Temperature and Light Sensors a)Thermocouple b)Thermistor c)Resistance-Temperature Devices (RTDs) d)Solid State (PN Junction Diode) Thermometry e)Fiber.

Temperature and Light Sensors

a) Thermocoupleb) Thermistorc) Resistance-Temperature Devices (RTDs)d) Solid State (PN Junction Diode) Thermometrye) Fiber Optic GaAs Bandgap Temperature Sensorf) Blackbody Radiation and IR Non-contact Thermometry (Pyrometer)g) Thermopile Radiation Detectorh) Optical Instruments: Spectrophotometersi) Snell’s Law and Optical Fiberj) Light Sources (Incandescent, LED, Laser)k) Photon Detectors

1. CdS Photocell2. Photodiode3. Phototransistor4. Photomultiplier

l) Wavelength Selectors (Optical Filters)

Practical Thermocouples

Thermocouples are manufactured in many different configurations for multitude of applications. Diameters, lengths, sheath material, lead lengths and sensor material are just a few of the variables that go into determining the style of thermocouple when manufactured. The main determining factors of what type thermocouple needs to be used in a application is temperature, environment, response time and accuracy. The junction of the thermocouple can be grounded, ungrounded or exposed. The thermocouple junction may be of many different styles, from tube type to a washer configuration. The lead length may vary depending on the distance of the temperature controller from the thermocouple sensor. The metal that the sensor is constructed of determines the type of thermocouple manufactured.

Applications:Plastic injection molding machineryFood processing equipmentEngine and turbine exhaust gasSemiconductor processingHeat treating and metals processingMedical equipmentAerospace industriesPackaging equipmentTest stands

Use of Thermocouple in Gas Furnace

Honeywell Standing Pilot Valve Body© Honeywell 2006

Furnace Thermocouple (actual wire junction is sheathed in protective copper tubing)

Standing Pilot Light (small gas flame that is always on --- used to light gas furnace). Thermocouple senses pilot light flame temperature. If pilot light goes out, thermocouple will close the pilot valve, not letting gas be sent to the furnace, thereby protecting homeowner from gas filling house!

Role Played by Thermocouple in Gas Furnace

• The thermocouple is the pilot's, well, co-pilot! It is the electronic device that senses if the pilot flame is hot enough to sustain burning the gas fuel from the burner.

• If the thermocouple thinks it's safe, then it keeps open the main gas valve located in the pilot assembly.

• If the thermocouple does not sense enough heat from the pilot flame (such as when the pilot is out), then the thermocouple shuts off the gas valve to the burners.

Typical “Standing Pilot” Gas Furnace

If thermocouple does not sense the heat coming from the flame of the pilot light, it shuts off the gas supply to the pilot flame AND to the main burner bed.

The Law of Intermediate Temperatures makes it possible to use the NIST thermocouple polynomials, which relate thermocouple voltage (x) in mV to temperature (T) in degrees C for each kind of thermocouple, assuming that one junction is held at 0 degrees C, and the other junction is at “T” degrees C.

Example: Here are the empirically obtained coefficients taken directly from the NIST document for the Type J (Iron/Constantan) thermocouple which hold over the temperature range of 0 - 760 degrees Centigrade, where one junction MUSTBE HELD at 0 degrees C using an “Ice Bath”, and the

other junction is held at temperature “T”,

mVFind x( ) 5.26785

c0 c1 x c2 x2 c3 x

3 c4 x4 c5 x

5 c6 x6 c7 x

7 100.0

Given

x 0MATHCAD needs and initial guess for the voltage

A. If T = 100 degrees C, use MAPLE to show that the measured voltage is V = 5.2679 mV

T c0 c1 x c2 x2 c3 x

3 c4 x4 c5 x

5 c6 x6 c7 x

7

The temperature T is in degrees Centigrade, and the voltage "x" is in millivolts are related by the following 7th degree polynomial:

c7 5.099890 1010c6 5.344285 10

8c5 3.585153 106c4 2.549687 10

4

c3 1.036969 102c2 2.001204 10

1c1 1.978425 101c0 0

• REMEMBER: to use the NIST polynomials, the reference junction MUST be held at 0 degrees C, which corresponds to an easily laboratory-duplicated standard temperature: the temperature of an ice bath at its “triple point” (gas/air/liquid).

