Notes on the Troubleshooting and Repair of Small Household Appliances and Power Tools


  7.41) Throbbing noise from ceiling fan

This could be due to a mechanical problem - bad bearings or blades out of
balance - or an electrical problem with the speed control.

(From: David Buxton (David.Buxton@tek.com)).

A quickie test.  Get the fan turning at a speed that demonstrates the 
throbbing noise.  Come up with a way to instantly remove power to the 
fan.  If the noise continues for a little bit until the fan has slowed 
down enough, then you know the noise is in the mechanical dynamics, 
perhaps blades out of balance.  If the noise quits instantly with power 
removal, then you need a better speed control better designed for fan 
motor control.

  7.42) Ceiling fan motor speed control and capacitor replacement

(From: Kevin Astir (kferguson@aquilagroup.com)).

Ceiling fans are normally multipole, capacitor-run  types.  They normally
run fairly close to stalled, the blades being big enough that the motor
never gets anywhere near synchronous speed.

Speed control in three speed types is by switching the value of the cap
in series with the quadrature windings. The caps normally have two
sections of 3 and 6 uF,  with a common connection between the two
sections allowing connections of 3, 6, or 9 (3 in parallel with 6) uF

I have seen some caps of slightly different value, but they should be
close, just translate my 3 and 6 to what you actually have in what

The higher the capacitance the higher the stall torque, so the faster
the fan runs against the non-linear (square-law) torque vs. speed
characteristic of the blades. (remember I said it is always pretty
much stalled)

If you miswired the cap, then you may be getting 3 or 6 and 2 (3 in
*series*  with 6 uF which would result in low speeds.  This *is* the
case if any 2 out of 3 speeds seem to be the same. The replacement caps
are usually marked with what terminal is which, but originals often are
not.  I don't know if there is a standard color code, but manufacturers
are under no obligation to adhere to it even if there was.  If you are
totally lost, there are only 6 possible ways to connect the capacitor.
2 of these will give you all 3 speeds (but one in wrong order).  So if
you keep good notes (essential here) then you could try all possibilities
in 20 minutes or so...yes, you're probably working with hands over head,
what you wanted easy too?

OK, here is how to get it in 3 tries max:

Identify the "common"  capacitor lead  (connects to both 3, and 6 uF
sections, hopefully your replacement is marked). It is currently
connected to the wrong place, so swap it with one of the other cap
wires.  If you now have three speeds in the correct order, then your
done.  If you have three speeds in the wrong order, then leave common
wire alone, but swap other two. (correct order is: off-hi-med-lo

If you *didn't* have three different speeds following the first wire
swap, then swap that common wire with the one wire you haven't moved
yet. Now you should have three speeds, now correct the order as
described, if needed.

If you currently have three speeds, but all are too slow, then it is
likely that your fan needed a higher value capacitor. another
explanation might be that the old cap was getting leaky when it
warmed up after start, and letting the fan have extra current, thus
giving extra speed.

In my experience, the three speed types should run from just slow
enough to follow with the eye, to fast, fairly noisy, and making a
fair amount of wobble on the mounting.

Continuously variable speed types put a  fixed 9 or 10 uF cap in
series with the quadrature winding, and regulate voltage to both
windings via lamp-dimmer style triac circuit.

  7.43) Mike's notes on ceiling fan installation

(From: morris@cogent.net (Mike Morris)).

Depending on what wiring you have and what new wiring needs to be installed,
I would install 14/3 cables for all ceiling lights. That way, you will be able
to control ceiling fan and light from two separate switches.

Each time a new light has to be installed in our house, I make sure a 14/3
wire is installed. For three-way switches, I make it two 14/3 wires, even
if I don't install a ceiling fan now. A 14/3 wire is not that much more
expensive, and 10 years down the road, it might be useful.

The local high-end lights-and-fans shops have a handout that recommends
that wherever a ceiling fan is to go have the following wiring:

1. Neutral

2. Ground (if local code requires it, good idea anyway)

3. Switched hot for the lights

4. Switched hot for the fan

5. An extra wire - some brands need 2 hots for the fan, and
   if your brand doesn't need it, an extra conductor doesn't hurt.

