So far, we have discussed the importance of switching, analyzing test specs, relay types, power, cabling and connectors. Now let’s start to put it all together. We'll walk through how switches are configured, their purpose, and the different types to deal with. 

Switch Types
Now comes the easy part. Switch types are as simple as A, B, C… literally!

    • Form A – A single-pole, single-throw switch (SPST). Form A switches are open circuit with no power applied to the coil.
    • Form B – A single-pole, single-throw switch (SPST). Form B switches are closed circuit with no power applied to the coil.

      spst relay diagram
    • Form C – A single pole, double throw switch (SPDT). In this instance, there is no open circuit condition. The common connection will always be connected to one or the other pins on the switch.

      spdt relay diagram

All switches are based on these three forms. You will also find multiple pole switches. So, a double pole, single throw switch (DPST) will consist of two Form A or Form B switches. A double pole, double throw switch (DPDT) will utilize two Form C switches.

There is a switch configuration that mainly applies to RF and Microwave applications. An absorption switch uses one Form C and two Form A switches. As you can see by the diagram below, the contact that is not being used is terminated in 50 ohms. This avoids the presence of transmission lines that are not correctly terminated and the resonant structures they can generate. It requires the presence of other (embedded) switches that connect the load to the unused contacts but may not be shown on the equivalent circuit. An important specification when selecting this type of switch is the wattage rating of the terminating load resistor. Too much power dissipated on the terminating load, and you can keep your test system warm and toasty…. until the load catches fire. Not a good scenario. 

absorption switches

In my next post, I will present the basic switch configurations and terminology and applications where they should be considered.

For more information, check out our SwitchMate ebook and watch the on-demand webinar: Maximizing Reliability in Signal Switching to learn the importance of switching in test systems.

Switchmate ebook

<< View the previous blog post in this series, "Electronic Testing 101: Cables and Connectors"

View the next blog post in this series, "Electronic Testing 101: Switch Types and Configurations - Part 2" >>