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


  6.8) About mercury wall switches

The types of mercury switches used for wall switches are quite clever and
provide in effect a snap action (called hysteresis) due to their construction
and the surface tension of the liquid mercury itself.  This despite the fact
that the motion of the toggle lever is totally smooth and silent.  It is not
possible to put the lever in such a position that there could be marginal
contact and random on-off cycles.  The mercury capsule inside such a switch
consists of a metallic shell with an insulating (glass or ceramic) spacer in
between the two halves.  Connection to the switch's wiring is made via sliding
contacts to the metal portion of the capsule.  There is a small hole toward
one side in the spacer.  Rotating the capsule results in the mercury flowing
through the hole to make contact:

* In the off position, the hole is above the level of the liquid mercury.

* In the on position, the hole is below the level of the liquid mercury.

* When turning the switch on, the hole is rotated below the surface and as
  soon as the mercury touches, surface tension quickly pulls it together.
  There is no 'contact bounce'.

* When turning the switch off, the mercury pulls apart as the capsule is
  rotated to raise the hole.  Eventually, surface tension is not sufficient to
  hold the two globs of mercury together and they part suddenly.

Problems are rare with these mercury switches.  In fact, GE mercury switches
used to carry a *50* year warranty!  I don't know if they still do.

In principle, these are also the safest type of switch since any sparking or
arcing takes place inside the sealed mercury capsule.  However, the contact
between the screw terminals and the capsule are via sliding contacts (the
capsule is press fit between the metal strips to which the screws are attached)
and with time, these can become dirty, worn, or loose.  For this reason, some
electricians do not like mercury switches, particularly for high current

  6.9) Relays - electrically activated switches for power or control

Relays are switches that are activated by an electrical signal rather
than a button or toggle.  They are used to switch power (as in an central
air conditioning system) or control signals (as in a telephone or modem).

* The most common relays are electromechanical - an electromagnet is used
  to move a set of contacts like those in a regular switch.

* Solid state relays have no moving parts.  They use components like thyristors
  or transistors to do the switching.

For more information on relays, see the document: "Notes on the Troubleshooting
and Repair of Audio Equipment and other Miscellaneous Stuff".

  6.10) Contact configurations

The arrangement of contacts on a switch is often abbreviated mPnT where:

* 'm' identifies the number of separate sets of contacts.

* 'P' stands for Poles or separate sets of contacts.

* 'n' identifies the number of contact positions.

* 'T' stands for Throw which means the number of contact positions.

In addition, you may see:

* NC (Normally Closed and NO (Normally Open) may be used to designate terminals
  when the switch is in the off or deactivated state.  This applies to power
  switches where OFF would be down or released and ON would be up or pushed in.
  It also applies to momentary pushbutton switches and relays.

* MBB (Make Before Break) and BBM (Break Before Make) designate how the
  connections behave as the switch is thrown.  Most switches found in small
  appliances will be of the BBM variety.

This also applies to relays except that the contact switching is activated
by an electrical signal rather than a finger.

The most common types are:

* SPST - Single Pole Single Throw.  Terminal (A) is connected to terminal (B)
  when the switch is on:

        A ______/ _______ B

  This is the normal light or power switch.  For electrical (house) wiring,
  it may be called a '2-way' switch.

* DPST - Double Pole Single Throw.  Terminal (A) is connected to terminal (B)
  and terminal (C) is connected to terminal (D) when the switch is on:

        A ______/ _______ B
        C ______/ _______ D

  This is often used as a power switch where both wires of the AC line are
  switched instead of just the Hot wire.

