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LargeGasValve t1_j7tpek1 wrote

it's cheaper and more efficient to use a relay.

They have effectively zero contact resistance, but can switch the current only so fast and only do that so many times before wearing out and are fairly noisy, but even if they fail, they are readily available and easily replaced. This means compromises have to be made on the switching speed

if you wanted to have more granular control you would need power transistors/triacs to be able to switcher the current fast enough to modulate power, these would be more expensive, and less efficient, wasting power and requiring heat sinks for the components and extra circuitry to drive them correctly, increasing costs with no benefit as the food doesn't really care about how power is regulated

induction stoves need high frequency switching to work so they must use electronics, so they always have actual power control rather than "bang-bang" control

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evilhamster t1_j7viqnh wrote

I've actually seen quite a few induction cook-tops that have bang-bang control. Which I don't really understand if they're using switching topologies, like you say. But maybe it's still cheaper to just disable the transistors entirely than to have to design around a variable frequency switching controller?

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Glasnerven t1_j85tcxs wrote

Resistive power dissipation is given by P = R * I^2 where R is resistance and I is current.

A transistor in an "off" state has a very high resistance but no current flow, so power dissipation is very low to zero.

A transistor in an "on" state--that is, fully on, has a lot of current flowing through it but very little resistance, so power dissipation is low.

A transistor in an intermediate state--the kind of state you'd use for modulating an analog signal--has significant resistance and significant current flow at the same time. That means they dissipate a significant amount of power, and they get hot.

For some things, like audio amplifiers, there's simply no way around this and you have to deal with it by using beefier transistors and providing cooling.

For a lot of applications, including stove burners, it's simpler and cheaper to use pulse width modulation or "bang-bang" controls.

(It could be argued that bang-bang control is just pulse width modulation with a really slow pulse frequency.)

Anyway, in engineering you'll see a lot of things where you wonder, "why don't they do X instead, it seems like it would be better?" In most cases (not all) the answer is that yes, it would be better to do X, but it would also be more expensive, and it wouldn't be enough better to justify the additional cost.

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Scott_Abrams t1_j7verrl wrote

Thank you for the explanation, I learned something new.

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