by Arno Holschuh
I distinctly remember the first time I saw a solidstate relay. It was the early 2000s, and my employer had just had their machine converted to digital temperature control. At the time, this was extremely cutting edge, sort of in the same ballpark as clones and flying cars. We gazed into the machine, looking at blinking lights, wires, and the temperature probes. I noticed the black box that had been glued to the frame.
“What’s that?” I asked.
“Not sure,” my coworker said. “Let’s not touch it.”
And so we didn’t.
It did its thing, silently switching on and off constantly to keep our brewing temperatures on an even keel. It wasn’t until years later that I thought back to that magic black box.
Why solid-state?
First, let’s take a second to consider the lowly relay. A relay is basically a little switch that closes a big switch. What happens in a traditional relay is that a little switch is used to power an electromagnet; the big switch is closed when that electromagnet pulls the contacts together,usually with a satisfying, audible >click<, or >thunk< if it’s large enough.
It seems like a lot of fuss over switch size, but switch size (and wire size, etc) matter a lot. Consider that the kind of switch that could safely control a car’s starter motor would probably take two hands to comfortably operate. Without relays, our lives would have a lot fewer tiny buttons and a lot more Frankenstein-laboratory knife switches. And relays were an essential building block in creating digital logic. Banks of old-fashioned relays were used to run roasters control cabinets into the 70s. (Opening one of these cabinets sounded like a battalion of power knitters clicking their way to a world record scarf.)
But traditional relays present problems. The inherent weakness is that they have moving parts. All components with moving parts will eventually fail, as the forces of metal fatigue, dirt and corrosion take their toll. The contacts on traditional relays also tend to get fouled with carbon due to arcing (as is the case with the closely related traditional pressurestat).
The solid-state relay,on the other hand,does away with all that motion, and is therefore muchmore reliable. There are no actual contacts to foul, no electromagnet that can wear out. And solid-state relays can switch incredible quickly, which makes them ideal for systems where a digital controller is trying to allow many tiny little bursts of electricity–like the PID controllers on our espresso machines. The primary downside to a solid-state relay is that when they break,they tend to break in the closed position–that is, in the position where they are delivering electricity. But it happens so infrequently, and modern machines have such robust safety designs, that this is acceptable.