Electronic – Why are relays still used in electric ovens

acmainsrelaytriac

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.

Best Answer

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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