* SPDT - Single Pole Double Throw.  A common terminal (C) is connected to
  either of two other terminals:

                 _______ NC
       C ______/
                 _______ NO

  This is the same configuration as what is known as a '3-way' switch for
  electrical (house) wiring.  Two of these are used to control a fixture
  from separate locations.
* DPDT - Double Pole Double Throw.  Essentially 2 SPDT switches operated by
  a single button, rocker, toggle, or lever:

                 _______ NC1
      C1 ______/
               : _______ NO1
               : _______ NC2
      C2 ______/
                 _______ NO2

* SP3T, SP4T, etc. - Single Pole selector switch.  A common terminal (C) is
  connected to one of n contacts depending on position.  An SP5T switch is
  shown below:
               _________ 1
                 _______ 2
       C ______/  ______ 3
                 _______ 4
               _________ 5

  6.11) Electrical overload protection devices - fuses and circuit breakers

The purpose of fuses and circuit breakers is to protect both the wiring
from heating and possible fire due to a short circuit or severe overload
and to prevent damage to the equipment due to excess current resulting
from a failed component or improper use (using a normal carpet vacuum
to clear a flooded basement).

Fuses use a fine wire or strip (called the element) made from a metal which
has enough resistance (more than for copper usually) to be heated by current
flow and which melts at a relatively low well defined temperature.  When the
rated current is exceeded, this element heats up enough to melt (or vaporize).
How quickly this happens depends on the extent of the overload and the type
of fuse.

Fuses found in consumer electronic equipment are usually cartridge type -
1-1/4" mm x 1/4" or 20 mm x 5 mm, pico(tm) fuses that look like green 1/4 W
resistors, or other miniature varieties.  Typical circuit board markings are
F or PR.

Circuit breakers may be thermal, magnetic, or a combination of the two.
Small (push button) circuit breakers for appliances are nearly always
thermal - metal heats up due to current flow and breaks the circuit
when its temperature exceeds a set value.  The mechanism is often the
bending action of a bimetal strip or disc - similar to the operation of
a thermostat.  Flip type circuit breakers are normally magnetic.  An
electromagnet pulls on a lever held from tripping by a calibrated spring.
These are not usually common in consumer equipment (but are used at the
electrical service panel).

At just over the rated current, it may take minutes to break the circuit.
At 10 times rated current, the fuse may blow or circuit breaker may open
in milliseconds.

The response time of a 'normal' or 'rapid action' fuse or circuit breaker
depends on the instantaneous value of the overcurrent.

A 'slow blow' or 'delayed action' fuse or circuit breaker allows instantaneous
overload (such as normal motor starting) but will interrupt the circuit
quickly for significant extended overloads or short circuits.  A large thermal
mass delays the temperature rise so that momentary overloads are ignored.  The
magnetic type breaker adds a viscous damping fluid to slow down the movement
of the tripping mechanism.

Common problems: fuses and circuit breakers occasionally fail for no reason
or simply blow or trip due to a temporary condition such as a power surge.
However, most of the time, there is usually some other fault with the
appliance which will require attention like a bad motor or shorted wire.

Testing: Fuses and circuit breakers can be tested for failure with a
continuity checker or multimeter on the low ohms scale.  A fuse that tests
open is blown and must be replaced (generally, once the circuit problem is
found and repaired.)  Of course, if the fuse element is visible, a blown
fuse is usually easy to identify without any test equipment.  A circuit
breaker that tests open or erratic after the reset button is pressed, will
need replacement as well.

  6.12) Fuse postmortem

Quite a bit can be inferred from the appearance of a blown fuse if the
inside is visible as is the case with a glass cartridge type.  One
advantage to the use of fuses is that this diagnostic information is
often available!

* A fuse which has an element that looks intact but tests open may have just
  become tired with age.  Even if the fuse does not blow, continuous cycling
  at currents approaching its rating or instantaneous overloads results in
  repeated heating and cooling of the fuse element.  It is quite common for
  the fuse to eventually fail when no actual fault is present.

* A fuse where the element is broken in a single or multiple locations blew
  due to an overload.  The current was probably more than twice the fuse's
  rating but not a dead short.

* A fuse with a blackened or silvered discoloration on the glass where the
  entire element is likely vaporized blew due to a short circuit.

This information can be of use in directly further troubleshooting.

  6.13) Fuse or circuit breaker replacement

As noted, sometimes a fuse will blow for no good reason.  Replace fuse,
end of story.  In this situation, or after the problem is found, what are
the rules of safe fuse replacement?  It is inconvenient, to say the least,
to have to wait a week until the proper fuse arrives or to tromp out to
Radio Shack in the middle of the night.