Why two switched hots?

* It allows to run the lights and fan separately. 

* You should not use a lamp dimmer as a motor speed control.  While it may
  work to some extent, the motor will likely hum and long term reliability
  and safety are questionable.

Note that this does not preclude using a fan with built-in controls - unused 
wire is just that. And pulling in 5 conductors during construction or 
remodeling costs just a little more that pulling in 2 or 3.

The handout sheet also point out that adding a extra brace to the ceiling 
during any remodeling or new construction sized for a 100 pound dead weight
is a good idea - it can be as simple as a couple of feet of 2x6" lumber and a 
couple of sheet metal fasteners. A wobbling fan can cause fatigue in a light 
duty metal brace rapidly.  The extra cost is minimal, and it can prevent a fan 
from landing in the middle of the bed! 

  7.44) Air cleaners

Simple air cleaners are just a motor driven fan and a foam or other filter
material.  HEPA (High Efficiency Particulate Air) types use higher quality
filters and/or additional filters and sealed plenums to trap particles down to
a specified size (.3 micron).  A clogged (neglected) filter in any air cleaner
is probably the most likely problem to affect these simple devices.  Failure
of the fan to operate can be a result of any of the causes listed above in the
section: "Portable fans and blowers".

Electronic air cleaners include a high voltage low current power supply and
oppositely charged grids in the air flow. A failure of the solid state high
voltage generator can result in the unit blowing air but not removing dust and
particulate matter as it should.  A typical unit might have 7.5 to 10 KV at
100 uA maximum (short circuit current, probably less at full voltage).  Actual
current used is negligible under normal conditions.  This voltage is
significant but the current would be just barely detectable, if at all.

The modules are usually quite simple: a transistor or other type of switching
circuit driving a step-up transformer and possibly a diode-capacitor voltage
multiplier.  See the sections: "Electronic air cleaner high voltage module schematic" and "Auto air purifier schematic" for an example of a typical

Where there is no high voltage from such a device, check the following:

* Make sure power is actually getting to the high voltage portion of the unit.
  Test the wall socket and/or AC adapter or other power supply for proper
  voltage with a multimeter.

* Excessive dirt/dust/muck/moisture or physical damage or a misplaced paper
  clip may be shorting it out or resulting in arcing or corona (a strong aroma
  of ozone would be an indication of this).  With such a small available
  current (only uA) it doesn't take much for contamination to be a problem.
  Thoroughly clean and dry the unit and check for shorts (with a multimeter
  between the HV electrodes and case) and then test it again.  Your problems
  may be gone!

* If this doesn't help and the unit is not fully potted (in which case,
  replacement is the only option), check for shorted or open components,
  especially the power semiconductors.

  7.45) Electronic air cleaner high voltage module schematic

At least I assume this cute little circuit board is for an electronic air
cleaner or something similar (dust precipitator, positive/negative ion
generator, etc.)!  I received the unit (no markings) by mistake in the mail.
However, I did check to make sure it wasn't a bomb before applying power. :-)

This module produces both positive and negative outputs when connected to 115
VAC, 60 Hz line voltage.  Each is about 5 KV at up to around 5 uA.

The AC line powered driver and HV multiplier are shown in the two diagrams,

                   D1                                            T1  o
  H o--------------|>|----+---+--------------------+                +-----o A
                 1N4007   |   |        Sidac     __|__ SCR1     |:|(
                          |   |   R3  D2 100 V   _\_/_ T106B2   |:|(
  AC                  C1  |   +--/\/\---|>|      / |   200 V    |:|(
 Line      Power  .15 uF _|_     1.5K   |<|--+--'  |   4 A    o |:|( 350 ohms
          IL1 LED   250V ---                _|_    |  +-------+ |:|(
        +--|<|---+        |              C2 ---    |  |        )|:|(
        |   R1   |   R2   |        .0047 uF  |     |  | .1 ohm )|:|(
  N o---+--/\/\--+--/\/\--+                  +-----+--+        )|:|(
           470      3.9K  |                                +--+     +--+--o B
           1 W      2 W   |                                |    R4     |

The AC input is rectified by D1 and as it builds up past the threshold of the
sidac (D2, 100 V), SCR1 is triggered dumping a small energy storage capacitor
(C1) through the primary of the HV transformer, T1.  This generates a HV pulse
in the secondary.  In about .5 ms, the current drops low enough such that the
SCR turns off.  As long as the instantaneous input voltage remains above about
100 V, this sequence of events repeats producing a burst of 5 or 6 discharges
per cycle of the 60 Hz AC input separated by approximately 13 ms of dead time.

