Switch Terms

A while back, I had a tech who was having some trouble finding a 35 PSI (2.41 bar) make-on-fall pressure switch. One adjustable switch in the catalog said (SPDT), but he didn't quite understand what that meant. In that case, it means single-pole, double-throw, and the “double-throw” part means that the switch has terminals in both the close-on-rise and close-on-fall directions. Another common example of this sort of switch is a “three-way” light switch.

switches

A single-pole, single-throw (SPST) switch is like a typical light switch. It only has one path (pole), and it is only closed or open.

A double-pole, single-throw (DPST) switch that is quite common is a two-pole contactor. It has two switches, but they only open and close in one direction.

A common double-pole, double-throw (DPDT) switch in HVAC/R is the 90-340 relay (and many other relays), where it has two circuits, and they alternate between closed and open terminals.

—Bryan

 

 

 

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Related Tech Tips

You're Grounded
One of the most common questions we get from techs is about using a voltmeter to diagnose a high voltage circuit. It's especially tricky when a tech is used to working on a low-voltage or 120V circuit, where there is a clear “hot” side of the circuit and a clear “grounded” side of the circuit. […]
Read more
I Really Want to Learn More!
This is an open letter sent to staff at Kalos Services. Hey Everyone, First, I want to remind you that you are in no way obligated to read company emails or do anything work-related when you aren’t working. So, if you don’t want to be bothered by this on your day or time off, then […]
Read more
Why Checking Gas Valve Leakage is Hard
This article is an extension of HVAC School’s “How to Measure Gas Pressure” video. Additional input was provided by Jim Bergmann. Bubble test procedures and standards were provided by Honeywell. Links to source materials are available at the end of this article. If there’s one complaint that HVAC techs should never take lightly, it’s a […]
Read more
loading

To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from