Do I need to de-rate an invertor for a dimmer

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cybermaus asked Mar 1, '24

Do I need to de-rate an invertor for a dimmer

If I use a triac based dimmer (or any leading/trailing edge dimmer) to reduce power of a resistive load from 1200W to ~500W (coffeemaker element), could I suffice with a 800W inverter.

My reasoning here is that one one hand, the overall power is resistive, so no weird power factor VA calculations. And in the inverter itself also, average heat should be fine.

But on the other hand, the crest of the sine still has the amps that go with it. But an continues 800W device can do 1600W peak, so all transistors should be able to work with it.

Any advise?

Phoenix Inverter

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Alexandra ♦ commented · Mar 01 at 08:27 AM

@cybermaus

Use a pure sine wave inverter without a transformer base.

As soon as you add the triac to say a phoenix the noise is interesting. And you will see overloads or overload warnings because of the messed up sine wave. (And the phoenix is 800VA so more like 600W.)

It would be better to either lower the voltage going to the element (maybe with a transformer) to reduce its output (probably more expensive than the triac idea) or use a smaller element.

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Answer 1 · 2024-03-01T09:38:11Z

cybermaus answered · Mar 01 at 09:38 AM

@Alexandra cost of the transformer is not ideal, but the increased footprint is more of a worry. Smaller elements are probably not available. They probably come only in 115V and 230V versions, so Americans could take a smaller element by installing a European part but not the other way around. So I may be stuck with the dimmer idea.

(Actually, I played with a 47uF capacitive dropper. And that does work well to reduce the power to 300W, but power factor makes it still use 800VA. Which is also still in range, but I wonder even more what unknown effect that may have on an inverter)

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Alexandra ♦ commented · Mar 01 at 09:52 AM

With the capacitive dropper it will make a weird noise also. The interaction with the transformer and a bunch of other things.

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Answer 2 · 2024-03-01T18:25:34Z

Michelle Konzack answered · Mar 01 at 06:25 PM

Do you know, that putting a Diode/Rectifier in series to the Heating element eleminate one half of the sinus wave and you have only the half power?

If your coffeemaker has 1200W and 600W would be OK, the try this.

But you did not say, if you have 120V or 240V.

Hence the Diode must support at least 6A at 120V or 3A at 240V.

This would not require any special tweaks.

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cybermaus commented · Mar 01 at 10:15 PM

Yes, I do know. In fact, I used this trick on my bathroom mirror anti-fog heater, halving its power.

However, the same question would exist as for the dimmer: Does it require derating the inverter? After all, you may be using average 600W, but really, you are pulsing 1200W with a 50% duty cycle on a very low 50 or 60 cycle.

For that reason also, the diode would have to be 6A on 230.

(Or in fact, doing it very detailed, it needs to be able to deal with the pulse current during the voltage of the crest of the sine, not merely the Vrms. So slightly over 8A at 230V)

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