The LED (IL1) is a power-on indicator. :-)

The transformer was totally potted so I could not easily determine anything
about its construction other than its winding resistances and turns ratio
(about 1:100).

                                            A o
                                     C3       |
          R5     R6      D3   |   D4     D5   |  D6     R7       R8
  HV- o--/\/\---/\/\--+--|>|--+--|>|--+--|>|--+--|>|---/\/\--+--/\/\--o HV+
         10M    10M   |      C4       |                220K  |  10M
                      +------||-------+                      |
       D3-D6: 10 KV, 5 mA            _|_                    _|_
       C3,C4: 200 pF, 10 KV          --- C5                 --- C6
       C5,C6: 200 pF, 5 KV            |                      |
                                 B o--+----------------------+

The secondary side consists of a voltage tripler for the negative output
(HV-) and a simple rectifier for the positive output (HV+).  This asymmetry is
due to the nature of the unidirectional drive to the transformer primary.

From my measurements, this circuit produces a total of around 10 KV between
HV+ and HV-, at up to 5 uA.  The output voltages are roughly equal plus and
minus when referenced to point B.

I assume the module would also operate on DC (say, 110 to 150 V) with the
discharges repeating continuously at about 2 KHz.  Output current capability
would be about 5 times greater but at the same maximum (no load) voltage.
(However, with DC, if the SCR ever got stuck in an 'on' state, it would be
stuck there since there would be no AC zero crossings to force it off.  This
wouldn't be good!)

  7.46) Auto air purifier schematic

Well, maybe :-).  This thing is about the size of a hot-dog and plugs
into the cigarette lighter socket.  It produces a bit of ozone and who knows
what else.  Whether there is any effect on air quality (beneficial or
otherwise) or any other effects is questionable but it does contain a nice
little high voltage circuit.

                                                                   DL1 +-+ |
                                                   o  T1 +-------+-----|o|
  +12 o---+--------+----------+---------------------+ ||(        |     +-+ |
          |        |          |                D 30T )||(        | DL2 +-+
          |        |        -_|_ 4.7uF           #30 )||(        +-----|o| |
          |        |         --- 50V         +------+ ||( 3000T  |     +-+
          |       _|_ C2    + |              |        ||( #44    | DL3 +-+ |
          |       --- 470pF   +--------------|------+ ||(        +-----|o|
          |        |          |              | F 30T )||(        |     +-+ |
        +_|_ C1    |          |       D1     |   #36 )||(        | DL4 +-+
         --- 33uF  +----------|---+---|<|----|------+ ||(        +-----|o| |
        - |  16V   |          |   | 1N4002   |     o     +--+          +-+
          |        /          /   |        |/ C           o |              | 
          |     R1 \       R2 \   +--------|Q1  TIP41       +--------------+
          |     1K /     4.7K /            |\ E             |            Grid
          |        \          \              |              |
          |        |          |              |              |
  GND o---+--------+----------+--------------+--------------+

T1 is constructed on a 1/4" diameter ferrite core.  The D (Drive) and F
(Feedback) windings are wound bifilar style (interleaved) directly on the
core.  The O (Output) winding is wound on a nylon sleeve which slips over
the core and is split into 10 sections with an equal number of turns (100
each) with insulation in between them.

DL1 to DL4 look like neon light bulbs with a single electrode.  They glow like
neon light bulbs when the circuit is powered and seem to capacitively couple
the HV pulses to the grounded grid in such a way to generate ozone.  I don't
know if they are filled with special gas or are just weird neon light bulbs.

