When selecting a reed switch for a specific application, one of several concerns is to determine how much load current the switch will be required to handle. If one is just switching a low-current signal, this is generally not a concern, as almost any reed switch can handle at least 100mA.
However, when switching a few hundred milliamps, or more, it begins to become an issue. When looking at the electrical ratings in a reed switch datasheet, one finds at least two different specifications for current:
At first, it may seem odd to have two different current specifications, and for some reed switches, these two values are the same. However, for many reed switches, the switching current is a lower value than the carry current.
So, what is the difference between switching current and carry current, and why does it matter?
Switching current is the current the reed switch’s internal contacts pass when they first make contact and the current they interrupt when the contacts open. Carry current is the steady-state current the reed switch passes once the contact closure has stabilized. The reason switching current is often a lower value, is that arcing can occur as the contacts approach each other or separate from each other, which can cause degradation of the contact material, and in extreme cases, can actually weld the contacts together. Once the contact closure is made, there is no opportunity for arcing to occur, as there is no gap between the contacts.
For most applications, the current the switch will be required to handle is the same, or higher, during the switching events than during stead-state operation. Therefore, when searching for a suitable reed switch, most designs should focus on the switching current rather than the carry current.