Even with circuit breakers, a short circuit may so damage the contacts or
totally melt the device that replacement will be needed.

Five major parameters characterizes a fuse or circuit breaker:

1. Current rating - this should not be exceeded (you have heard about not
   putting pennies in fuse boxes, right?)  (The one exception to this rule
   is if all other testing fails to reveal which component caused the fuse to
   blow in the first place.  Then, and only then, putting a larger fuse in
   or jumpering across the fuse  **just for testing** will allow the faulty
   component to identify itself by smoking or blowing its top!)  A smaller
   current rating can safely be used but depending on how close the original
   rating was to the actual current, this may blow immediately.

2. Voltage rating - this is the maximum safe working voltage of the circuit
   (including any inductive spikes) which the device will safety interrupt.
   Thus, you may see fuses where the elements look like [|------|] versus
   [|==--==|].  It is safe to use a replacement with an equal or high voltage

3. AC versus DC - fuses rated for AC and DC may not be the same.  For a given
   voltage, a shorter gap can be used to reliably interrupt an AC circuit
   since the voltage passes through zero 120 (100) times a second.  For
   example, a fuse rated 32 VDC may look similar to one rated for 125 VAC.

4. Type - normal, fast blow, slow blow, etc.  It is safe to substitute
   a fuse or circuit breaker with a faster response characteristic but
   there may be consistent or occasional failure mostly during power-on.
   The opposite should be avoided as it risks damage to the equipment
   as semiconductors tend to die quite quickly.

5. Mounting - it is usually quite easy to obtain an identical replacement.
   However, as long as the other specifications are met, soldering a normal
   1-1/4" (3AG) fuse across a 20 mm fuse is perfectly fine, for example.
   Sometimes, fuses are soldered directly into an appliance.

  6.14) Thermal protection devices - thermal fuses and thermal switches

These devices protect against excessive temperature due to either a fault
in the appliance (locked motor overheating), improper use (blow dryer air

There are three typical types:

1. Thermal fuses.  This is similar to an electrical fuse but is designed
   to break the circuit at a specific temperature.  These are often found
   in heating appliances like slow cookers or coffee percolators or buried
   under the outer covering of motor windings or transformers.  Some also
   have an electrical fuse rating as well.  Like electrical fuses, these
   are one-time only parts.  A replacement that meets both the thermal and
   electrical rating (if any) is required.

   CAUTION: When replacing a thermal fuse, DO NOT SOLDER it if at all
   possible.  If the device gets too hot, it may fail immediately or be
   weakened.  Crimp or screw connections are preferred.  If you must solder,
   use a good heat sink (e.g., wet paper towels, little C-clamps) on the leads
   between the thermal fuse and the soldering iron, and work quickly!

2. Thermal switches or thermal protectors (strip type).  These use a strip of
   bimetal similar to that used in a thermostat.  Changes in temperature
   cause the strip to bend and control a set of contacts - usually to
   to break a circuit if the set temperature is exceeded.  Commonly found
   in blow dryers and other heating appliances with a fixed selection of
   heat settings.  They may also be found as backup protection in addition
   to adjustable thermostats.

3. Thermal switches or thermal protectors (disk).  These use a disk of bimetal
   rather than a strip as in most thermostats.  The disk is formed slightly
   concave and pops to the opposite shape when a set temperature is exceeded.
   This activates a set of contacts to break (usually) a circuit if the rated
   temperature is exceeded.  They may also be found as backup protection in
   addition to adjustable thermostats.  A typical thermal switch is a small
   cylindrical device (i.e., 3/4" diameter) with a pair of terminals and a
   flange that is screwed to the surface whose temperature is to be monitored.

In some applications, device types (2) and (3) may be used as the primary
temperature regulating controls where adjustment is not needed.

  6.15) More on thermal fuses

(From: Paul Grohe (grohe@galaxy.nsc.com)).

