicable load" mean? When and how do I need to consider this value?

Answer 1

The primary reason that almost all relays have a minimum load requirement is that the mechanical action of closing coupled with an actual current flow are required to 'whet' the contact and break through a layer of oxidation that invariably builds up.

That is one reason that small signal relays generally use expensive contact alloys which resist oxidation, but as the phone company found out decades ago, even pure gold contacts can have issues in a high humidity environment. While oxidation doesn't affect the gold contacts, repeated cycles of moist/dry air would deposit an insulating layer.

Answer 2

Mercury wetted (reed) relays are used where you cannot guarantee minimum load currents. These have a film of mercury on the contacts that will make contact even with no current as it forms a liquid junction. Generally the contact resistance will be slightly higher than the best gold contact resistance. The switching lifetimes may be much higher that other types. The load currents are generally lower as they are only usually needed for small signals.

In your case where the live loads are of a respectable value almost any dry relay type will work reliably.

The longevity of a dry contact will not appreciably change even if no current flows but age and lack of load current will slowly increase the low current contact resistance as mentioned by others. This means that marginal low current signals will suffer higher than specified contact resistance until such time that an adequate current has been switched.

EDIT:
It is also possible for the thin oxide layer that forms on the relay contacts to cause the low current resistance to increase and remain high until reasonable load currents have been switched that will 'burn' it away.

Answer 3

Relay will work at any load (below maximum of course). However if you use less than minimum permissible load, it may fail before it should (less switching operations). The reason for this being after time some corrosion can get on the contact. Minimal current is measured/calculated to be able to break through contact corrosion and it also slows down the corrosion process itself.

You can use the relay with less current, but if you are using it in a device that needs high reliability or you can't change the relay if it fails, it's not a good idea.

Answer 4

Relay contacts, even special alloy contacts and gold contacts will coat with a layer of poorly conducting contamination over time, hence possible premature failure. The manufacturer has empirically found what current is needed to ablate away the contamination with each operation just enough to keep the contacts clean while not ablating away the contact material too fast - a trade off. That minimum current is likely to allow the longest relay life. Operating at higher current does wear out the contact metal faster and maximum current yields minimum specified relay life. What actually matters is the energy of the spark/discharge that occurs when the contacts open, and in some circuits close. That energy is rarely specified because few engineers and technicians know how to compute the energy dissipated in the transients. It involves stray and intentional energy storage in inductances and capacitances throughout the circuit, and is a function of current and voltage over time up to the moment of switching. I've seen minimum and maximum energy specs on spacecraft and mil spec relays with guaranteed MTBFs.

Answer 5

The relay may not work as expected below the minimum load. If the current you're switching is below 10uA, you may find that the relay still conducts a significant current even in open state, due to internal capacitance. In a similar way, an open relay can easily pick up 10mV from EMI, so if your actual signal is that small, the relay may still seem to pass it through.

Of course, such phenomena depend on the frequency of the signals you're switching and your environment, which the datasheet stresses out:

Minimum applicable load is reference value. Please perform the confirmation with actual load before production, since reference value may change according to switching frequencies, environmental conditions and expected contact resistance...

So your relay will work just fine even if the signal you're switching is not actually present. Your signal must simply be above the threshold when it's there.

Answer 6

Either your application or explanation don't make sense. If there is no current flow, then you have a permanent open, this means you can remove the relay and save a component, time, and money. However, if you are controlling a 10mA load (per your comment), then you can use the relay and not "worry" about the "minimum applicable load."