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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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hairdrier drops to half-speed
My handheld hairdrier suddenly dropped to half-speed while in use. Does
the DC motor have two sets of parallel windings such that one can burn out yet leave the other to continue operation but at half speed? There was a slight "overheated paint/plastic" odour, but that could be accounted for simply by the heater power not having similarly halved thus causing the heater elements to glow red hot until I backed off the heat setting. It is a good quality appliance, so I disassembled it to investigate. All appears in order, visually, nothing blackened. I traced the circuit. The heater has 3 elements, one is permanently in series with the small DC motor, the other 2 elements are switched in/ out by a 3 position heater switch (0-1-2) and there's a series diode associated with one of this pair of elements. A separate 3 position switch selects the speed of the motor: OFF-MED-HI by adding a diode in series with the motor (and, incidently, all elements of the heater circuit so this means that in normal operation heater power drops commensurately with the lowered fan speed). Ah ha! Obviously it's part of the bridge-rectifier that has gone faulty, I mused. But I connected a 30 volt DC wall wart to the 240 VAC pins of the hairdrier and with the speed switch set to "HI" the motor runs smoothly and with no discernible speed difference FOR EITHER POLARITY (though obviously a lot slower than with 240 volts!). The bridge rectifier is a 4 pin epoxy device mounted on the motor. With speed setting on "MED" a reversal of the DC supply polarity causes the motor to not run because the series diode blocks the current. So, should I focus on the motor? Could there be part of a small DC motor that burns out and causes its speed to halve? Otherwise, what else is there to account for the fault? With the heating switched to the "0" position there exists just a series string comprising the 3-position speed switch, its diode, the motor, one heating element, and the overheat bi-metal cutout switch. Ideas are eagerly sought!! -- John Savage (my news address is not valid for email) |
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