The following is From Microtemps' literature (`95 EEM Vol.B p1388):

"The active trigger mechanism of the thermal cutoff (TCO) is an electrically
 non-conductive pellet. Under normal operating temperatures, the solid pellet
 holds spring loaded contacts closed. When a pre-determined temperature is
 reached, the pellet melts, allowing the barrel spring to relax. The trip
 spring then slides the contact away from the lead and the circuit is opened.
 Once TCO opens a circuit, the circuit will remain open until the TCO is

Be very careful in soldering these. If the leads are allowed to get too hot, it
may "weaken" the TCO, causing it to fail prematurely. Use a pair of needle-nose
pliers as heat sinks as you solder it.

I have replaced a few of these in halogen desk lamp transformers.  The
transformers showed no signs of overheat or overload. But once I got it
apart, the TCO's leads had large solder blobs on them, which indicated
that the ladies that assembled the transformers must have overheated the
cutouts leads when they soldered them.

The NTE replacement package also comes with little crimp-rings, for high-temp
environments where solder could melt or weaken (or to avoid the possibility
of soldering causing damage as described above --- sam).

  6.16) Controls 1 - adjustable thermostats and humidistats

Thermostats are use to regulate the temperature in heating or cooling type 
appliances.  Common uses include heaters, airconditioners, refrigerators,
freezers, hair dryers and blow dryers, toaster ovens and broilers, waffle
irons, etc.  These are distinguished from the thermal switches discussed
above in that they usually allow a variable temperature setting.

Four types are typically found in appliances.  The first three of these
are totally mechanically controlled:

1. Bimetal strip.  When two metals with different coefficients of thermal
   expansion are sandwiched together (possibly by explosive welding), the
   strip will tend to bend as the temperature changes.  For example, if the
   temperature rises, it will curve towards the side with the metal of lower
   coefficient of expansion.

   In a thermostat, the bimetal strip operates a set of contacts which make
   or break a circuit depending on temperature.  In some cases the strip's
   shape or an additional mechanism adds 'hysteresis' to the thermostat's
   characteristics (see the section: "What is hysteresis?").

2. Bimetal disk.  This is similar to (1) but the bimetal element is in the
   shape of a concave disk.  These are not common in adjustable thermostats
   but are the usual element in an overtemperature switch (see the section:
   "Thermal protection devices - thermal fuses and thermal switches").

3. Fluid operated bellows.  These are not that common in small appliances
   but often found in refrigerators, airconditioners, baseboard heaters,
   and so forth.  An expanding fluid (alcohol is common) operates a bellows
   which is coupled to a set of movable contacts.  As with (1) and (2) above,
   hysteresis may be provided by a spring mechanism.

Other variations on these basic themes are possible but (1)-(3) cover the
vast majority of common designs.

Testing of mechanical thermostats: examine for visible damage to the
contacts.  Use a continuity checker or ohmmeter to confirm reliable
operation as the knob or slider is moved from end to end if it will
switch at room temperature.  Gently press on the mechanism to get the
contacts to switch if this is not possible.  Use an oven on low or a
refrigerator or freezer if needed to confirm proper switching based
on temperature.

4. Electronic thermostats.  These typically use a temperature variable
   resistance (thermistor) driving some kind of amplifier or logic circuit
   which then controls a conventional or solid state relay or thyristor.
Testing of electronic thermostats:  This would require a schematic to
understand exactly what they are intended to do.  If a relay is used, then
the output contacts could perhaps be identified and tested.  However,
substitution is probably the best approach is one of these is suspected
of being defective.

Humidistats, as their name implies, are used to sense relative humidity
in humidifiers and dehumidifiers.  Their sensing material is something
that looks kind of like cellophane or the stuff that is used for sausage
casings.  It contracts and expands based on the moisture content of the
air around it.  These are somewhat fragile so if rotating the control
knob on a humidifier or dehumidifier does not result in the normal 'click',
this material may have been damaged or broken.

Testing of mechanical humidistats: examine for visible damage to the
contacts.  Use a continuity checker or ohmmeter to confirm reliable
operation as the knob or slider is moved from end to end.  Gently press
on the mechanism to get the contacts to switch if this is not possible.
Gently exhale across the sensing strip to confirm that the switching point

  6.17) What is hysteresis?