  7.47) Bug zappers

You know the type - a purplish light with an occasional (or constant) Zap!
Zap! Zap!  If you listen real closely, you may be able to hear the screams of
the unfortunate insects as well :-).

The high-tech versions consist of a high voltage low current power supply and
fluorescent (usually) lamp selected to attract undesirable flying creatures.
(Boring low-tech devices may just use a fan to direct the insects to a tray of
water from which they are too stupid to be able to excape!)

However, these devices are not selective and will obliterate friendly and
useful bugs as well as unwanted pests.

Here is a typical circuit:

         S1        R1         C1            C2            C1-C4: .5 uF, 400 V
  H o----o/ o--+--/\/\--------||---+--------||---------+  D1-D5: 1N4007
               |  25K        D1    |   D2        D3    |   D4
               |         +---|>|---+---|>|---+---|>|---+---|>|---+
              +-+        |        C3         |        C4         |
 AC Line      |o| FL1    +---+----||----+----+---+----)|----+----+--o + 
              +-+ Lamp   |   |    R3    |        |    R4    |        500 to
               |         |   +---/\/\---+        +---/\/\---+        600 V
               |   R2    |       10M                 10M             to grid
  N o----------+--/\/\---+------------------------------------------o -

This is just a line powered voltage quadrupler.  R1 and R2 provide current
limiting when the strike occurs (and should someone come in contact with the
grid).  The lamp, FL1, includes the fluorescent bulb, ballast, and starter (if
required).  Devices designed for jumbo size bugs (or small rodents) may use
slightly larger capacitors!

(From: Jan Panteltje (pante@pi.net)).

I have one, bought it very cheap: they are only $10 here :)

It comes with a 25 W blue lamp inside, with wires around it.  The lamp did not
last long, so I replaced that with a 7 W electronic fluorescent type, that now
just keeps going and going and going.  The bugs do not care, they just go for
the light.  Then they hit the wires.

Here, we have 230 V, in the lamp is a voltage doubler, with 2, 220 nF
capacitors, 2 silicon diodes, and a 10 K Ohm series resistor in the mains.
The whole thing cannot be touched by humans from outside.  The voltage between
the wires is something like 620 V.  If an insect shorts the wires, the 10K
limits the current until it is destroyed (the insect that is).  The insect
actually explodes, the 600 V cap discharges into it.

  7.48) Electric fences

Yes, I know, this isn't a common small appliance but....

(From: John Harvey (johnharvey@bigpond.com)).

Most DIY fence energizers use an automotive ignition coil and kits (generally
minus coil) are available in Australia and probably elsewhere.

Commercial units operate on the capacitor discharge principle and are fired at
a 1.2 second interval.  Voltage O/P needs to be around 5 to 8 KV (which will
drop under load).  The energy O/P (pulse duration) is determined by the
capacitor and 10 to 20 uF is about right for a small unit (up to 2km or so).
They must use a pulse grade capacitor (which has a high dV/dt) to be reliable.

  7.49) Appliance and light timers

There are two basic types: mechanical and electronic.

* Mechanical timers are simply a synchronous timing motor and gear reducer
  controlling a two prong (usually polarized) outlet.

  The most common problems relate to failure of the timing motor or gear
  train.  With time, the oil and grease used inside the timing motor may
  gum up.  Eventually, it gets so stiff that the motor stops - or more likely -
  doesn't start up after a power failure or the unit has been unplugged for
  a while.

  The cheap plastic gears may also break, chip, or loose teeth.

  Sometimes, disassembly, cleaning, and lubrication, will get the motor
  going - possibly for a long time.  However, replacement parts are rarely
  worth the cost compared to a complete new timer.

* Fully electronic timers use digital clock-type circuitry to control a triac
  or other solid state switching device.

  These may fail in the same way as other electronic controls such as dimmers.
  Most likely problems are that they are either stuck off or stuck on.  Aside
  from testing for bad connections or shorted or open components (with power
  OFF or disconnected!), repair is probably economical.  Assuming it can be
  opened non-destructively at all, check the triac and other parts in its
  vicinity.  The rest of the circuitry is probably in a proprietary chip - but
  these don't fail much.

