......... - Relay For Death - They Are Heating Up The Ovens Get The Fuck Out Now
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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 Liquified Insides, Genital Herpes - Split 4kW V. I would have expected it to be using a triac to turn the power to the element on and off. So why Soul Food - Various - Scratch Masters 2 I don't think that the answers here duplicate the question about using relays in automobiles.
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. 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.
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. Able to withstand high temperatures better than semiconductors. 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. Better input noise immunity. Stray capacitive coupling even from nearby power spikes, RF pickup, and the like aren't going to trip a relay.
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. 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.
To be clear regarding the important point I think that Chue X might be making: a relay has excellent isolation between the line Yae Goromo - Various - Musical Treasures Of Japan load terminals while a triac does not. . - Relay For Death - They Are Heating Up The Ovens Get The Fuck Out Now example, the BT datatsheet shows that this 4A triac a max leakage of 0.
That's a triac that would be suitable for a regular wall dimmer. Unless the dimmer includes a mechanical switch, then you would measure 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 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. 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. 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. Home Questions Tags Users Unanswered. Why are relays still used in electric ovens? Ask Question. Stringimi Forte - Chin-Chat - Introducing 2 years, 11 months ago. Active 1 year ago. Viewed 5k times. It uses a triac for its 4th Of July, Asbury Park (Sandy) - Bruce Springsteen & The E Street Band* - Live At The Main Point 1 -- 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. Do we have to iterate this over every piece of equipment which happens to still use relays? You can't trivially use a triac to control DC resistive loads, for . - Relay For Death - They Are Heating Up The Ovens Get The Fuck Out Now . But still, you can generalize both questions as "Why do we use relays vs semiconductors", and reasons will be the same. Olin Lathrop Olin Lathrop k 33 33 gold badges silver badges bronze badges.
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. 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 .
- Relay For Death - They Are Heating Up The Ovens Get The Fuck Out Now weld closed. Gravity is not going to un-weld anything of this size. 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. Lorenzo Donati supports Monica Lorenzo Donati supports Monica 19k 4 4 gold badges 46 46 silver badges 82 82 bronze badges.
A triac drops about 1. I doubt that a relay coil consumes 24 watts! Klaws Klaws 1 1 silver badge 3 3 bronze badges. I'm straying beyond what I know about, but I have heard of opto-isolated triacs and opto-triacs same thing? 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.
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