• But it is not practical to carry an ice bath around in a portable temperature measuring instrument!

• Instead of an ice bath, the portable instrument has its reference junction tied to a “thermal mass” (heat sink) with a relatively long thermal time constant that assumes the average temperature of the surrounding environment (like a cave assumes the average yearly temperature of its environment.) Let’s call this thermal mass temperature “Tavg_env”.

• The Law of Intermediate Temperatures allows us to relate the voltage produced by a thermocouple whose junction temperatures are (Tx, Tavg_env) to the voltage produced by an identical thermocouple whose junction temperatures are (Tx, 0 deg C), which is the voltage predicted by the NIST polynomial equation.

TxTavg_env

Tavg_env0 Degrees C

(Ice Bath)

Tx0 Degrees C (Ice Bath)

= Emeas (Actual measured Voltage)

= Ecal (Calibration Voltage – may be calculated from NIST coefficients and Tavg_env)

E_corrected represents the voltage that WOULD HAVE BEEN MEASURED if the thermal block were replaced by the Ice Bath reference. This is the voltage that must be used (as “x” in millivolts) with the NIST polynomials to find the unknown temperature, Tx (in degrees C)

E_corrected = Emeas + Ecal

Practical Thermistors NTC => “Negative Temperatur Coefficient” => Resistance

goes DOWN with increasing temperature

The Resistance Temperature Detector (RTD)

RTD materials. Pt is the most popular, but also the most costly, because of its high resistivity, relatively high melting point, and because it is a “noble metal” (it is not willing to combine with other elements easily, so its resistance remains stable since it is not easily “contaminated”!)

Improved RTD measurement circuits: Minimizing inaccuracy caused by resistance of cables that connect the RTD element to the instrument.

RTD temperature sensors are used when accurate and repeatable temperature sensing is needed. They are manufactured from platinum and are available in a 2 or 3 wire configuration. As with thermocouples the RTD sensor is available in a large number of different configurations.

Applications:Air conditioning and refrigeration servicingFurnace servicingFood service processingMedical researchTextile productionPlastics processingPetrochemical processingMicroelectronicsAir, gas and liquid temperature measurement

PN Junction (Integrated Circuit) Linear Thermometry

National Semiconductor LM35 Linear IC Temperature Sensor

Audio Amplifier Application of LM35 Temperature Sensor (LM3886 is a 60 Watt IC Audio Power Amp IC)

Fiber Optic Temperature Sensor

Measures temperature inside body without danger of introducing electrical currents into body that could harm patient, nor can electrical currents during defibrillation harm temperature measuring electronics at ends the transmit and receive fibers.

See: Text Fig. 2.20, p. 75

Opsens’ OTG-A fiber optic temperature sensor offers the highest performances in the industry. The OTG-A sensor uses the well proven technique based on the temperature-dependent bandgap of GaAs crystal as the temperature transduction mechanism. Its small sensing GaAs crystal located at the tip of the optical fiber makes it convenient for tip measurement applications.Combined with Opsens’ GaAs signal conditioning technology and with the inherent advantages of fiber optic, the OTG-A delivers unprecedented repeatability and reliability in the most adverse conditions such as high level of EM, RF, MR and microwave field environments.The standard operating range of the OTG-A is from -40 °C to +250 °C. Higher temperature ranges up to 350 °C are available upon request.

KEY FEATURESSmall and robust designGood accuracy and outstanding repeatabilityEMI/RFI and microwave immuneIntrinsically safeOEM-type and custom version

APPLICATIONSEM, RF and microwave environmentsHigh voltage environmentsNuclear and hazardous environmentsMicrowave assisted chemistrySterilization applicationsIn-situ process controlRF and microwave drying applications

Non-Contact Infrared Thermometer (Pyrometry)

• Based upon Planck’s Law that relates temperature of a “blackbody” to the IR radiation that it emits.

• A blackbody is an object that absorbs all incident radiation, it usually has a rough surface.

• Radiation from any object may be found by multiplying the blackbody radiation by a “correction factor”, called the emissivity of the object “ε”, which varies between 1 (perfect blackbody) and 0 (perfectly reflective object).

