Hi
Controls 1 - adjustable thermostats and humidistat
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.
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.
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-------------+
|
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.
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