Coffee at 12V or 110V

Why does it care if it's AC or DC? The 100x power difference between

120V and 12V may get a little tricky, though. A diode in series would drop the difference to "only" 50:1. Perhaps there are three wires and the adapter changes the connections?

snip

Now I look a bit closer there are indeed three wires in the cord. The 12V adapter has a metal pin offset from the centre so you can only connect it one way, ie with correct polarity. So the pin is in circuit only on 12V. I failed to notice on first look that the pin was for more than just getting the polarity right.

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** So the unit has two heating coils, one for 120VAC and another for 12VDC with a common connection. The 12VDC coil uses less watts than the AC coil.

You could easily measure the resistances of each.

..... Phil

Yeah I should have done some resistance checks first then I may have figured out quicker that there had to be two heating elements.

12V

it

V.

VDC

red

I idly wondered if it was possible to have one element run off either suppl y. Tungsten wouldn't give enough resistance change. A bimetal strip to cycl e power would give too high consumption during on times. An inductor heatin g element could work, though it would be a right mess of compromises and no t the cheapest/sensible choice. A relay to switch element sections between series & parallel could work, but relay contact failure would be fun, and a gain not the practical option. Wide ranging smpsus are the logical choice w here something really has to run off whatever.

NT

They could be using a whopping big PTR, but a practical and cheap solution would be a voltage dropping inductor in series with the heater element. Big drop at line frequency, no drop at DC.

Say it makes a quarter litre of coffee at a time. The power of the thing must then be ballpark 100W and the 12V heater resistance would then be about 1.2 Ohms. To get the same power at 110V AC, you'd need a 30 mH inductor in series, with a wire gauge and core size good for 10A, roughly. That's a big thing. Not practical at all. An additional 120 Ohm heater element is cheaper.

Jeroen Belleman

You don't know what you're talking about. I see this for speed control on induction run motors all the time, the inductors are not large, and it's very practical.

The 'whopping big PTR' could be one of those ceramic heater elements that is intended to self-limit below the combustion temperature of fabrics. Such heaters DO have enough power output, but I'm unsure of the operating voltage range.

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** Really?

Using the same heating element in both cases is not practical.

The AC and DC currents must be nearly the same, making the AC current 17amps just for 200 watts of heat. The needed inductor would be huge too.

Using two heater coils is the answer.

..... Phil

No true coffeephile would brew with DC. No timbre.

Mark L. Fergerson

You're not an industrial engineer, you know nothing about manufacturing.