• Rough objects (like the human skin) are a very nearly blackbodies (skin has an approximate emissivity of ε = 0.98)

Infrared Emissivity of common materials

Spectral radiant emittance vs. wavelength for a blackbody at 300K on left vertical axis & Percent of total power radiated on the right vertical axis.

Spectral transmission for several optical “window” materials

Spectral sensitivity of photon detectors and thermal detectors.

Inside a silicon radiation thermopile microsensor Thermocouple junctions are made from Phosphorus (N) Doped and Boron (P) Doped PolySilicon, resulting in a high Seebeck Coefficient. Cold junctions are located under the reflecting areas, which are in contact with the silicon wafer (heatsink), while the Hot junctions are located under the absorbing polyimide material, and are NOT in contact with the silicon wafer, since holes have been etched in the silicon (white areas below).

Hot-cold junction pairs are connected in series in a 2-dimensional array over the active area of the IR microsensor.

180 degree phase

difference if Tobject < To

V = K(T4-To4)

Bandpass Filter

Chopping blade used to make signal AC in order to avoid 1/f noise, which is largest near dc, and also to avoid the effects of dc amplifier drift. A bandpass filter (BPF) centered on the chopping frequency can be used to reject all noise that is not at the chopping frequency.

OmegaScope Handheld IR Thermometer

Built-In Patented Laser Sighting is Switchable Between Laser Dot or Circle Patterns •New Series Includes High Performance, General Purpose, Close Focus and High Resolution Models•Models Available with Temperature Ranges to 870°C (1600°F) •Emissivity Adjustable from 0.1 to 1.00 in 0.01 steps •Backlit LCD Display •Dual Digital Display Indicates Current with Min, Max, Average, or Difference Temperatures•°C/°F Selectable •1 mV/Degree Analog Output Standard •RS-232 Output Models Include FREE Data Logging Software•Audible and Visible Alarms •Integral Tripod Mount •Type K T/C Input Available •Temperature Data Storage Available •Electronic Trigger Lock •Last Temperature Recall•All Models Include Built-In Laser Sighting, Display and Lens Protective Bumpers, Wrist Strap, Soft Carrying Case, 4 Long Life Lithium Batteries, and User’s Manual with Emissivity Reference ChartReference Chart •Distance Measuring Option

Introduction to Infrared Pyrometry

Why should I use an infrared pyrometer to measure temperature in my application?

Infrared pyrometers allow users to measure temperature in applications where conventional sensors cannot be employed. Specifically, in cases dealing with moving objects (i.e., rollers,moving machinery, or a conveyer belt), or where non-contact measurements are required because of contamination or hazardous reasons (such as high voltage), where distances are too great, or where the temperatures to be measured are too high for thermocouples or other contact sensors.

What should I consider about my application when selectingan infrared pyrometer?

The critical considerations for any infrared pyrometer include field of view (target size and distance), type of surface being measured (emissivity considerations), spectral response (for atmospheric effects or transmission through surfaces), temperature range and mounting (handheld portable orfixed mount). Other considerations include response time, environment, mounting limitations, viewing port or window applications, and desired signal processing.

FIELD OF VIEW--- What is meant by Field of View, and why is it important?

The field of view is the angle of vision at which the instrument operates, and is determined by the optics of the unit. To obtain an accurate temperature reading, the target being measured should completely fill the field of view of theinstrument. Since the infrared device determines the average temperature of all surfaces within the field of view, if the background temperature is different from the object temperature, a measurement error can occur (figure 1).

IRt/c MONITORS TIRE TEMPERATURES FOR RACING PERFORMANCETire temperature is of critical concern in automotive racing for two reasons: the tire temperature directly affects its adhesion and its wear characteristics; and tire temperature patterns provide valuable information on the set-up and performance of the suspension. For example, excessive loading of a tire caused by out-of-tune suspension will cause that tire to become considerably warmer than the others.

The IRt/c is an ideal measuring device for on-board data acquisition, due to its small size, ruggedness, and low cost. It may be connected to standard thermocouple read-out systems. Installation should include connecting the shield to a suitable ground in order to avoid interference from the electrically harsh environment of a racing automobile. Mechanical installation should include attention to air flow patterns to minimize dirt building on the lens. The OS36-2 or OS36-5 are recommended due to their narrower field of view, thus allowing you to position it further away.

