Contents:
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 loads.
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".
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
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.
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.
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 rating. 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.
These devices protect against excessive temperature due to either a fault in the appliance (locked motor overheating), improper use (blow dryer air blocked). 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.
(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 replaced....." 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).
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 changes.
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.
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-------------+
|
V
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
dips.
* 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-------------+
|
V
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.
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 removed). 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.
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.
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.
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 ohms). 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 name. 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.Go to [Next] segment
Go to [Table 'O Contents]