  Also see the section: "Warnings about using compact fluorescent lamps on electronic timers".

  7.50) Warnings about using compact fluorescent lamps on electronic timers

You may have seen these warnings in the instructions or on the package of
electronic (not mechanical) timers and/or compact fluorescents.

There are two issues:

1. Providing the trickle current to operate the clock circuitry in the timer.

   Where a solid state timer is used to replace a normal switch, there is
   usually no connection to the Neutral so it must derive all its operating
   power from current through the load (though at a very low current level).

   The type of circuitry in a compact fluorescent with an electronic ballast
   (or other equipment with a switching power supply like a TV, some VCRs,
   computer, etc.) may result in this current being too low or erratic.  The
   result will be that the timer doesn't work properly but damage isn't that
   likely (but no guarantees).

   If it is installed with 3 wires (Hot, Neutral, Load), then this should not
   be a problem.

   In addition, interference (e.g., spikes) from the CF ballast may feed back
   into the electronic timer and this may either confuse or actually result
   in failure.

2. Damage to either or both of the devices dues to incompatibility.

   The solid state switching device - usually a triac - in the timer unit
   may be blown by voltage spikes or current surges when the power goes on or
   off into an inductive or capacitive load like an electronic ballast (or
   normal magnetic ballast, for that matter.

In short, read and follow label directions!  Although a given combination may
actually work reliably for years even if it is not supposed to but you should
be able to find a pair for which this shouldn't be a problem.

  7.51) Wall thermostats

These can be divided into several classes depending on:

* Normal or setback (electronic or electromechanical).
* 24 VAC or self powered (thermopile or thermocouple) valve control.
* Heating, airconditioning, or combination.

It is not possible to cover all variations as that would require a
complete text in itself.  However, here is a summary of possible
problems and solutions.

Conventional thermostats usually use a bimetal strip or coil with a
set of exposed contacts or a mercury switch.  In general, these are
quite reliable since the load (a relay) is small and wear due to
electrical arcing is negligible.  On those with exposed contacts, dirt
or a sliver of something can prevent a proper connection so this is
one thing to check if operation is erratic.  The following description
assumes a single use system - heating or cooling - using 24 VAC control
which is not properly controlling the furnace or airconditioner.

1. Locate the switched terminals on the thermostat.  Jumper across them
   to see if the furnace or airconditioner switches on.  If it does, the
   problem is in the thermostat.  If nothing happens, there may be a
   problem in the load or its control circuits.  Cycle the temperature dial
   back and forth a few times to see if the contacts ever activate.  You
   should be able to see the contacts open and close (exposed or mercury)
   as well.

   (CAUTION: on an airconditioner, rapid cycling is bad and may result in
   tripped breakers or overload protectors so ideally, this should be done
   with the compressor breaker off).

2. Check for 24 VAC (most cases) across the switched circuit.  If this
   is not present, locate the control transformer and determine if it
   is working - it is powered and its output is live - you may have the
   main power switch off or it may be on a circuit with a blown fuse or
   tripped breaker.  I have seen cases where the heating system was on
   the same circuit as a sump pump and when this seized up, the fuse blew
   rendering the heating system inoperative.  Needless to say, this is not
   a recommended wiring practice.  The transformer may be bad if there is
   no output but it is powered.  Remove its output connections just to make
   sure there is no short circuit and measure on the transformer again.

3. If 24 VAC is present and jumpering across the terminals does nothing,
   the heater valve or relay or airconditioner relay may be bad or there
   is a problem elsewhere in the system.

4. Where jumpering the terminals turns on the system, the thermostat contacts
   may be malfunctioning due to dirt, corrosion, wear, or a bad connection.
   For a setback unit, the setback mechanism may be defective.  Test and/or
   replace any batteries and double check the programming as well.  On
   those with motor driven timers operating off of AC, this power may be

5. Where jumpering the terminals does not activate the system, check the 
   load.  For a simple heating system, this will be a relay or valve.  Try
   to listen for the click of the relay or valve.  If there is none, its
   coil may be open though in this case there will be no voltage across the
   thermostat contacts but the 24 V transformer will be live.  If you
   can locate the relay or valve itself, check its coil with an ohmmeter.