CONTROLLING WEB ROLLER TEMPERATURE

The IRt/c infrared thermocouples have quickly become the sensors of choice for monitoring and controlling both web and roller temperatures. Tips on accurate roller temperature measurement:

Uncoated Metal or Chrome Rolls – Shiny, uncoated metal rolls are difficult for any infrared sensor to properly sense the true temperature (the sensor will see too many environmental reflections). The solution to the problem is to simply: paint a small black stripe on an unused end of the roller. Aim the IRt/c sensor at the black paint stripe. It will then measure the temperature accurately and reliably regardless of changes in the surface conditions of the rest of the roller.

ASPHALT TEMPERATURE MONITORING

Asphalt properties are particularly sensitive to temperature, and it is important that the asphalt is applied at the correct temperature in order to perform to its specifications. Accordingly, temperature monitoring is a common requirement, but the thermocouples normally used have severe breakage problems due to the harsh abrasiveness of the material, and must constantly be replaced at high cost and interruption of production.

The IRt/c solves this problem directly, since the temperature is monitored without contact. The normal thermocouple controller can be used – simply calibrate offset if necessary. The OS36-2 and OS36-5 models are recommended due to their built-in air purge, which will keep the lens clean by preventing vapors from condensing on the lens. The OS36-2 can be mounted in the chute to view the asphalt through a small hole, while the OS36-5 can be mounted some distance away due to its narrow 5:1 field of view.

Optical Instrument: Spectrophotometer – measures optical absorbance (or transmittance) through a sample in the cuvette.

1 Focal Length

1 Focal Length

1 Focal Length

1 Focal Lengt

h

Any Length

Any Length

Convex Lens has a “focal length”, which is the length at which light rays diverging from a point source will be collimated (be made parallel). Conversely, the focal length may be thought of as the distance at which parallel rays incident on the lens converge to (are focused to) a point.

“f-number” of a lens = focal length / diameter

Blood Oximetry --- Based on the Absorptivities of Oxygenated Hemoglobin (Hb02) and Unoxygenated (Reduced) Hemoglobin (Hb).

)log()(water

Hgbx

I

ItyAbsorptivi

Isosbestic Wavelength λ2= 805 nm

Wavelength of maximum Absorptivity difference

λ1= 660 nm

Hb02

Hb

Beer’s Law

)log()(water

blood

I

IA Absorbtivity “A” can be measured at

different wavelengths, where “I” is the detector output.

)()()()( 2022 WLaCCWLaA r

Cr = 1-Co

Only Co is unknown, everything else is known!

805 nm(or 910 nm)

660 nm

Principles of Pulse Oximetry Technology:

The principle of pulse oximetry is based on the red and infrared light absorption characteristics of oxygenated and deoxygenated hemoglobin. Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through. Deoxygenated (or reduced) hemoglobin absorbs more red light and allows more infrared light to pass through. Red light is in the 600-750 nm wavelength light band. Infrared light is in the 850-1000 nm wavelength light band.

Pulse oximetry uses a light emitter with red and infrared LEDs that shines through a reasonably translucent site with good blood flow. Typical adult/pediatric sites are the finger, toe, pinna (top) or lobe of the ear. Infant sites are the foot or palm of the hand and the big toe or thumb. Opposite the emitter is a photodetector that receives the light that passes through the measuring site.

There are two methods of sending light through the measuring site: transmission and reflectance. In the transmission method, as shown in the figure on the previous page, the emitter and photodetector are opposite of each other with the measuring site in-between. The light can then pass through the site. In the reflectance method, the emitter and photodetector are next to each other on top the measuring site. The light bounces from the emitter to the detector across the site. The transmission method is the most common type used and for this discussion the transmission method will be implied.

After the transmitted red (R) and infrared (IR) signals pass through the measuring site and are received at the photodetector, the R/IR ratio is calculated. The R/IR is compared to a "look-up" table (made up of empirical formulas) that convert the ratio to an SpO2 value.

