Interposing Relays and the Men Who Love Them

I recently found myself with cause to control two different 20A+ fan motors with NodeMCU microcontrollers… a utility blower I’m using as an attic fan, and a 2 HP dust collector modified to act as a central vacuum. What I didn’t find was an affordable relay that would reliably switch fans of this amperage signaled by an ESP8266 board.

In the end, the solution was to use two relays: a $1.40 standard Arduino-style relay was triggered by the MCU and controlled a 12v load, which in turn triggered a high-amp solid state relay.

Having just built my house and thus being even less interested in burning it down than the average homeowner, I took a look around the web to see if such shenanigans have been done before with a reasonably survival rate, and it turns out that the practice is very common, and for the very reasons I had.

There’s apparently not an accepted term for this arrangement. Such obvious candidates as “daisy-chained relays” and “nested relays” already enjoy common usage with completely different meanings. The function is sometimes described as an “interposing relay,” which turns out to be so vague as to describe 95% of all relays used anywhere:

“An interposing relay is simply an auxiliary relay that is used to isolate two different systems or devices from one another.”

– rspsupply.com

It’s a grandiose enough term, though, to convince my wife the idea was a stroke of genius rather than a stroke of desperation, so I’m going with “interposing relay” until something even more intimidating comes along.