An intuitive explanation of hysteresis is that it is a property of a
system where the system wants to remain in the state that it is in -
it has memory.

Examples of systems with hysteresis:

* Thermostats - without hysteresis your heater would be constantly
  switching on and off as the temperature changed.  A working thermostat
  has a few degrees of hysteresis.  As the temperature gradually increases,
  at some point the thermostat switches off.  However, the temperature then
  needs to drop a few degrees for it to switch on again.

* Toggle switches - the click of a toggle switch provides hysteresis to
  assure that small vibrations, for example, will not accidentally flip
  the switch.

Examples of systems which ideally have little or no hysteresis:

* Audio amplifiers - input vs. output.

* Pendulums on frictionless bearings - force vs. position.

Hysteresis is usually added thermostats by the use of a spring mechanism
which causes the mechanism to want to be in either the open or closed
position but not in between.  Depending on the appliance, there may be
anywhere from 0 hysteresis (waffle iron) to 5-10 degrees F (space heater).
Sometimes, the thermal mass of the heated device or room provides the
hysteresis since any change to the temperature will not take place
instantaneously since the heating element is separated from the thermostat
by a mass of metal.  Therefore, some overshoot - which in effect performs the
same function as a hysteresis mechanism - will take place.

  6.18) Controls 2 - rheostats and potentiometers

These controls are usually operated by a knob or a slide adjustment and
consist of a stationary resistance element and a wiper that can be moved
to determine where on the fixed element it contacts.  In some cases, they
are not actually user controls but are for internal adjustments.  In other
cases, they are operated by the mechanism automatically and provide a means
of sensing position or controlling some aspect of the operation.

* Rheostats provide a resistance that can be varied.  Usually, the range is
  from 0 ohms to some maximum value like 250 ohms.  They are used to
  control things like speed and brightness just by varying the current
  directly, or via an electronic controller (see the section: "Electronic controllers - simple delay or microprocessor based").

                 B o-------------+
                 A o--------/\/\/\/\/\-----
                         250 ohm rheostat

  In the diagram above, the resistance changes smoothly from 0 to 250 ohms
  as the wiper moves from left to right.

  Very often, you will see the following wiring arrangement:

                 B o-------------+------+
                                 |      |
                                 V      |
                 A o--------/\/\/\/\/\--+
                        250 ohm rheostat

  Electrically, this is identical.  However, should the most common failure
  occur with the wiper breaking or becoming disconnected, the result will be
  maximum resistance rather than an open circuit.  Depending on the circuit,
  this may be preferred - or essential for safety reasons.

  Testing: Disconnect at least one of the terminals from the rest of the
  circuit and then measure with an ohmmeter on the appropriate scale. The
  resistance should change smoothly and consistently with no dead spots or

* Potentiometers are either operated by a knob or a slide adjustment and
  implement a variable resistance between two end terminals as shown below.
  This can be used to form a variable voltage divider.  A potentiometer (or
  'pot' for short) can be used like a rheostat by simply not connecting one
  end terminal.  These are most often used with electronic controllers.

                 B o-------------+
                 A o--------/\/\/\/\/\--------o C
                       1K ohm potentiometer

  In the diagram above, the resistance between A and B varies smoothly from
  0 to 1K ohms as the wiper moves from left to right.  At the same time, the
  resistance between B and C varies smoothly from 1K to 0 ohms.  For some
  applications, the change is non-linear - audio devices in particular so that
  the perceived effect is more uniform across the entire range.

  Testing: Disconnect at least two of the terminals from the rest of the
  circuit and then measure with an ohmmeter on the appropriate scale.
  The resistance should change smoothly and consistently with no dead
  spots or dips.  Try between each end and the wiper.  Check the resistance
  across the end terminals as well - it should be close to the stamped
  rating (if known).

Rheostats and potentiometers come in all sizes from miniature circuit board
mounted 'trimpots' to huge devices capable of handling high power loads.
The resistance element may be made of fine wire ('wirewound') or a carbon
composition material which is silkscreened or painted on.

