How do motherboards control case fans (electrical question)?

[kendalf]

Does anyone know what is the method(s) used by motherboards that allow BIOS or software control of fans attached to a motherboard fan header? I'm excluding PWM fan headers in this question, as PWM control is relatively straightforward to me.

I assume that the motherboard is reducing the voltage output by the header, but how is this being done? I know that some add-on fan controllers use rheostats or variable resistors to reduce the voltage, but this excess energy that isn't going to the fan must then be dissipated elsewhere, which means some extra heat produced by the controller itself. One of the issues with rheostat fan controllers is that the farther you undervolt the fan, the more excess energy (and heat) needs to be dissipated, which is why some controllers (like the Sunbeam Rheosmart have such large heatsinks. If this is how motherboards control the output to fan headers, does this mean that when we use the motherboard to turn down the fan speed that we are producing excess heat in the motherboard? Or do they have circuity that actually produces whatever voltage is being used by the fan, rather than just reducing the initial 12V?

[johno]

Good question. I've always assumed the 3-pin motherboard headers worked by placing the PWM driver on the motherboard - essentially implementing a simple Buck converter. That would be cheaper to implement than D/A conversion and linear power amp. I'd be surprised if they were dumping much heat to regulate the fan voltage because its actually easier to do it the efficient way.