Whenever a relay is used to hot switch a signal its expected life (number of operations) will degrade. Hot switching is most commonly used to avoid unwanted disconnection of signals which might cause a test to be not representative or to speed up the test process (by avoiding power down cycles). See more on hot switching >>
Reed relays are a an excellent choice for lower power switching applications because of their long life and fast operation. However, if they are used to hot switch some of these advantages may be lost as reed relays tend to have lower hot switch capacity than EMR's. The hot switch rating (expressed as a power in Watts) has to be defined into resistive loads since this is the only test that can be consistently applied when life testing.
The presence of inductance or capacitance (referred to as reactive loads) in the switching system load (or source in the case of inductance) can also impact relay life since opening and inductive load tends to create a back EMF spike and closing a contact on a capacitive load tends to create high inrush currents. Again reed relays tend to be more vulnerable than larger EMR designs. Capacitance is often present in switching systems because of the number of relays and density of tracks in the switching system layout and the attachment of cables. For capacitive loads the life is very sensitive to voltage (at least a square law dependency) and for inductive loads by current (again at least a square law dependency).
For resistive only loads a reed relay life has the following typical dependency:
- For load powers less than 10% of rating the life is broadly the mechanical life stated on the data sheet
- For load powers at 30% of rating life is degraded by a factor of ten compared to the mechanical life
- For load powers at 100% of rating the life is as stated on the data sheet for the switching system.
These are guidelines only, failure will show as an increasing path resistance as the relay contact wears which in turn may eventually lead to welded contacts. The hot switching events to create uneven surfaces on the contacts concentrating the hot switch energy into smaller regions. Welding occurs because the metal becomes soft or even liquid and creates a sticking contact. If the weld is "light" the spring pressure or mechanical shock may release the contact.
Capacitive inrush current can also cause welds to occur as the inrush current heats the contact metals and softens or even melts them. When the contacts come together a weld can result. Different reeds have differing ability to withstand inrush currents, it is critically dependent on the contact size and the heat sinking materials present. Typically the assumption is made that capacitive switching capacity, provided the hot events are not frequent, is related to the energy stored or taken from the capacitor during the hot switch operation..
Capacitive loads are the most common reactive loads seen in test systems.