38 With a DPDT switch contact, two independent wires can be routed to two separate sets of independent contacts. While the switch contacts are electrically isolated, they are mechanically linked together so that they change states at the same time. This nomenclature can represent nearly any type of switch contact; for instance, a 4PST switch would have four wires on separate circuits, and only two possibili- ties: on or off for all four wires simultaneously. Contact Options Part 1 · An Introduction Double Pole Double Throw (DPDT) In addition to the contact arrangement, there are a number of other options to consider when selecting a contact closure device. For instance, you need to consider whether the contacts should be "momentary," returning to a "normal" position after the actuating force (e.g., an operator pushing the button or the machinery moves away) is removed, or "maintained," where the contacts stays in the actuated state until some other action resets it. There are dozens of other considerations too numerous to detail here, but keep in mind that the seemingly sim- ple act of selecting something like an operator control can involve quite a bit of work. Relays/Contactors While computers and controllers can typically pro- duce small electrical currents for control, we often want to run larger loads. A simple electro-mechani- cal device that will do this is a "relay," or a "contac- tor." Relays generally refer to devices that carry smaller currents and contactors generally are devices that carry large currents. Both work in the same way: A small control current is used to energize an electromagnet, which, in turn, pulls a set of contacts together. These contacts can be designed to handle very large amounts of current, and, therefore, large, high-power devices can be operated from small control signals. Solid-state relays, using tran- sistors, silicon-controlled rectifiers, IGBTs, and other devices are also available and offer the same functionality with no moving parts. Care must be taken to suppress the inductive spikes caused by electromagnetic coils found in control electronics devices. "Snubbers" are typically used for this purpose. Photo: Andrew Gitchel AB