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Repair - hair dryer

Problem

There was a hairdryer. One day it stopped working, bit the dust. So it was donated for parts or repair. Physical examination shown that the most probable event happened - the heater filament failed, locally overheated and melted. Repair of such broken wire is usually considered impossible.

Solution

It was decided to attempt to repair the device. The wire had to be reconnected using a reliable way.

Attempt to spot-weld the wire failed. The presumably nichrome alloy refused to melt. The wire was therefore twisted together, to achieve mechanical integrity. The joint was heated and dipped into borax, which was then melted into glass serving as a flux. A small piece of silver was attached to the joint and molten, however it refused to wet the wires and after a while dripped off. The attempt was repeated, this time with a small piece of brass wire wrapped around the twisted joint - this time it worked. A literature check revealed that it was to be expected - silver does not wet nickel and iron, which are the main components of nichrome. Copper, the main ingredient of brass, however wets those metals easily. Brass is therefore suitable for brazing nichrome wire. Its melting point above 900 °C is also suitable for this application, as it can survive red-hot temperature of the heating spirals, which is above what is needed for a hairdryer.

The mounting screws that hold the unit together are of a proprietary nature, using a triangular hole. A slit was cut into their heads, to allow using a flat screwdriver. Manufacturers who do this should be shot.

Device description

The device is very simple; consists of a DC motor with a fan, a rectifier and smoothing capacitor to provide the DC power for the motor, a pair of chokes to reduce EMI from the motor, and two heater coils, one of which is tapped and serves as a resistive voltage divider for feeding the motor with lower voltage.

The heater is wound with a nichrome wire on a mica body. The wire is wound in two parallel spirals, 26 turns and 150 Ω each, connected on one end and fed with power via a bimetal-based overheat protection switch. One of the spirals is tapped at fifth turn, resulting in a 0.2:0.8 resistive divider. (The ohm values in the schematics look wrong, should be 29 Ω and 121 Ω. TODO: check.)

The motor is the RS-365SH-2080 type - a carbon-brush DC motor running on 6-20 (nominally 18) volts. 19400 rpm/0.25 amps at no load, 15910 rpm/1.14 amps/0.00766 N.m (78.1 g/cm) at optimum efficiency, and 5.2 amps and 0.0426 N.m (434 g/cm) on stall.

The device has three power levels: 1, 2, and cold (C). At power level C the power for the motor and heater is cut to half using a series diode, resulting in lower motor speed (and lower airflow and noise) and only gentle heating of the air. At power level 1 the motor and the first heater run at full power. At power level 2 the auxiliary heater (the other spiral) is also enabled.

Sister project

The fan was AWFULLY noisy. For use in workshop setting, a quieter operation was desired. The motor was disconnected and removed and the airflow supplied with a larger, mains-powered fan with larger blades, lower speed (most of the noise is caused by the airflow over the fan blades) and sufficient airflow. The result was the Hot Air Blower.

Images


Hairdryer, outside view

Hairdryer, brand detail

Hairdryer, outside

Removed cover

Rotating cable

Rotating cable, contacts detail

Motor and heater

Motor and heater, without outer cover

Motor and heater, without outer cover

Motor and heater, without outer cover

Proprietary screws

Proprietary screws, modded

Brazed joint

Brazed joint

Brazed joint

Schematics

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