6. If the previous tests are ok, there may be bad connections in the wiring.

Type of control - most systems use a 24 VAC circuit for control.  However,
some use low voltage self powered circuits that require special compatible
(sometimes called thermopile or thermocouple) thermostats with low resistance
contacts and no electronics directly in series with the control wires.
Erratic or improper control may result from using the wrong type.

Setback thermostats - these may be controlled electromechanically by a
timer mechanism which alters the position of the contacts or selects
an alternate set.  Newer models are fully electronic and anything beyond
obvious bad connections or wiring, or dead batteries is probably not
easily repaired.  However, eliminate external problems first - some of
these may need an additional unswitched 24 VAC or 115 VAC to function
and this might be missing.

Heat anticipators - in order to reduce the temperature swings of the heated
space, there is usually a small heating element built into the thermostat
which provides some more immediate feedback to the sensor than would be
possible simply waiting for the furnace to heat the air or radiators.  If
this coil is defective or its setting is misadjusted, then erratic or much
wider than normal temperature swings are possible.  There will usually be
instructions for properly setting the heat anticipator with the thermostat
or furnace.

Units that control both heating and airconditioning are more complex and
will have additional switches and contacts but operate in a similar manner
and are subject to similar ailments.

  7.52) Electric space heaters

All types have one thing in common - they are nearly 100% efficient which
means that just about every watt of power utilized is turned into heat.
The remainder is used for any built in fans or the wasted light produced
by glowing elements or quart lamps.

However, this does not mean that these are the most economical heating
devices.  Heat pumps based on refrigeration technology can be much less
costly to run since they can have coefficients of performance - the ratio
of heat output to energy input - of 3 or more to 1.  Thus, they are in
effect, 300% or more efficient.  Note that this does not violate any
conservation of energy principles as these simply move heat from one place
to another - the outdoors is being cooled off at the same time.

Space heaters come in 3 common varieties:

* Radiant - heating element with polished reflector.
* Convection - heating element and small fan to circulate heated air.
* Oil filled radiator - heating element heats oil and metal fins.

Problems with space heaters are nearly always related to bad heating elements,
problems with the thermostat, interlock switches, or fan (if any), or bad
connections.  Blown fuses or tripped circuit breakers are very common with
these appliances as they are heavy loads - often the maximum that can safely
be plugged into a 15 A outlet - and thus overloads are practically assured
if **anything** else is used on the same circuit.  Since we rarely keep
track of exactly what outlets are on any given circuit, accidentally using
other devices at the same time are likely since the same circuit may feed
outlets in more than one room - and sometimes some pretty unlikely places.

  7.53) Radiant space heaters

These use a coiled NiChrome, Calrod(tm), or quartz lamp heating element.
There is no fan.  A polished reflector directs the infra red heat energy
out into the room.  Radiant space heaters are good for spot heating of
people or things.  They do not heat the air except by convection from the
heated surfaces.

Of course, first check that the outlet is live.

As with other heating appliances, the most likely problems are with burned
out heating elements; defective on/off switches, thermostats, or safety
interlock or tip-over switches, bad cord or plug, or bad wiring connections.
Your continuity checker or ohmmeter will quickly be able to identify which
of these are the problem.

Warning: do not be tempted to bypass any interlock or tip-over switches
should they prove defective.  They serve a very important fire and personal
safety function.  Never, ever cover the heater in any way as a serious fire
hazard will result.

  7.54) Convection space heaters

A small fan blows air over or through a heating element.  This may be a
NiChrome coil, Calrod(tm) element, or ceramic thermistor.  This type
is probably the most popular since it can quickly heat a small area.
The ceramic variety are considered safer than the others (of this type)
since they are supposed to operate at a lower surface temperature.

In addition to the problems covered in the section above: "Radiant space heaters", the fan can also become sluggish or seize up due to
gummed up lubrication (as well as other fan-motor problems).  Since
it is running in a high temperature environment, disassembly, cleaning,
and lubrication may be needed periodically despite what the manufacturer
may say about permanently lubricated parts.

