How Is the Switching Capacity of a Relay Defined and What Determines It?

Issue Details

Solution

Switching capacity (or switching load) is typically specified in terms of both voltage and current. The voltage rating corresponds to the load voltage that appears across the relay’s terminals when the relay is open, while the current rating refers to the load current that flows through the relay and the load when the relay is closed. 

Electromechanical relays often specify not only a maximum switching capacity but also a minimum switching capacity (sometimes called the minimum switch load). Meeting this minimum is important for the long-term reliability of the relay. Over time, contaminants and particulates can accumulate on the contacts of armature relays. A small but sufficient current is needed to "clean" the contacts by burning off this buildup whenever the relay closes.

This requirement applies only to electromechanical armature relays. Reed relays, for example, are sealed in a noble gas environment and therefore immune to particulate buildup, while solid-state relays (SSR) and FET relays contain no mechanical contacts at all and are unaffected.

Additional Information

In theory, relays dissipate almost no power since P=V×I: when open, current is zero, and when closed, the voltage drop across the contacts is nearly zero, limited only by the load current and the relay’s on-resistance. Ideally, opening would cause current to fall to zero instantly while voltage rises instantly to the full load value, meaning the relay’s switching capacity would be constrained only by the current-carrying capability of its internal conductors, essentially equivalent to a wire of the same gauge. 

However, when the relay is in the process of switching, there is a finite time when both values are nonzero values. During this time, power will be dissipated in the relay. Depending on the inductance and capacitance of the load and relays, there may be large voltage and current spikes during this period of time when the relay is switching. This may generate a lot of heat in a very small area and consequently melt the contacts of the relays or leave corrosion that will shorten their life. With larger voltages and currents on your load, the switching time will be longer, and you will increase the energy (the time integral of power) dissipated in your relay. You may also have larger power spikes as the relay is in transition.

Another factor of the switching capacity has to do with the mechanical construction of the relay. When the relay is just beginning to switch, the force of the contact decreases which results in an increase of resistance. The load current flowing through this resistance will heat the contacts. There is also an instant when some parts of the contact are touching and some are not. By decreasing the surface area that the current is flowing through, the resistance is again increased. At the instant just before there is no contact, all of the current is flowing through an extremely small area. This last point of contact will get so hot that it will usually boil or even vaporize.

In practice, this issue often comes up with usage of the device in unexpected scenarios. For example, a PXIe-2532B in a 4 by 256 matrix will not be able to do a Fault Insertion Unit (FIU) Test because it's limit is sixty drives continuously. This is a direct example of the above mentioned theory and can be resolved by using a specific FIU scheme or be using an alternative PXI-2536 FET that can take multiple drives.