Power reducer for 13A resistive AC load (like a PV diverter but with totally dumb controls)

OT perhaps but couldn’t think of a better place.

has anyone come across a product which packages the “variable AC power output” elements of a solar pv diverter such as Eddi, into a simple switch controlled device? no sensing of what power output level is required whatsoever, all to be human set.

the use case is camping and water heating.
some campsites have really low amps (5A is common in france), so for camping one tends to buy a 1000w kettle and put up with a kettleful taking forever.
but some campsites have really good amps (16A on a good uk site). but you’ve left your 3kw kettle at home and the water boils really slowly and it can get really infuriating.
a dual element kettle would solve it but there is no such thing.

If I could plug a (safe) “thing” in between the supply and a 3kw kettle, that’d safely downrate it to 1kw, (even better if a switch to set 1,2,or 3 kw) - just as the AC output of the Eddi does with my immersion heater - that’d be a really useful “thing” to have? does it exist?

Yes, we need a “Miscellanous” category.

Do you really mean this? If so, it can’t be done - not with something lightweight and electronic.

Your problem is the resistance of the kettle element is roughly constant, so you’re up against P = V² / R = I²R. What this means is to reduce the current drawn by an 18 Ω element (3 kW) to 4.5 A say, you can only have about 80 V on it, and this gives you about a third of the power you’re asking for - only about 360 W.

You’ll stand a much better chance with a transformer, because this transfers power and so you control voltage and current in inverse proportion, but you’d need to have one wound to your specification, and a lot of copper and iron will be very heavy (~ 15 kg for 1 kVA, 25 kg for 2 kVA). To deliver 1 kW into your 18 Ω element, you need a voltage of 133 V and it will run at 7.5 A but still drawing the same 4.5 A from the 230 V mains.

You could of course have a SMPS - these are almost “constant power” devices (like the transformer), but I very much doubt you’ll be able to get one ‘off the shelf’ with a variable (or even switch-selected) voltage output at the voltages you want. Even bearing in mind, the output could be d.c. (the kettle will be quite happy with d.c, but I’d prefer not to rely on its own switch).

my understanding is that the Eddi and similar PV diverters, use a form of PWM. they don’t have heavy transformers. I’m no power modulation expert but I don’t think it varies the voltage.

A SMPS is PWM. If you go down this route, you then have all the difficulties associated with filtering switching noise and either preventing the kettle acting as an aerial and radiating it, or it being conducted back up the supply and interfering with other consumers.

If you can get full information about one of those devices (which I suspect will be exceedingly unlikely) then you might be able to modify one to your requirements. Designing such a thing from scratch would not be too difficult, getting it to perform acceptably from an EMC perspective would be a completely different matter.

How about something like this?

that looks exactly like the sort of concept I was thinking about. thanks!

there’s chinese things like these
on amazon/ebay but don’t look particularly trustworthy, lots of bare wires and lack of earth.

How about getting a portable induction stove that shows it’s power in Watts. Then you can just stay under that level when you are on a limited supply.

Note that not all display power in watts, and many cheap units tend to taper the power down to a modest level (from memory about 600 Watts on the new ikea model) then pulse on and off to do lower power levels, which is annoying doing absorption rice in pans with a low thermal mass for example.

I’ve actually got one of those cheaper type standalone inductions already. As you say it pulses when on low power setting - a noticeable full blast / off / full blast / off behaviour with a periodicity of several seconds. the full power duration is likely to be enough to overload a limited supply so I avoid using it in those situations. Do you know any particular brands that do a better non-pulsed turn-down?

the ideal cost-no-object design is a inverter like a victron multplus with lithium batteries that can boost a limited incoming feed. but obviously not cheap or small or light!

I really wish someone made kettles with two individually switched elements.

Have you tried putting it on a power meter to measure it’s usage at each level? I haven’t seen one that pulses full power on and off. It’s usually much less.

I’m also searching for the perfect portable induction without pulsing and with fine grain control. Nuwave Gold gets good reviews and has temperature based control and has three levels for peak draw 900, 1500, 2000W and can run from a 1000W generator. So that might fit the bill. (Controls look busy though)
Or look for other camping inverter friendly units and that should limit the peak draw at lower levels.