  6.19) Interlocks - prevent operation with case or door open

Most of these are simple switches mechanically activated by the case or door.
Sometimes, optical or magnetic interlocks are used (rare on small appliances
but common on things like printers).  Line cords that are firmly attached to
the case and disconnect automatically when the case is removed are another
example of an interlock.  Interlocks may be designed to prevent injury during
normal operation (e.g.. food processor blades will not start when cover is
removed) or during servicing (remove AC power to internal circuits with case

1. Interlock switches.  Various kinds of small switches may be positioned
   in such a way that they disconnect power when a door is opened or cover
   is removed.  These may fail due to electrical problems like worn or
   dirty contacts or mechanical problems like a broken part used to activate
   the interlock.

   Testing: Use an ohmmeter or continuity checker on the switches.  The reading
   should either be 0 ohms or infinite ohms.  Anything in between or erratic
   behavior is indication of a bad switch or cord.

2. Attached cordset.  Should the case be opened, the cord goes with the case
   and therefore no power is present inside the appliance.  To get around
   this for servicing, a 'cheater cord' is needed or in many cases the
   original can be easily unfastened and used directly.

   Testing: Use an ohmmeter or continuity check to confirm that both wires
   of the cord are connected to both AC plug and appliance connector.  Wiggle
   the cord where it connects to the appliance and at the plug end as well to
   see if there might be broken wires inside.

  6.20) Light bulbs - incandescent and fluorescent

Small incandescent light bulbs are often used in appliances for interior
lighting or spot illumination.  The common 'appliance bulb' is simply
a 'ruggedized' 40 W incandescent light bulb in a clear glass envelope.
Other types are found in vacuum cleaners, microwave overs, makeup mirrors,
and so forth.

Testing: visual inspection will often reveal a burnt out incandescent light
bulb simply because the filament will be broken.  If this is not obvious,
use an ohmmeter - an infinite resistance means that the bulb is bad.

Small fluorescent lamps are often found in makeup mirrors, plant lights, and
battery powered lanterns.

Testing: The best test for a bad fluorescent bulb is to substitute a known
good one.  Unfortunately, there is no easy go-no go test for a fluorescent
lamp.  Other parts of the lamp or fixture (like the ballast or starter) could
also be bad.

See the sections on the appropriate lamp type for additional information.

  6.21) Indicators - incandescent or neon light bulbs or LEDs

Whereas lighting fixtures using incandescent or fluorescent bulbs are designed
to illuminate a room or small area, an indicator is simply there to let you
know that an appliance is on or in a specific mode.

There are three common types of electrical indicator lights:

1. Incandescent bulbs.  Just like their larger cousins, an incandescent
   indicator or pilot light has a filament that glows yellow or white hot
   when activated by a usually modest (1.5-28 V) source.  Flashlight
   bulbs are very similar but usually have some mechanical method of
   keeping the filament positioned reasonably accurately so that the
   light can be focussed by a reflector or lens.  Since the light spectrum
   of incandescent indicators is quite broad, filters can be used to obtain
   virtually any colored light.  Incandescent indicator lamps do burn out
   just like 100 W bulbs if run near their rated voltage.  However, driving
   these bulbs at reduced voltage can prolong their life almost indefinitely.

   Incandescent indicator lamps are often removable using a miniature screw,
   bayonet, or sliding type base.  Some are soldered in via wire leads.  Others
   look like cartridge fuses.

   Testing: Visual inspection will often reveal a burnt out incandescent light
   bulb simply because the filament will be broken.  If this is not obvious,
   use an ohmmeter - an infinite resistance is means that the bulb is bad.

2. Neon lamps.  These are very common as AC line power indicators because
   they are easy to operate directly from a high voltage requiring only a
   high value series resistor.

   They are nearly all the characteristic orange neon color although other
   colors are possible and there is a nice bright green variety with an
   internal phosphor coating that can actually provide some illumination
   as well.  While neon bulbs do not often burn out in the same sense as
   incandescent lamps, they do darken with age and may eventually cease to
   light reliably so flickering of old Neon bulbs is quite common.

