Compact video cameras using CCD (charge-coupled device) sensors are now widely available at low cost, and as
a result they find many uses around the home, office and factory. Typical uses including monitoring babies, keeping an
eye on kids playing in the yard or swimming pool, viewing callers at the front door and general surveillance of office
and factory areas. In this data sheet we.ll explain how CCD cameras work, and give you the information you.ll need to select the most appropriate camera (and lens) for any particular job and get the best performance from it.
CCD imagers At the heart of this type of camera is the CCD imager, a specialised type of integrated circuit (IC) which is located just behind the camera’s lens. The lens focuses a small image of the scene in front of it directly onto the
CCD imager chip, which is behind an optical glass window in its package. The CCD imager then ’scans. the image, and with the help of a few support chips generates a complete standard video signal from it, ready to feed into your TV, video monitor or VCR.

The detailed operation of a CCD imager chip is fairly complex, but here’s a simplified explanation of how they work. Over the active image-sensing area of the chip, there’s an array of tiny sensor cells, each typically measuring 10 x 5um (micrometres) or less. The array of a typical CCD sensor has 297,984 of these cells, arranged in 582 horizontal rows and 512 vertical columns. Inside each cell there’s a light sensitive element . essentially a very tiny photodiode . together with a charge-transfer area which forms part of a long vertical shift register. There are also two control
elements, called the readout gate and the overflow gate, and a short section of a long vertical structure called the overflow drain. All parts of the cell apart from the sensor element are covered by metallisation, so they.re .kept in the dark.. When light falls on the sensor element (as part of the image), the photons generate charge carriers and as a
result a small quantity of charge builds up in that part of the cell. How much charge builds up depends on the amount of light reaching the cell, of course. The area directly under the sensor element is designed to contain this charge, as a kind of .bucket.. Then after a short time, a voltage pulse is applied to the readout gate. This has the effect of lowering the .retaining wall. on that side of the bucket, allowing the accumulated charge to flow out of the sensor bucket and into the
charge-transfer area. So after the readout pulse, the charge that was generated in the sensor element by the incident light has been shifted into the charge-transfer area alongside. And as mentioned earlier this area is actually part of a long vertical shift register, which links all of the charge-transfer areas in a complete column of cells. This shift register is used to transport the charges in each of the charge-transfer areas down the columns, and ultimately out of the chip.
How does the shift register work? By passing the charge in each charge-transfer area down to the one below it, using exactly the same kind of process that was used to shift the charges into them from the sensor elements. There’s
another set of gates between each pair of adjacent transfer areas in the column, and by pulsing these the charges are transferred from each one to the one below. It’s like a traditional bucket brigade’. Along the bottom of all the columns, there’s yet another of these bucket brigade shift registers’  only this time it’s horizontal. So by pulsing the transfer gates linking the bottom row of charge-transfer elements, the charges in them can be shuffled out of the image array, in serial order. Here they’re passed through a charge-to-voltage amplifier stage to produce the output video signal. Fig.2 shows the overall charge flow paths in the image sensor array. Getting back to the basic imager cell of Fig.1 for a moment,
you’re probably wondering what that overflow gate and rain are for. Basically, they’re to prevent the sensor
elements from accumulating too much charge, if the light falling on them is too great (i.e., over exposure).
The idea here is that the overflow gate is held at a voltage level where the retaining wall’ on that side of the sensor
bucket’ is a little lower than on the charge-transfer region side. This means that if the charge builds up in the bucket
to reach that level, any further charge simply flows over the wall’ into the overflow drain, where it’s drained away. This
system prevents the photosensor elements from ever completely filling with charge  which would tend to make the CCD imager saturate and its output video wash out’ in highlight areas.
By the way the type of CCD imager we’ve described here is known as the interline transfer type, because of the way
the charges from the sensor elements are shifted first sideways into their own charge-transfer region, then down
the vertical shift registers and finally out via the horizontal shift register. This is the type of CCD imager used in most
home video cameras, camcorders and digital still cameras. There are other types of CCD imager, which use a different
system to shuffle the charges out of the array. The frametransfer system has a second complete storage array underneath the sensor array, which allows charges from the next image to be built up while the first charges are being
processed. However these chips are roughly twice as complex as the interline transfer type and also tend to need a mechanical shutter for exposure control, so they’re more costly.