The other thing to consider is their default start-up power. I have a cheap Aldi/Ambiano model and it starts up at 1600W. That would have you diving for the ‘-’ button before the upstream limiter trips. If that’s a slow blow breaker you might be in with a chance but it would still be prone to user error.

I our kitchen we actually have 2 kettles now, a small 1kW travel kettle marked with 1 and 2 cup marks, use for cups of tea during the day as well as the bigger 3kW kettle. Ensures we both boil just the right amount of water and are also more likely to be below the solar capacity.

How the modulation works Robert and other experts on here can explain in much more detail, but I use one of these modules.


It has a potentiometer used to set the power, you could probably replace by three fixed resistors and a switch if you wanted. They also make a module that allows it to be controlled via a 5V DC PWM signal from arduino or similar, although I have never tired this.

It has worked fine on a 2kW washing machine heating element (2kW) and a 3kW kettle. to keep the power below my solar generation, there is a another challenge on washing machine though - if water does not get hot fast enough the PLC goes into error!

I have never had any noticeable EMC issues with it, either in the kitchen or with other equipment on the wiring

Photo is during testing, before anyone says anything the final result is boxed up!

As others have said, my view is if connecting to the mains I buy reputable brands from reputable suppliers like Farnell/RS/DigiKey etc not unknown things on eBay/Amazon.

PS for my immersion I have a Solar iBoost - this drives the heating element with PWM DC. It’s pretty heavy on account of a big toroidal transformer inside it.

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that looks even better! many thanks.
I’m much more comfortable with a 20 quid unit from farnell than something chinese. I will probably build an exact copy of what you’ve got there.
does it get hot (the power controller I mean not the kettle!)

Running in the test configuration you see in the photo, ambient ~20degC, with the module screwed to the plywood - it never got more than slightly warm, that’s with a 3Kw Load modulated between 1 and 2kw. I also used the it and kettle as a dump load on the house/inverter when I calibrated the home made meter on my diverter so it’s done perhaps 2 or 3 kettles worth in a row with no problems. Perhaps if you used for hours and hours it might, if you had the chance to screw to a metal body I probably would.

That is presumably a burst fire controller which will cause some illegal flickering on your and your neighbours’ mains. Especially on a campsite which won’t have very thick cables running to each pitch, so the volt drop caused by the bursts of power will be significant. Last time I checked, any load greater than 1kW was likely to be illegal which is why induction hobs, at about 1kW for each “element”, can get around the flickering regulations. But for the sake of a few minutes to boil a kettle no one is likely to complain.

3kW solar diverters which use burst firing are questionably legal in my opinion. I tried one once and the neighbouring house immediately complained that their lights were flickering.

One cheaper alternative, which is also likely not completely legal, but should be safe, is to just insert a single heatsinked diode of >10A in the supply to half wave rectify the mains. This will cut it to about 1500W. I use this to dim down a soldering iron. But the distortion on the mains of dimming a 15W iron and a 3000W kettle is somewhat different!

Are you are about that? I thought they used phase control with the torroid maybe there for EMC filtering.

I’m reasonably sure the weight will be due to the filters that will be necessary.

And when you add filters on the load side of the switch (as is likely to be necessary) then you create the problem of what to do with the energy stored in the magnetics when current is switched. Rather than the flywheel diode you need in parallel with a relay coil run off d.c., you need an “a.c. flywheel diode”, which turns out to be a pair of transistors, and then the timing between these and the series transistors switching the load becomes critical in the extreme, because if there’s any overlap, it’s a short-circuit on the supply, and if there’s a gap, the voltage (from E = L dI/dt) breaks down the active devices.

The Farnell part above looks out of stock.
A quick google, I see a 1.5kw unit for about 15 best British quids.

If it worked, you would need to make a way to limit it to max of 50%. Could add a resistor in the patentiometer cct. Or even a simple mechanical stop on the knob.

The basis of my statement is the manufacturers literature that came with the unit

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Ah, so the iBoost gets around the need to cater for circulating the current in both halves of the a.c. wave by (first, presumably) rectifying it and then chopping up the d.c. However, it will almost certainly still need filters, and probably on both the a.c. and the d.c. sides.

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How about a Variac, although it might be too heavy and a bit of an overkill :grinning: