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Comment by frabert | Hacker News Reader

What you're hearing when your coffee maker cycles on and off may actually be a bimetallic strip used as temperature control so technically not a relay

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Bimetallic strips are pretty slow, I can't imagine them being audible. I'd guess what you're hearing is almost certainly a relay, very likely driven by the bimetallic strip

edit: Replies have pointed out that they sometimes do "snap", so i guess i'm wrong there. I'm still pretty sure anything in a kettle/coffee pot/toaster etc would be a relay to avoid arcing

edit 2: i'm just going to shut up now

It's totally possible to shape a bimetallic strip in a way that makes it bistable, thus clicking between the two states, just like an electrical switch which uses springs inside which are normally not noisy either.

One construction I've seen is with a permanent magnet for hysteresis; the strip jumps to the magnet when it gets close, and then requires extra tension to release. The thermostat in my first apartment worked this way (and it looked to me like the strip was actually carrying the current to operate the gas heater; presumably a NC valve).

Indeed, this is how (non semi-conductor) motor overload relays (aka ‘heaters’) work, a simple bimetallic strip with two states.

https://www.c3controls.com/white-paper/what-is-overload-prot...

This is actually the only type that I've ever seen.

They used to use a coiled strip with a mercury switch on the end of it.

Right!!! In the old thermostats. I forgot about those.

A bimetallic strip making a noise when it flexes is the origin of the original blinker noise. Cars these days just emulate the sound, but that’s where it came from.

Technology connections covered them as part of this video: https://youtu.be/2z5A-COlDPk?t=330

> I'm still pretty sure anything in a kettle/coffee pot/toaster etc would be a relay to avoid arcing

You'd be wrong again. The Bi-Metal strip switches the current directly.

ahh well, guess i should probably just stop saying anything at this point

It depends on the kind: a lot of bimetallic strips are designed to have hysteresis, so they snap to a different shape at one temperature and then stay like that until the cool down further (at which point they snap back), and this can produce a click.

Also not great for higher voltages and power.

You can in fact purchase right this very moment bimetallic temperature switches for nearly arbitrary voltage / current combinations.

Right.

Mouser (a general electric/electronic component distributor in the USA) has 'em on the shelf rated for 600 V, 50 A.

I'm sure that specialty distributors offer a much wider range.

I've opened up my water kettle before and it uses a little bimetalic strip to shut itself off once the water is boiling. According to the label it sucks 1500 watts and it's just shoving mains voltage through the heating coil.

Interesting; I always thought of bimetallic strips as being continuous, and silent. I'd open the machine up but unfortunately, it's my production coffee maker and the users would not be happy if there was downtime.

I opened it up, causing a production outage that was noticed by the users. Anyway, the relevant part seems to be a Univen T250 XB thermostat, but I can't figure out what the technology inside is.

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