Electronic shutter
The basic interline-transfer CCD imager provides a fairly simple way of controlling the exposure for each image: by
EDC Distribution Reference Data Sheet: CCDCAMS.PDF (1) Fig.1: The basic structure of one picture element (pixel)
cell of a CCD imager, typically measuring about 10 x 6um (micro-metres). Typical imagers have an array of 297,984
of these cells, in 582 rows by 512 columns.

UNDERSTANDING & USING CCD CAMERAS
varying the length of time that the charge can build up in each sensor element, before it’s shifted out into the charge-transfer region. So by adjusting the timing of the readout pulses, the control circuitry effectively controls the exposure
time. This property of CCD imagers is usually described as their electronic shutter, and most CCD cameras use it to provide a simple means of allowing the camera to deliver clear video signals over a fairly wide range of lighting levels.
With most CCD imagers, this automatic electronic shutter’ or AES function has an effective range from about 10us (1/100,000th of a second) up to almost 20ms (1/50th of a second) the video field period. This gives an exposure
control range of almost 2000:1.
B&W or colour
The photosensor elements of a CCD imager respond to any light in a fairly wide range of wavelengths. In other words, they can’t distinguish between colours. So a basic CCD imager forms what is essentially a B&W (black and white) video camera. Two different systems are used to produce a CCD colour camera. In the single chip system used in most low cost video cameras, camcorders and digital still cameras, tiny strips of colour filter material are laid on the top of the CCD imager, covering the sensor columns in a repeating green-red-blue sequence. This restricts each column of sensor elements to responding primarily to the colour passed by that filter, so that the video signal that emerges from the imager has colour information multiplexed into it. All of the video is used by the processing circuitry to generate the luminance signal, but the information corresponding to each trio of sensor bits can also be used to generate the
chrominance (colour) signal. The alternative way of producing a CCD colour camera is to use three separate CCD imagers, each receiving its light via a filter for one of the three primary colours. The three imagers are mounted around an optical prism/splitter system behind the lens, so that all three receive exactly the same image. This three-chip colour system can deliver higher quality colour signals than the single-chip system, but tends to be much more expensive because of the three imagers and more complex optical system. It’s used mainly in broadcasting and professional TV cameras.
Imager size
The majority of domestic and industrial CCD video cameras use one of two main sizes of CCD imager. These are usually called the 1/3” type and the 1/4” type, and both are made in either B&W or colour versions. Other sizes are made, including a smaller 1/5” type and a larger 1/2” type, but they’re much less common. The active image size of a nominal 1/3” CCD imager is actually 4.8 x 3.6mm, while that of a nominal 1/4” imager is 3.6 x 2.7mm. In each case the larger of
the two dimensions is image width. Note that the ratio of the two is 1.33:1 in each case. This is known as the aspect ratio, and matches that of a standard CCIR/PAL TV signal (usually expressed as 4:3).

