# Need help designing a hairdryer

I'm designing a hairdryer as project for a university class, but I'm having trouble regarding the choice of the motor. I need it to have two possible speeds. My first idea was to use an AC motor and a diode as in the schematic below, when the current flows through the diode, half of the AC wave gets cut, so the motor runs at half speed. My first question is: Will it work?

I also tried to use a DC motor instead with a rectifier bridge, but using the aforementioned diode to cut the wave in half doesn't seem to change to voltage on the motor very much, how can I make it work with I choose to use a DC motor?

Thanks in advance.

• Thanks for the comment, sadly I have no choice, but I'm aware of the danger and safety precautions and only have to come up with the schematic. – gabs136 Oct 21 '20 at 12:54
• A hairdryer uses to have a simple circuit... the basic circuit is well stablished at, maybe, 30 or 40 years. You can open a hairdryer to see what there is inside. There is a lot of schematics on the web, just Google for "hair dryer schematic" – mguima Oct 21 '20 at 12:58
• @mguima - Hairdryers are like that as well. The motor is driven from a tap on the heating element through a rectifier. The tap is changed to change the speed. – Kevin White Oct 21 '20 at 14:17
• @KevinWhite I deleted the comment saying that hairdryers always have the heating element and motor wired in parallel, trusting in what you said about the tap in the heating element. But, in my limited experience, there is also a lot of hairdryers with this configuration shematic, all of them use AC motors, and there is no rectifier. The only diode in the circuit is used to permit half-wave operation. I've personally opened several hairdryers like that. – mguima Oct 22 '20 at 13:30
• @gabs136 That schematic was provided by the manufacturer, I'm sorry that the schematic was not canonically drawed. The key to understanding this circuit is understanding how those switches work. The switches have three positions: (A) all pins unconnected; (B) pin 1 connected to pin 2; (C) all three pins interconnected. The diode is there for halving the wave that goes to motor (it's an AC motor); when the current for the motor passes through the diode, the motor receives half AC wave and the fan is low. When the diode is out of the circuit, the fan runs at full speed. – mguima Oct 24 '20 at 19:19

## 1 Answer

From this question you can see the typical schematic of a two-speed inexpensive hair dryer.

The heater forms a voltage divider to feed the low voltage DC motor (via a bridge rectifier), and you can switch an additional rectifier in series for 'low'. The fuse (FU = 熔断器) and thermostat (ST = 恆溫器) are for obvious safety reasons. If the air flow is obstructed for any reason or the fan fails the heater temperature will rapidly increase so some form of safety cutoff is required. The "FU" is likely a one-time thermal fuse that acts as a backup bricking the appliance if it seriously overheats, and the "ST" is a thermostat that closes once the dryer cools down a bit.

If you want to see a higher end dryer, have a look at tear-down videos of the Dyson hair dryer. It costs about $500 CAD ($400 USD) and uses a high speed (110,000 RPM so the racket is mostly ultrasonic) BLDC motor.

• I like the differential AND gate driving it all... :-) – TonyM Oct 21 '20 at 13:18
• Thanks for the comment. Are you suggesting adding a half wave rectifier to reduce the speed on the "Low" setting ? I tried putting it in series with the bridge and it didn't seem to work. – gabs136 Oct 21 '20 at 19:47
• It needs to go on the mains side in series with the whole mess as shown on the schematic at the switch position marked '冷'. After the bridge it will do nothing much. It cuts the heater power by 50% and reduces the motor speed. – Spehro Pefhany Oct 23 '20 at 17:54