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Edit: The reason this question exists is because I once had a cheap Amazon relay fail on me during something important (switched on and off randomly then on permanently), but apparently well-made relays are actually good and don't fail as often as the cheap ones.

I was able to pick up a decent, stainless steel air fryer for a pretty cheap price from a big name brand store recently. It was a medium-sized air fryer and stainless steel, so it wasn't as cheap as the smaller plastic ones, but I thought it was a good compromise between quality and price.

However, now every time I hear it working I hear a distinct "click" sound when the heater turns on, and another "click" sound as the heater turns off. It's a typical programmatic heating system, where the heater goes on and off to keep the temperature around a set point, with some degree of variance around that point. 3D printers use a similar system and call it "PID tune" but it's basically just as simple as a thermometer/thermocontroller deciding to turn on the big red heaters when the oven falls a few degrees lower than the temperature you set it at, and turning off the heaters when the temperature reaches a few degrees higher than the temperature you set it at, over and over until the cook time is done. The "click" sound is the sound of the automatic "switch" being turned on and off as this happens, providing current to the heaters. The "switch" is just like a switch on a surge protector or wherever else, except it's controlled programatically by the thermocontroller.

But what surprises me is the fact that I can hear a "click" ing sound at all. Aren't there solid state relays now, which are way safer than a mechanical one? Solid state relays operate similarly except they don't have a "clicking" sound and are slightly more expensive, plus I have had better luck with them not-failing. Plus, this relay system is literally inside a hot box. Don't mechanical relays fail way more often at higher temperatures?

How can I be sure that this appliance is safe? It seems like if the relay fails, and the temperature goes up higher than it should be, the appliance could catch on fire or at least burn my food, setting off the smoke detectors. But now that I think about it, I guess cheap toaster ovens that I have had in the past also had a mechanical relay in them.

Shouldn't heat-based appliances, similar to kilns, be advised to spend the extra $4 to get a solid state relay in their internal circuitry? Or is this a special mechanical relay that will be fine?

Do more expensive toaster ovens and air fryers use solid state relays?

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    Is it really a relay? It could be an ordinary mechanical thermostat (a switch directly controlled by thermal expansion of a metal/fluid instead of electricity flowing through a coil). If you set the temperature by turning a knob, it's likely a thermostat with no electronics involved (like in a simple electric oven).
    – TooTea
    Commented Jan 19, 2022 at 14:57
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    Actually, heat is more likely to negatively affect SSRs than mechanical relays.
    – DoxyLover
    Commented Jan 19, 2022 at 15:41
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    3d printers heat via DC, which is easier to switch solid-state using mosfets than AC is using triacs.
    – dandavis
    Commented Jan 19, 2022 at 17:53
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    also, there will be one-time thermal fuses on the product that will blow shortly after a switching element (mechanical or triac) fails closed.
    – dandavis
    Commented Jan 19, 2022 at 18:05
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    Target sells it. Retail shops don't sell things that aren't UL listed because the consumer safety agencies will crush them if they do. So you're all set. However that should also be on the equipment nameplate. Amazon is immune to the safety agencies because they're "only a marketplace connecting buyers to 3rd party sellers" and their warehouses are "free trade zones whose products might ship to Canada". Commented Jan 19, 2022 at 23:56

3 Answers 3

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There are two issues here:

  • Solid State vs. Traditional Relay

A relay may actually be more resistant to heat than a solid state component. For comparison, consider light bulbs. An incandescent light bulb is very heat resistant, doing well even inside an oven. An LED, on the other hand, can have its lifetime shortened significantly if exposed to high temperatures, far lower than an oven, to the extent that some larger (light produced/power consumed, not physically larger as that would result in better heat dissipation) LED fixtures include a thermostatically controlled fan to help dissipate heat.

A solid state component may have significantly faster reaction time or be able to switch using a smaller trigger than a regular relay. But a regular relay is simple and reliable. Newer is not always better.

  • Location of Relay

A relay does not need to be in a hot location. In fact, whether electro-mechanical or solid-state, relays are likely to be located together with other controls a significant distance away from the heated area. The only part that needs to be in the heated area is a temperature sensor of some sort, which it then sends through (typically) a pair of wires to the control panel, where it triggers the relay.

As far as cost, it is actually quite possible that the solid state relay costs less and that cheaper devices use solid-state relays to save money and more expensive devices use electro-mechanical relays for reliability.

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    But the ssr may need mounting on a heat sink etc which pushes the cost up...
    – Solar Mike
    Commented Jan 19, 2022 at 14:44
  • @SolarMike True. In the end, cost depends on a bunch of design factors. Commented Jan 19, 2022 at 14:46
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    +1 Relays also rarely fail closed. That happens pretty much only when a massive overcurrent (short) welds the contacts together. Semiconductors, on the other hand, can fail short for a myriad of reasons.
    – TooTea
    Commented Jan 19, 2022 at 15:00
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    "Newer is not always better." +100!
    – FreeMan
    Commented Jan 19, 2022 at 16:41
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"How can I be sure it's safe.." You buy UL listed appliances. Mechanical relays can be built to withstand high temperatures. We used them all the time in a steel heat treating company I worked at. The Clicking is just the solenoid kicking in and out and the spring opening the circuit. Both have got to have the strength to make and break higher amp current so it will be louder than the relays you find in your car. Personally, I'd feel much safer with a mechanical relay that I could hear operating...

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    Speaking of car... Turn signals used to be operated by capacitors and relays. The relay was what you heard clicking to remind you that the signal was on. Since they're all electronically controlled now, the manufacturers emulate the clicking sound digitally and have to install an extra speaker (or play it through the car's radio, er, in-car entertainment system) to get the same sound that used to come for free. "Newer is not always better", to quote another answer...
    – FreeMan
    Commented Jan 19, 2022 at 16:43
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    It's a slow Friday, @manassehkatz-Moving2Codidact. I just watched that. Yup!
    – FreeMan
    Commented Jan 21, 2022 at 17:26
  • I remember taking apart a turn signal relay 25-30 years ago on a wrecked Suzuki truck, it had the heater coil style turn signal, pretty cool to see how it worked as a kid.
    – rtaft
    Commented Jan 21, 2022 at 18:30
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They are different, and I wouldn't say one is superior to the other, it depends on your use case. But specifically for an air fryer, this isn't heavy duty use, and the relay will likely last for decades. Here is a list of some of the advantages for each.

EMR advantages:

  • It is on or off, no current leakage occurs. SSRs can leak current in the off state.
  • It is at least 50% cheaper than an SSR.
  • It can handle a much wider range of temperatures than SSRs, which in turn means they can generally handle more current.
  • Most likely will fail in the off state, SSRs are more likely to fail in the on state.
  • Some uses (like thermostats) people want to hear the click when it engages.
  • They don't need heat sinks and because of that, generally have a smaller profile.
  • They aren't picky about the current as long as its below its rating and doesn't care whether it's AC or DC current. The current and/or voltage can also change/vary during operation without issues.

SSR advantages:

  • Lasts longer than EMRs. EMR contacts can wear out over time, more likely to fail. EMRs are even more likely to fail in environments that have a lot of vibrations (like a vehicle).
  • Quiet Operation
  • Faster, more reliable/predictable switching speed. (1ms vs 10-20ms)
  • No electrical interference is generated during switching.
  • Uses less energy to switch. An EMR requires constant current to engage the switch.
  • Can't be accidentally triggered, dropping an EMR can temporarily switch it on, magnets and electromagnetic interference could engage an EMR as well.

Source: https://www.controldesign.com/articles/2015/should-you-use-solid-state-or-electromechanical-relays-/ (paywalled)

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