Resolution
Broadly speaking, the image clarity or picture sharpness’ delivered by a CCD camera depends on its resolution  how well it reproduces or resolves’ fine details in the image. However there are a number of ways of describing the resolution, which can make things a bit confusing. For example there’s the basic resolution of the camera’s CCD imager: how many rows and columns of sensor elements it uses, which determines the number of picture elements or pixels
that it uses to analyse the image. Most low cost CCD video cameras use an imager with a basic resolution of either 512 or 500 columns across the picture, and 582 rows down the picture. This gives roughly one row of sensor pixels for each active line of a nominal 625-line video image, and the potential of 500 or more pixels along each line. But the final horizontal resolution of the image isn’t determined only by the CCD imager. It’s also influenced by the frequency response of the other chips used to process the video signal from the imager, and these inevitably reduce the effective resolution to some extent. To give you a better idea of the final image resolution from a camera, manufacturers usually also specify an effective horizontal resolution figure as well as the imager’s raw pixel figures. This resolution figure is usually quoted in terms of the number of alternating black and white lines that can be resolved across the width of the image  i.e., along each line. This figure usually turns out to be rather lower than the potential 500- or 512-line resolution you’d expect from the imager: typical cameras provide figures ranging from 330 to 420 lines. However a figure of 400 lines or more will generally give images that most people find quite clear and sharp’.
Note, though, that the final clarity of the images produced y any camera will also depend on the performance of the
video monitor or TV receiver it’s displayed on. If the monitor has relatively poor video response, the image from the best camera will still look soft’ or furry’.
Spectral response
The sensor elements of a basic CCD imager (B&W) respond to wavelengths covering the complete range of visible light, and beyond (see Fig.3). The peak response is usually between about 500 and 550nm (nanometres), corresponding to green-yellow light. However the sensors often still have 20% or more of their peak sensitivity at 780nm, which is the start of the infra-red (IR) part of the spectrum and outside the range visible to the human eye. This wide spectral sensitivity of CCD imagers has both advantages and disadvantages. On the plus side, it means that CCD cameras can be used with IR illumination to monitor areas that seem to the human eye to be in total darkness. This makes them very suitable for surveillance. On the other hand, the fact that a CCD imager responds to IR as well as visible light can degrade image quality when a camera is viewing a scene where there’s significant IR radiation as well as visible light. This is because many lenses have a different focal length at different wavelengths  so a focus setting that’s correct for visible light tends to result in a defocussed (blurry) IR image, and vice-versa. So with many CDD cameras, the only way to get a really  sharp and clear image of some scenes is to use an IRrejection filter to block out the IR components in the image. This tends to be more of a problem with B&W CCD cameras than with colour cameras, as the colour filter stripes tend to reduce the imager sensitivity to IR wavelengths. However some colour cameras still have a significant sensitivity to IR, especially if they’ve been designed to be sensitive down to very low light levels. CCD cameras
Currently there are two broad types of low cost video camera based on CCD imagers: the naked board’ type, usually with a built-in lens, and the fully encased type. The latter can have either a built-in lens or be designed to accept replaceable screw-in lenses. Both types are available in either B&W or colour, and the fully encased type often consists of a board-type camera in a sturdy but compact metal case, fitted with a lens mount at the front and power/output connectors at the rear. Whether of the naked-board or encased type, most of the latest CCD cameras are fully automatic in operation and have virtually no manual controls or adjustments apart from focusing via the lens mounting. Exposure control is automatic and based on the CCD imager’s AES function. This typically copes with a 2000:1 range in light level, and allows the use of low cost fixed-aperture lenses. If a camera needs to operate at a very high light level, a
neutral-density filter can often be used to prevent overload. When a CCD camera does need to be used where lighting levels vary over a range of much wider than 2000:1, an auto iris lens can be used to allow it to cope with the
larger range. These lenses are not cheap (often costing as much or more than the camera itself), but they give somewhat better performance than the AES system. When such a lens is fitted the camera’s own AES function is often
disabled.

Nowadays both the naked-board and fully enclosed types of camera are often equipped with an electret microphone insert and preamplifier, so they deliver an audio signal as well as the video from the CCD imager. Some enclosed cameras are also provided with a number of forward-facing IR emitting LEDs, to give the camera built-in IR scene  illumination. This makes them especially suitable for covert surveillance work. Of course IR illuminators (usually just an array of IR LEDs) are available at quite low cost anyway, so it’s also possible to use these with cameras that don’t have the inbuilt illumination, to achieve the same result.

Power supply
Most small CCD cameras are designed to be powered from a fairly well regulated source of 12V DC (typically +/-10%). This makes them very suitable for operation from a battery supply, for example, but they can malfunction or even be damaged if the voltage rises much above 13.5V. That’s why it is unwise to attempt running them from low cost unregulated 12V plug pack’ mains adaptors, as the output from these can easily rise to 16-17V or more. A small number of cameras do have internal regulation circuitry and are able to cope with a wider range of input voltages  say 9-15V. However in general, when perating any CCD video camera from mains power it’s safest to use an electronically regulated 12V power adaptor or power supply.