Most manufacturers have their own look-up tables based on calibration curves derived from healthy subjects at various SpO2 levels. Typically a R/IR ratio of 0.5 equates to approximately 100% SpO2, a ratio of 1.0 to approximately 82% SpO2, while a ratio of 2.0 equates to 0% SpO2.

The major change that occurred from the 8-wavelength Hewlett Packard oximeters of the '70s to the oximeters of today was the inclusion of arterial pulsation to differentiate the light absorption in the measuring site due to skin, tissue and venous blood from that of arterial blood.

At the measuring site there are constant light absorbers that are always present. They are skin, tissue, venous blood, and the arterial blood.

However, with each heart beat the heart contracts and there is a surge of arterial blood, which momentarily increases arterial blood volume across the measuring site. This results in more light absorption during the surge.

If light signals received at the photodetector are looked at 'as a waveform', there should be peaks with each heartbeat and troughs between heartbeats. If the light absorption at the trough (which should include all the constant absorbers) is subtracted from the light absorption at the peak then, in theory, the resultants are the absorption characteristics due to added volume of blood only; which is arterial.

Since peaks occur with each heartbeat or pulse, the term "pulse oximetry" was coined. This solved many problems inherent to oximetry measurements in the past and is the method used today in conventional pulse oximetry. 

Light Source Radiant Energy vs. Wavelength

• Tungsten (W) at 3000K has a broad spectral output, at 2000K, output is lower at all wavelengths and the peak output shifts to longer wavelengths. Note that much of the radiation from an incandescent (W) lamp is NOT visible, but rather is infrared.

• LEDs yield a relatively narrow spectral output, with color determined by the bandgap of the semiconductor. GaAs is IR LED, GaP is RED LED, GaAsP is green LED.

• Monochromatic outputs from lasers are shown by dashed lines: Ar, 515 nm; HeNe, 633 nm; ruby, 693 nm; Nd, 1064 nm

Filter Transmission vs. Wavelength

•Crown glass passes a wide range of ultraviolet, visible, and near infrared wavelengths.

•A Corning 5-56 glass filter passes a blue wavelength band.

•A Kodak 87 gelatin filter passes infrared and blocks visible wavelengths

•Germanium (Ge) lenses pass long IR wavelengths that cannot be passed by glass, and thus Ge lenses are useful in IR pyrometers

•Hb and HbO unoxygenated and oxygenated hemoglobin pass equally at 805 nm and have maximal difference at 660 nm.

Detector Sensitivity vs. Wavelength

• Thermal (Thermopile or Pyroelectric detector) has very wide response, using absorbing areas to convert light into heat.

• S4 is a typical phototube response.• Human eye has relatively narrow response, with colors indicated by

VBGYOR.• CdS photocell has response similar to eye.• PbS photocell is a sensitive near IR detector (burning flame thermometry).• InSb photocell is useful in far IR (body temperature thermometry).

Source/Filter/Detector Combined Wavelength Response

• Indicated curves from (a), (b), and (c ) are multiplied at each wavelength to yield the overall wavelength response of the entire system.

• This indicates the range of wavelengths over which a particular instrument, such as a spectrophotometer could be expected to give accurate readings.

Halogen Lamps A halogen lamp is a special kind of incandescent lamp. The light output is more consistent than a standard incandescent lamp and the life is longer. Size is smaller because it is important for the halogen cycle to have a high bulb wall temperature, which requires quartz or hard glass to be used. Better beam control is possible because of the small source size.

Halogen Lamp Construction Construction of a halogen lamp is similar to a standard incandescent lamp with a few key differences. The light is still produced by a coiled filament of tungsten. The filament is protected by a quartz or hard glass bulb which is deliberately small to ensure the correct bulb wall temperature. The bulb is filled with a halogen gas rather than nitrogen and argon. The halogen gas is either iodine or bromine, or in some cases a mixture of both. The bulb shape is tubular so it is close to the hot filament. Most GE halogen lamps use quartz bulbs because this is a stronger material both mechanically and thermally.

Figure 8.1. Typical halogen lamp.

