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agate_ t1_j7u1wsk wrote

If they didn’t, the control device would heat up as much as the hob.

Using the “water analogy” for electricity, voltage is like water pressure inside a pipe, current is like water flow. The power consumed by an electrical device is the voltage change across it times the current through it.

If a switch is turned off, it holds back the full voltage from the mains, but no current flows through it, so the switch consumes no power, because anything times zero is zero. If it’s turned on, the current is high but it doesn’t hold back the voltage at all, so again no power consumed by the switch.

But if you turn the switch on “halfway”, so it blocks half the mains voltage and lets the other half pass through to the hob, then hob and switch carry the same current across the same voltage change, so the switch consumes as much power as the hob. This is wasteful, but more importantly it’s dangerous, because the switch will produce as much heat as the hob.

This technique is called “pulse modulation”, and it’s incredibly common, not just in stoves. Any time a digital device is controlling a smooth variation of something, the device is usually just switching it on and off. Often the switching happens too fast for humans to notice (like dimmable lighting) or the signal goes through a filter that smooths out the pulses.

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uh-okay-I-guess t1_j7u5cnc wrote

There are fully analog ways to get fractional power that don't require a voltage divider. For example, a variable transformer would work just fine in an electric stove. But a variable transformer is also much larger, more expensive, and less efficient than a cheap relay.

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theperfectsquare t1_j7ulvdj wrote

hi, follow-up question to help me understand; would another method (using the dimmable light example) be to have several sets of lights at different light intensities and using say a rotary knob pass current to each set of lights to get sort of a stepped change in light?

using the above method could a switch be turned, say 1/8th of the way on, to waste a minor amount of energy to ease the transition between steps?

thanks for the previous answer!

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evilhamster t1_j7vparn wrote

This is a little incomplete... using a resistor divider or a variac ("halfway" switch) is not the only way to limit current -- switched mode power supplies (AC/DC or DC/DC converters) can limit current and/or voltage to arbitrary, programmable levels with efficiencies of as high as 98% (eg only 2% of energy lost as heat). They turn the output all the way on and all the way off according to a clock signal, with feedback loops connected to sensors at the output to regulate the output voltage or current.

It's still pulse modulation, just done really fast. So instead of using bang-bang at multi-second intervals, you do it at millisecond intervals (or in a modern DC/DC converter, down to ~500 nanosecond intervals)

Why this is not done on a hob/electric stovetop is purely a cost of manufacturing thing. There are solid-state relays that can switch 10 amps and reasonable service lifetimes but they cost $10+ each, and require implementing electronic controls (eg you have to use touch-buttons to set the heat level, or have analog-digital converters reading a rotary knob position...) and various other components, like needing to address current leakage from the solid-state-relay when it's off. The general guideline is you can expect retail value of a piece of electronics to be about 3x the component costs, so for a 4 burner stove it'd likely add at least $150 to the retail price. The benefit to the consumer is almost nil, so justifying the extra cost and complexity is not worth it.

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