hairdrier drops to half-speed

I would stick with the full wave bridge, even though it works, it sounds like just one diode has failed, and will still rectify ok, but it will be half wave and not full wave rectification. The motor will certianly run slower with one diode out of action.

Peter Dettmann

and

I would say not the DC motor. Check switches and all diodes/rectifier (out of circuit if possible).

In the week since that happened, twice while it has been in use the speed of the hairdrier has slowly risen to almost normal operating speed with it running at that speed for about 20 to 30 seconds before dropping back to half-speed operation. During this normal speed operation there was a slight odour of an overheating electronic component (quite different from a burnt rubber smell).

If it was an open circuit diode in the bridge rectifier then the motor would not operate with reversed polarity DC input like I tested it with. If it was a short-circuited diode, I'd expect the bridge package to self-destruct with a bang.

The mystery continues.

-- John Savage (my news address is not valid for email)

Does

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)

Hi John...

Just thinking out loud...

There should be some sort of safety thermostat in there, I think usually buried inside heater coils. That doesn't describe well enough - let me say in the center of the air flow, close to the heater coil.

Possible that the contacts are sufficiently cooked that there's a few ohms worth, perhaps a bit intermittent?

Take care.

Ken

If the user of the dryer has long hair, and brushes her hair while blow-drying it, it is not uncommon for deteached hairs to get sucked into the intake end of the blower. When that happens, they suddenly wind around the blower shaft, slowing the blower almost instantly. If they are caught on something and stick in place, the blower shaft will spin within a cocoon of wound-up hair, and the heat of the friction can cause the cocoon to heat up and smell funny. As the hairs in the cocoon burn up, they grip the shaft less tightly, allowing the blower to slowly speed up.

I ran into this several times when my wife had long hair.

Bill