  7.55) Oil filled electric radiators

These are also considered convection heaters but they do not have any fan.

The typical unit consists of a pair of heating elements providing 600, 900,
or 1500 Watts depending on which are switched on.  A simple bimetal
adjustable thermostat is used for temperature control.  The heating
elements are fully submerged and sealed inside an oil filled metal finned
replica of an old style radiator.  The whole affair is mounted on wheels
as it is quite heavy.

Common problems with these have been the pair of power switches which tend
to fail resulting in no or erratic operation.  Note: if your heater is a
Delongi, there has been a free (well $5 S&H) upgrade to replace the failure
prone power switches and thermostat on some common models.

The heating elements are replaceable (as a set).  Since they are immersed in
the oil, you MUST have the radiator on its end with the terminals straight up
while changing them or else there will be a mess.  Replacement will be worth
the cost and effort only if you require the high settings as it is unlikely
for both elements to fail.  If testing reveals an open element, you will just
not have the heat ranges that use it.  If an element shorts to the case, it
must be disconnected to prevent a shock hazard though the other one can still
be safely used.  Parts should be available.

  7.56) So, what about the Pelonus Disk furnace?

(From: Kirk Kerekes (kkereke@iamerica.net)).

It is a portable electric heater, using high-power thermisters as the heating
elements. This technology was originally developed by TDK a few decades ago.
The premise is that the power thermisters will automatically control the
heating element temperature (the thermister), so that if the air flow is
blocked, the heater won't cook. The manufacturers make efficiency claims, but
these seem to be bogus.  (All space heaters are nearly 100% efficient.  See
the section: "Electric space heaters" --- sam.)

I have a bathroom version of this device, and it works.

  7.57) Electric pencil sharpeners

AC powered pencil sharpeners consist of a small shaded pole induction
motor, pencil sense switch, and some gears and cutter wheels.  Aside
from pencil shavings crudding up the works - which can be cleaned - the
most common failure is of the cheap plastic gears.  These can be easily
be replaced if you can get them - the original manufacturer is likely the
only source.  The switch contacts may become dirty or level/bar may become
misaligned or worn.  Some clever repositioning or the addition of a shim
may help in these cases.

Battery operated pencil sharpeners use a small DC motor for power.  These
tend to be whimpier than their AC counterparts but all other comments
apply.  Always try a fresh set of batteries first.

  7.58) Blenders

A blender really is just a high speed motor mounted inside a base.  Units
with 324 speeds accomplish (this more or less useless marketing gimmick)
through a combination of diodes, resistors, and multiple windings on the
motor.  Without addressing the ultimate utility of thousands of speeds,
problems with these units are more likely to be in the motor itself - open
or shorted windings, or bad bearings.  However, the selector switches and
electrical parts can fail as well.

The motors are typically of the series wound universal type.  These have
carbon brushes which are prone to wear.  However, given the relatively short
total usage of a blender, this is not usually a problem.

Disconnecting (and labeling!) connections one at a time may permit the
source of a problem to be localized.  Diodes can be tested with a multimeter
(they should read open in one and only one direction) and resistors checked
as well.  Shorts in a motor with multiple taps on its windings may be
difficult to identify or locate.  Shorted windings can result in overheating,
incorrect speeds, or even a blender that runs with the power switch supposedly
in the off position as the wiring is sometimes sort of strange!

Bad bearings will result in any number of mechanical problems including
excessive or spine tingling noise, vibration, a seized rotor or very sluggish
rotation.  Sometimes, disassembly, cleaning, and oiling will be effective
but since these rotate at high speed, don't count on it.  Unfortunately,
cheap bronze bushings are often used instead of ball bearings.  However,
substituting a set from another similar unit might work since it is usually
the bronze bushing and not the motor shaft that fails.

The most sophisticated units will have a variable speed control - similar
to a light dimmer.  If this goes bad - the blender always runs at full
speed - then the active element (triac) has probably blown.  Replacement
is possible and the part types should be readily available.

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Written by Samuel M. Goldwasser. | [mailto]. The most recent version is available on the WWW server http://www.repairfaq.org/ [Copyright] [Disclaimer]