Why Isn’t There a Relay in This?

Relays are known for the bigger switching jobs, but they’re not the only game in town. There is a middle-ground where your five volt control board would fry and where a relay would be overkill or too slow to do the job. We turn to another special component for these “higher” but not necessarily “high” voltage jobs.


The Relay’s Weaknesses

There are jobs where a relay just isn’t the answer. Relays come with two pretty big weaknesses: they’re mechanical and they’re slow. If we were to take apart pretty much any relay, we’d find perhaps some springs, an electro-magnetic coil, and some contacts. Whenever we actuate that relay, parts inside of it move.

Moving parts are bad for speed and reliability. Eventually, the switching mechanism, contacts, or actuator inside that relay will degrade and fail. At the same time, we can only force that relay to move so fast. If the contacts inside are a half-inch apart, then we need to cover that half-inch before power will pass through. It’s a small distance, but consider that standard AC power in the US runs at 60hz. If we wanted to change the relay position based on the electrical frequency, it would need to move in 1/60th of a second. We need to accelerate to 2 mph and stop within 16 milliseconds, and that still costs us an entire 1hz cycle. Even then, chances are it will take longer to de-actuate. When the contacts move apart, there’s going to be a tiny arc until we’ve traveled far enough to break that arc. You see this phenomenon with car batteries, which might spark when you attach jumper cables.

We accept these trade offs for the high voltage capacity of a relay. We can spec relays up into hundreds of amps and volts. It’s a giant switch, so there’s not much limit to the amount of power a relay can be built to handle. However, if you’re not trying to run the electrical grid, do you really need something that beefy?


Going Solid State

The key way to get rid of a relay is with a MOSFET or an IGBT. These are solid-state chips that do the same thing as a relay: they turn power on and off. That solid-state bit is key though. Solid state devices can be insanely fast and long-lived. The processor in your computer is a solid-state device. It has little transistors changing state billions of times a second and we have computers from the 70s and 80s, with early integrated circuits, which still happily work today. Designed right, solid-state equipment can last forever.

This creates some obvious incentives to use MOSFETs, IGBTs, and other alternatives. They’re long lived, generally smaller, and faster than a mechanical switch. We can use them to step down voltages in power supplies or very crucially, control variable-speed motors. As we move to increasing power efficiency, being able to vary the speed of a compressor or fans will play a bigger role in how we increase comfort and reduce costs.

If you look inside your equipment, you’ll likely spot these little chips in your thermostat, power supplies, and possibly in fan and motor controllers. This is both good and bad. The increase in presence means we’re moving to more efficient and less wasteful technologies but in most cases, you don’t have a say over relays, MOSFETS, or IGBT in your equipment.


All Built In

The major switching components are attached directly to circuit boards. The biggest exception perhaps is the drivers for fans and compressors, but  that too becomes tricky. Your equipment was designed with a single, dual, or variable speed compressor and you can’t change that after the fact without voiding your warranty. If your system uses chunky relays, then that’s the best way to make that system operate.

That said, we are perhaps on the verge of the next big thing in AC development. Solid-state tech has improved, variable drive tech has improved, and in the near future we may move entirely away from single and dual stage compressors to efficient, precise variable-speed systems.

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