I'm working on a side project where I need to figure out how a hair dryer works...so I took apart a hair dryer and made a circuit diagram (below). I am much more mechanically inclined than electrically, but would love to have a basic understanding of what is happening in this circuit.

Things I know...

  • The motor is AC (120V, 60Hz)
  • Switch #1, the 'ON / OFF' switch has three positions; OFF, LOW, and HIGH
  • Switch #2, the 'Heating Element' switch has three positions; COOL, WARM, and HOT
  • Switch #3, the 'Cool Shot' switch, some how short circuits / turns off the heating elements
  • When Switch #1 is in its third position, the 'input' is connected to both 'outputs'
  • When Switch #2 is in its third position, the 'input' is connected to both 'outputs'
  • Switch #3 is normally closed

Things I would like to know...

  • How does the motor change speeds? Are the diodes responsible? Are the heating elements responsible? What are they doing to cause this change?
  • Do the heating elements change the voltage seen at the motor?


2 Answers 2

  • The "low" motor speed is instead realized by blocking every negative half cycle of the incoming AC with Diode 1. The "high" speed is realized by bypassing the diode.

  • In this particular hair dryer the motor is never in series with the heating elements.

I found your schematic particularly confusing, so I redrew it. In general, try do draw high voltage elements on the top and low voltage ones on the bottom side of the schematic. In this particular case the device is AC powered, so pick an orientation as you wish but stay with it. Signals (not present here) should generally flow from left to right.


simulate this circuit – Schematic created using CircuitLab

Regrettably SW1, SW2, the thermal fuse, the thermal cutout, the electrode and the motor have the wrong schematic symbols:

  • SW1 is a three position rocker switch, model SR-71, function type "K". Its three positions are off(open circuit) - fan low(right side only) - fan high(both right and left sides connected to common).

  • SW2 is an identical three position rocker switch. Its three positions are cool(open circuit) - warm(right side only) - hot(both connected).

  • \$\begingroup\$ Thank you for taking the time to make the diagram and chart! After digesting this for a day, the chart really helped my understanding of what was going on. \$\endgroup\$ Mar 6, 2016 at 2:47

For full motor speed or full heater output the AC mains voltage feeds through the respective switch and goes to the load. The full 120VAC rms voltage is applied to the motor or heater.

For lower speed or heater level the AC mains voltage is fed through a diode to each of the loads. The diode has the characteristic in that it only conducts current in one direction so only the positive parts of the AC voltage waveform makes it to the motor or heater. The negative parts of the AC voltage waveform are cut off by the diode and so thus do not deliver power to the load. The net effect of this is that the motor or heater get about half the power and thus the motor turns slower or the heater does not emit as much heat.


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