Halogen Bulb Operating Principles •When a tungsten filament is hot, minute particles of tungsten evaporate - much like steam leaving boiling water. •With standard incandescent lamps, these particles are deposited on the inside surface of the bulb - just as steam will condense on cold glass. This deposit gradually reduces the transmission of light passing through the bulb as the bulb blackens. This change in light output is called lumen maintenance.•However with halogen lamps, the tungsten particles combine with the halogen gas and are prevented from condensing on the bulb, provided the bulb is above 200°C. •The combined tungsten and halogen remains as a gas which circulates within the bulb, and when it approaches the filament where the temperature is much higher, the combination becomes unstable and reverts to the two separate constituents of tungsten and halogen. •The tungsten will condense on the coolest local point, usually around the end of the filament. This process is called the halogen cycle.•As the inside surface of the bulb remains clean the lumen maintenance remains high throughout the lamp life.

15 kW Xenon High Pressure, High Voltage High Intensity Arc Lamp

•The electric arc in an arc lamp consists of gas, which is initially ionized by a high voltage and therefore becomes electrically conductive. •To start an arc lamp, a very high voltage is pulsed across the lamp to "ignite" or "strike" the arc across the gas. This requires an electrical circuit with an igniter and a ballast. •The ballast is wired in series with the lamp and performs two functions:First, when the power is first switched on, the igniter/starter (which is wired in parallel across the lamp) sets up a small current through the ballast and starter. This creates a small magnetic field within the ballast windings.

•A moment later the starter interrupts the current flow from the ballast, which has a high inductance and therefore tries to maintain the current flow (the ballast opposes any change in current through it); it cannot, as there is no longer a 'circuit'. •As a result, a high voltage appears across the ballast momentarily - to which the lamp is connected, therefore the lamp receives this high voltage across it which 'strikes' the arc within the tube/lamp. The circuit will repeat this action until the lamp is ionized enough to sustain the arc.

•When the lamp sustains the arc, the ballast performs its second function, to limit the current to that needed to operate the lamp.•Lightning is a similar principle where the atmosphere is ionized by the high potential difference (voltage) between earth and storm clouds.•The temperature of the arc in an arc lamp can reach several thousand degrees Celsius. The outer glass envelope can reach 500 degrees Celsius, therefore before servicing one must ensure the bulb has cooled sufficiently to handle. •Some lamps (mainly fluorescent tubes/energy saving lamps) can be restruck as soon as they are turned off (called hot restrike lamp).•Often, if these type of lamps are turned off or lose their power supply, one cannot restrike the lamp again for several minutes (called cold restrike lamps).

Carbon Arc Lamp (Low Pressure)

In a carbon arc lamp, the electrodes are carbon rods in free air. To ignite the lamp, the rods are touched together, thus allowing a relatively low voltage to strike the arc. The rods are then slowly drawn apart, and electric current heats and maintains an arc across the gap. The tips of the carbon rods are heated to incandescence, creating light. The rods are slowly burnt away in use, and need to be regularly adjusted to maintain the arc.

• Fluorescent Lamp Parts Basically, a fluorescent lamp is made up of five components. See Figure 5.1.

• GLASS TUBE, coated on the inside with fluorescent powders called PHOSPHORS.

• Two ELECTRODES (or cathodes) coated with EMITTER, supported by a glass mount structure, and sealed at the ends of the tube.

• FILLING GAS - usually a low pressure of Argon or Krypton/Argon mixture.• Small amount of MERCURY (less than 20 mg), which vaporises during the

lamp operation.• LAMP CAP cemented to each end of the tube to connect the lamp to the

lighting circuit.

Fluorescent Lamp Operation When the circuit is energized, electricity heats the cathodes. See Figure 5.2. The cathodes are coated with material which, when heated, emits electrons. The electrons establish an electric arc between the cathodes at opposite ends of the tube. The electrons collide with the mercury atoms, causing mercury to emit invisible ultra-violet radiation. The ultra-violet is absorbed by the phosphor coating on the tube and re-radiated as visible light.

Fluorescent Lamp Circuit Operation The circuit must contain a ballast (choke or inductor) to limit the current and a starter to provide the pre-heat conditions. Initially the starter switch closes so the two cathodes are connected in series. Current flows and the cathodes heat up emitting electrons. After a short time the starter switch opens so voltage is applied across the tube. If sufficient electrons are available an arc is struck and the starter plays no further part until the next starting operation. If there are insufficient electrons, the tube will flicker, fail to start, and the starter will repeat the heating of the cathodes. The ballast limits the current to a safe and appropriate level for the power of lamp. Without the ballast, the current would increase to a high level and the lamp would destroy itself. 

