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

Hardware: SCXI, Modular Instruments, FieldPoint>>Discrete I/O Modules>>FP-RLY-420, FieldPoint>>Discrete I/O Modules>>FP-RLY-422

Problem: One of the specifications on switches and relays is the switching capacity. What does this mean and how is it determined?

Solution: Switching capacity is usually rated with both voltage and current. The voltage is the load voltage which will be across the terminals of the relay when it is open. The current is the load current that will flow through both the load and relay when the it is closed.

Relays theoretically do not dissipate any power. The equation for power is Power=Voltage*Current. When the relay is open, the current is zero. When it is closed, the voltage across the terminals is almost zero (depending on the load current and "on" resistance of the relay). In both cases, power is equal or very close to zero. Ideally when the relay opens, the voltage will instantaneously go to the load voltage and the current will instantaneously go to zero. If this were true, then the switching capacity would be as large as the gauge of wire used in the relay.

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.

For more information on how to improve the switching capacity of your relay, view the pdf file linked below.

Related Links: Measuring and Improving the Switching Capacity of Metallic Contacts by Tony Lee and Edmund O. Schweitzer III, PH.D.