After some googling, I've found several explanations of what is going on in a dual-voltage hair dryer, but I've been unable to glue them together into a solid understanding of what is probably a very simple circuit. (I should specify that I'm talking about the cheapest type of hair dryer, which has a single high/low switch, rather than separate speed & heat switches.) One of the suggestions I've found is that the 110/220v "switch" does nothing but physically lock out the High setting, leaving the circuit exactly the same - meaning the high/low switch is the relevant one.

Conflicting results suggest: 1) the low/high switch simply puts a diode in series with the AC input, chopping off half the wave & thereby reducing power by half. 2) the low/high switch moves the AC input from one end of the heating element to the middle, putting the two halves in parallel instead of series.
The diagram at the bottom of this page shows a setup like #1, with the motor in series (well, behind a bridge rectifier) with half of the coil. But a doubling of the input voltage here seems like it would lead to a doubling of the current through Coil 2, and hence 4x the power/heat, rather than 2x. So maybe that diagram doesn't apply to a dual-voltage dryer.

My ultimate goal here is to predict how a dual-voltage dryer will behave on 220v. Will the air speed on Low be equivalent to the High setting on 110v? Will the heat produced be equivalent/more/less?

Lacking one that I can crack open, I'll settle for a diagram/explanation of a generic device, and just roll the dice based on the result.

 

Halving the voltage is a Quarter of Wattage,

W = V*V/R

 

Halving the voltage is a Quarter of Wattage,

Thanks for the reply. And yes, I mentioned at the end of my second paragraph that 2x V leads to 2x I leads to 4x P... Though making use of that result to determine the heat and airflow output would still depend on how the circuit is set up, and in particular, how the Low/High switch changes the circuit, no?

That is, if the goal is to say "at 220V, the Low setting on this dryer will produce equal/less/more heat and equal/less/more air than the High setting at 110V", then it's still necessary (and preferable!) to know how the circuit works.

 

Most of these Cheap hairdryers use a single diode to drop the supply to 120V, mainly for the heaters, they all put the motor in series with the heater.

 

By "drop the supply to 120V", I assume you're talking about the diode in the AC path that cuts out the negative cycles? So, the one that's switched by the Low/High switch.

In the interim, I was able to take power readings from a few hair dryers. It looks like I got a couple of different types:

    low    high    220+low    220+high
B    430    870     111        222
D    500    1000    130        260
C    350    1320    350        locked

So dryers B & D had Low settings that drew 1/2 as much power as their High settings. When turning the 110/220 switch to 220v, the power was reduced for both low & high by 1/4. For dryer C, the low setting was 1/4 of the high setting, and the 110/220 switch locked out the high setting altogether.

My guess is that cheap dryer C is doing the diode-half-wave thing to get its Low setting, and slightly less cheap dryers B&D are doing some variation of the series/parallel coil thing mentioned in my original post.

I guess most importantly, all of them seem to suggest that when used on 220v, the power will be the same as on 110v. Presumably, this means both the same amount of heat and the same amount of air. I guess I'll find out.