60 Hz, 120 Vrms Power Line, N is neutral (white) grounded side of line, and L is the hot (black) ungrounded side of the line.

1. The starter (which is simply a timed switch) allows current to flow through the filaments at the ends of the tube for a few seconds, and then it suddenly breaks the connection..

2. Most starters incorporate a Neon Glow Tube with an internal bimetallic switch, which is normally open. When power is applied, a glow discharge takes place which begins to heat the bimetallic switch inside the glow tube. A second or so later, the bimetallic contacts heat up and bend (due to different coefficients of expansion of the two metals) and the contacts close thereby permitting current to flow through the starting filaments at each end of the tube. The closing of the switch also shorts out the Neon Glow, and the glow is extinguished, therefore there is no longer any heating of the bimetal, and so after a few seconds, the bimetallic switch contacts once again open.

3. An inductive kick voltage spike (vL = Ldi/dt) is generated at the instant the contacts are opened, since the ballast (choke) current suddenly falls toward zero as the contacts open.

4. This high voltage pulse is coupled through the capacitor, so it appears across the two ends of the fluorescent tube, thereby ionizing the mercury vapor in the tube, and the tube lights.

5. Since the lighted fluorescent tube has a low resistance, the ballast now serves as a current limiter. Note that now no current flows through the fluorescent tube’s filaments. They are only used to heat and vaporize the mercury in the tube before the starter cuts the current through the ballast and generates the high voltage pulse that starts the arc..

Conventional Neon “Glow Tube” Starters (This classic design has been in use for more than 50 years!)

The situation before the light switch is turned on. No power is applied and the lamp is OFF

Power switch turned on. 120 VAC power voltage ionizes Neon in glow tube and the glow tube lights and heats up the bimetal switch contacts inside the glow tube. As they heat up, the contacts bend toward bend toward each other until they touch, shorting out the Neon Glow arc discharge. The glow is extinguished, and the glow tube cools.

While the contacts are closed, a large current builds up in the ballast and the tube heater filaments (at both ends of the tube) heat up and vaporize the mercury in the tube.

A few seconds later, the bimetal contacts cool enough to once again separate, and the current though the ballast is suddenly interrupted.

As this happens, a large vL = L*di/dt inductive voltage kick is generated, which appears across the tube (in series with the 120 VAC power supply.)

1. If the glow bulb starter contacts happen to open when the 60 Hz is passing through (or near) a voltage 0 in its 60 Hz cycle, the inductive kick may not be strong enough to ionize the mercury vapor in the main tube, and the main fluorescent tube will not light.

2. However, in this case, the voltage across the glow tube will be high (since the fluorescent tube itself is NOT ionized and conducting) and so the glow tube will once again light and the whole starting cycle will be automatically be repeated until an arc is successfully struck in the fluorescent tube and it lights.!

Automatic Restart

Note that as Neon glow tube starters age, they get black and eventually stop working and must be replaced (see the old one on the right). A new glow tube starter appears on the left. This one also shows a capacitor connected across the starter to help absorb radio frequency (RF) emissions caused by the spark across the contacts as the contacts separate.

Motion detector used in home security. Detects only moving living things at body temperature that move past the field of view of the two pyroelectric IR sensors, which are wired in a series-opposing fashion.

Side view showing the Fresnel lens focusing the incoming parallel IR light rays onto the surface of the PIR detector

Front view of the Fresnel Lens, which looks like a circularly symmetric grooved plastic sheet.

Fresnel lens is made with a material that passes far IR radiation in the 8 to 14 um region.

Idx

Vdx

Load LineSlope = 1/RL

Passes Thru Origin

Vd

Slope = 1/RL

+

=> Passes Throug Origin

R L L o a d R e s is t o r

Load Line Equation

-

Id = Vd/RL

D 1P H O TO V O L TA I C C E L L

Id

Power Delivered to Load Resistor for 10 mW/cm2 light level:

PRL = -Vdx*Idx

Note that different load resistances extract different amounts of power from photocell