   Some Neon bulbs come in a miniature bayonet base.  Most are soldered
   directly into the circuit via wire leads.

   Testing: Inspect for a blackened glass envelope.  Connect to AC line
   (careful - dangerous voltage) through a series 100K resistor.  If glow
   is weak or absent, Neon bulb is bad.

3. Light Emitting Diodes (LEDs).  LEDs come in a variety of colors - red,
   yellow, and green are very common; blue is just appearing.  These run
   on low voltage (1.7-3 V) and relatively low currents (1-20 mA).  Thus,
   they run cool and are easily controlled by low voltage logic circuits.
   LEDs have displaced incandescent lamps in virtually all electronic
   equipment indicators and many appliances.  Their lifetime easily exceeds
   that of any appliance so replacement is rarely needed.

   LEDs are almost always soldered directly into the circuit board since they
   rarely need replacement.

   Testing: Use a multimeter on the diode test scale.  An LED will have a
   forward voltage drop of between 1.7 and 3 V.  If 0 or open, the LED is
   bad.  However, note: some DMMs may not produce enough voltage on the diode
   test scale so the following is recommended:  Alternative: Use a 6 to 9 V DC
   supply in series with a 470 ohm resistor. LED should light if the supply's
   positive output is on the LED's anode.  If in doubt, try both ways,  If the
   LED does not light in either direction, it is bad.

  6.22) Heating elements - NiChrome coils or ribbon, Calrod, Quartz

All heating elements perform the same function: convert electricity into
heat.  In this they have one other characteristic in common: they are all
nearly 100% efficient.  The only electrical energy which does not result in
heat is the slight amount of light (usually red-orange) that is produced
by a hot element.

There are 3 basic types of heating elements.  Nearly every appliance on
the face of the planet will use one of these:

1. NiChrome coil or ribbon.  NiChrome is an alloy of Nickel and Chromium
   which has several nice properties for use in heating appliances - First,
   it has a modest resistance and is thus perfect for use in resistance
   heating elements.  It is easily worked, is ductile, and is easily formed
   into coils of any shape and size.  NiChrome has a relatively high melting
   point and will pretty much retain its original shape and most importantly,
   it does not oxidize or deteriorate in air at temperatures up through the
   orange-yellow heat range.

   NiChrome coils are used in many appliances including toasters, convection
   heaters, blow-dryers, waffle irons and clothes dryers.

   The main disadvantage for our purposes is that it is usually not possible
   to solder this material due to the heating nature of its application.
   Therefore, mechanical - crimp or screw must be used to join NiChrome
   wire or ribbon to another wire or terminal.  The technique used in the
   original construction is may be spot welding which is quick and reliable
   but generally beyond our capabilities.

   Testing: Visual inspection should reveal any broken coil or ribbon.  If
   inspection is difficult, use a multimeter on the low ohms scale.  Check
   for both shorts to the metal chassis as well as an open element (infinite

2. Calrod(tm) enclosed element.  This encloses a fine coiled NiChrome wires
   in a ceramic filler-binder inside a tough metal overcoat in the form of a
   shaped rod with thick wire leads or screw or plug-in terminals.

   These are found in toaster oven/broilers, hot plates, coffee makers,
   crock pots and slow cookers, electric range surface elements, conventional
   and convection ovens and broilers.

   Testing: When these fail, it is often spectacular as there is a good
   chance that the internal NiChrome element will short to the outer casing,
   short out, and melt.  If there is no visible damage but the element does
   not work, a quick check with an ohmmeter should reveal an open element or
   one that is shorted to the outer casing.

3. Quarts incandescent tube.  These are essentially tubular high power
   incandescent lamps, usually made with a quartz envelope and thus their

   These are found in various kinds of radiant heaters.  By running a less
   than maximum power - more orange heat - the peak radiation is in the
   infra-red rather than visible range.

   Testing: Look for a broken filament.  Test with an ohmmeter just like
   an incandescent light bulb.

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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]