120 V ac to 20 V Dc

A simple corded 120VAC portable hair dryer uses a nomimal 20V DC fan motor. I understand that the input AC is converted to DC by the 4-diode full-wave bridge rectifier, but I don't understand how, or even if, the 120V is converted to a lower voltage that won't destroy the motor. Can you explain? Thanks.
- Eddie
Houston, TX, USA

My guess is that there's a step-down transformer on the ac before the 4-diode rectifier. Hair dryers conk out pretty frequently, so you should have a chance to take one apart and see if that guess is right,

Mike W.

(published on 03/02/2008)

TO MIKE W. Re 120VAC to 20VDC corded hairdryer fan motor- Of course I've taken it apart; unfortunately I didn't keep it. There is no step-down transformer; of that I am sure. However, the V IN to the motor (via bridge rectifier) comes from the end of, or a point on, the highly resistive NiCr heating coil. Does that help you arrive at an answer? Thanks.
- Eddie
Houston, TX. USA

Yep. It sounds like the coil may be used as a voltage divider. In  other words, you can tap any voltage from 0 to 120 V by picking some point from one end to the other of the coil. It would get trickier if there are multiple heat settings.

Mike W.

(published on 03/05/2008)

Right, Mike. I had just confirmed that from another source. The V-in to the motor is from taps on the heating coil. The coil serves as 2 series resistors, tapped at the appropriate intervals. Thanks. And, keep me away from electricity!!
- Eddie
Houston

Thanks for the update.

(published on 03/06/2008)