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I bought a new electric fan oven recently. It has a digital thermostat and control system. Yet much to my surprise, I can hear a relay clicking on and off inside it to control the power to its heating element. The oven is rated at 4kW (230V).

I would have expected it to be using a triac to turn the power to the element on and off. So why not?

I don't think that the answers here duplicate the question about using relays in automobiles. The design criteria for switching 230V AC are very different for 12V DC. To start with, LVDC would use a MOSFET whereas mains AC would use a Triac. Considerations concerning voltage drop across the semiconductor device and dissipating the waste heat are different. Safety regimes are different. The operational environment is different. And so on.

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    \$\begingroup\$ I bought a combination microwave / convection oven that was only 1.2kw. It uses a triac for its element -- and I know this because I was standing right there when the stupid thing burnt itself out. I expect a relay to have been a bit more robust. \$\endgroup\$ – Bryan Boettcher Jan 11 '17 at 22:20
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    \$\begingroup\$ Relays might be a penny cheaper with no added cost for a heatsink \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 11 '17 at 22:26
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    \$\begingroup\$ Possible duplicate of Why are there so many relays used in a car, instead of transistors?. Do we have to iterate this over every piece of equipment which happens to still use relays? \$\endgroup\$ – Dmitry Grigoryev Jan 12 '17 at 11:28
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    \$\begingroup\$ Would have thought reasons for 12V DC and reasons for 230V AC have little in common. You can't trivially use a triac to control DC resistive loads, for starters. \$\endgroup\$ – nigel222 Jan 12 '17 at 11:48
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    \$\begingroup\$ @nigel222 Indeed, in that case, you'd use a MOSFET (which is easier than a triac to work with). But still, you can generalize both questions as "Why do we use relays vs semiconductors", and reasons will be the same. \$\endgroup\$ – dim Jan 12 '17 at 16:17
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Advantages of relays over triacs:

  1. Very little voltage drop when on. This means they don't dissipate much power. For high power devices, the cost of dealing with the heat often outweighs the cost of the component that dissipates the heat.

  2. Good isolation. The relay coil is inherently electrically separated from the relay switch. Making that isolation withstand normal power line voltages is pretty easy and cheap.

  3. Able to withstand high temperatures better than semiconductors. Silicon stops being a semiconductor at around 150 °C. It's not too hard to make relays that can withstand substantially more. That can be quite useful when in a device that is intended to get hot.

  4. Better input noise immunity. Stray capacitive coupling even from nearby power spikes, RF pickup, and the like aren't going to trip a relay.

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    \$\begingroup\$ One additional reason is the failure mode. On AC it's very unlikely a relay would ever stick on. A semiconductor however can always fail that way. You don't want such a failure mode in a powerful heater. \$\endgroup\$ – Janka Jan 11 '17 at 23:01
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    \$\begingroup\$ And dealing with heat dissipation in a device whose purpose is to reach temperatures above what semiconducors can stand is even more problematic. \$\endgroup\$ – dim Jan 12 '17 at 0:22
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    \$\begingroup\$ We have contact-closed relay failures all the time here, on various ovens, stirplates, chillers etc. Of course it's with very old units, where the contacts are far past their rated life. That, or with low-grade relays in poorly-made products. Yes, the almost-destroyed contacts usually fail open, but sometimes they'll weld closed. \$\endgroup\$ – wbeaty Jan 12 '17 at 0:26
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    \$\begingroup\$ @Joshua: as wbeaty said, sometimes the contacts WELD closed. Gravity is not going to un-weld anything of this size. \$\endgroup\$ – Mister Mystère Jan 12 '17 at 12:48
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    \$\begingroup\$ @Josh: And in addition, the spring in a typical relay acts on the switch much more strongly than gravity, in part so that the relay can be oriented arbitrarily in a 1 g gravity field. Some relays even have the moving parts designed so that there is no net gravity effect in any direction. This is often the case for "vibration resistant" relays. \$\endgroup\$ – Olin Lathrop Jan 12 '17 at 12:55
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Adding to the points of Olin's answer:

If you don't need the fast switching times of semiconductor devices, relays are pretty robust and cheap, compared with the circuitry needed to implement a solid state switch capable of switching the same amount of power.

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    \$\begingroup\$ Plus, for an electric stove the increased power required to drive a relay coil vs a solid-state switch is not worth bothering about. \$\endgroup\$ – WhatRoughBeast Jan 12 '17 at 0:15
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    \$\begingroup\$ @WhatRoughBeast it may actually be the other way around w.r.t power. A triac drops about 1.5V and it's switching 16A so 24 watts. I doubt that a relay coil consumes 24 watts! \$\endgroup\$ – nigel222 Jan 12 '17 at 16:25
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Additionally, when a triac fails, it is often "stuck" in the conducting state. It won't turn off anymore.

Might not be a good idea to have a semiconductor which, when damaged by (for example) a voltage or current spike, turns on your oven at full power while you are on vacation.

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To be clear regarding the important point I think that Chue X might be making: a relay has excellent isolation between the line and load terminals while a triac does not. For example, the BT136-600 datatsheet shows that this 4A triac a max leakage of 0.5 mA. That's a triac that would be suitable for a regular wall dimmer. Unless the dimmer includes a mechanical switch, then you would measure 120 VAC on the load side when the triac is off if there is no connected load. If there is a load connected you will measure a much lower voltage which would be equal to the leakage current times the load resistance.

As a rule of thumb, you would expect that a higher power triac capable of 4 KW would have higher leakage current due to its much larger active area. That would create a substantial shock hazard in the oven when the element is burned out or needs to be removed for service. There would be 230 VAC with significant current capability exposed on the heating element connections. Using a relay insures that the element is safely isolated from the line when the oven is off.

Regarding opto-isolated triacs: that is referring to isolation between the line/load connections and the control connections. That is needed to prevent dangerous voltages and currents flowing back through the control input signal to electronics that are controlling it. A good overview of optocouplers, including opto-isolated triacs is available here Optocoupler tutorial. Opto-isolated triacs still have substantial leakage current and are frequently not suited for controlling certain loads. This is also the sort of leakage that a relay provides between its coil and load, as mentioned by Olin's answer.

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Adding to the point of Olin's answer, there is insulation between switched and the control sides. Whereas a triac requires a small amount of current between the two circuits.

Ref: Triac versus Relay

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  • \$\begingroup\$ No current between sides with opto-isolation. I'm straying beyond what I know about, but I have heard of opto-isolated triacs and opto-triacs (same thing? ) \$\endgroup\$ – nigel222 Jan 12 '17 at 16:27
  • \$\begingroup\$ Opto-isolated triac is a common name for an integrated output module that includes both the triac and opto-coupler in a single module. It's also most commonly epoxy potted to insure dielectric isolation. You can't make a monolithic part with optical isolation built in but these modules are quite common in industrial control. \$\endgroup\$ – wilk Jan 12 '17 at 20:56

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