Lenses
Naked-board and very compact enclosed CCD cameras usually come complete with an integral lens and holder,  fitted directly over the CCD imager on the front of the board. These lenses are generally one of two main types: the fixed-focus pinhole’ type or the adjustable focus three-element type. The pinhole’ type lens isn’t a true pinhole, but a
low cost single-element lens with a short focal length and a small fixed aperture (often 2-3mm), so that it provides a depth of field extending from about 2m to infinity. However the single lens  element limits image quality, and the small aperture restricts such cameras to fairly high lighting levels. Better image quality is generally available from the type of camera using a three-element lens, not only because of the additional elements but also because of the adjustable focus. The aperture is usually somewhat greater too, making the camera more useful at lower lighting levels.

Although the built-in lenses fitted to naked-board and compact cameras can deliver quite good image quality, much greater flexibility is available from the larger enclosed type of camera, which generally offers the ability to use interchangeable lenses. In most cases these lenses are of the screw-in CS’ type, which is a modified version the Cmount’ originally developed for 16mm movie film cameras.  The CS’ version is made with a shorter length extending behind the mounting thread, to ensure they clear the CCD imager.

Focal length
Whether they’re built into the camera or are of the screw-in interchangeable type, the key parameter used to describe camera lenses is their focal length. This is basically a measure of the spacing needed between the lens’s centre of focus and its focal plane (here, the active surface of the CCD imager), when the lens is producing a properly focussed image of an object at infinity. The focal length of CCD video camera lenses is usually given in millimetres (mm), although sometimes the horizontal viewing angle is given instead.

Viewing angle
Because of the way lenses work, the focal length of a lens determines how wide an angle it views’, when producing an image of a certain width  here, the width of the active image area of the CCD imager it’s being used with.  The viewing angle is narrower when the lens has a focal length (f) that’s relatively long compared with the active image width of the sensor (s). Conversely it’s wider when the lens has a relatively short focal length. So longer focal length lenses tend to give a telephoto’ or close-up effect, while those with shorter focal length give a .wide angle. effect. The actual viewing angle of a lens when used with a particular CCD imager can be found using this expression: f = 2 x tan-1(s/2q) where f is the horizontal angle of view, s is the width of the CCD sensor’s active image area and q is the distance between the centre of focus of the lens and the CCD imager plane.

Note that when the lens is focussed at objects further away than about 1m, q will be very close to f, the focal length of
the lens. So for most purposes you can simply substitute f for q in this expression, to give: f = 2 x tan-1(s/2f) However this isn.t true if you use the lens to focus on very close objects, because q then becomes significantly longer than f. In these cases you need to use the first expression. Another point to note from these expressions is that a lens with a particular focal length will give a wider angle of view with a CCD imager having a larger active image width, and vice-versa. For example a lens of 4mm focal length will give a 48° angle of view with a camera using a 1/4. CCD imager, but a 62° angle of view with a camera using a 1/3. imager. Typically the lenses built into CCD cameras have a focal length of about 2.5mm, which tends to give a fairly wide angle of view: around 90° with a 1/3. imager or 70° with a 1/4. imager. Cameras designed especially for .front door viewer. use are fitted with a special type of lens with a very wide angle of view . typically 170°, which is almost a hemisphere. This type of lens is often called a .fish eye..

Scene width
Although it’s handy to be able to visualise the angle of view of a lens when used with a particular camera and its imager,
often it’s more important to be able to work out the best lens to use in order to cover a particular scene width, at a known distance from the camera. This is also quite easy to work out. The ratio of scene width W to the camerascene
distance D is the same as the ratio between s, the active width of the CCD imager and q the lens-imager distance. In other words, W/D = s/q And as before q will be almost exactly the same as f the focal length of the lens, for objects and scenes more than about 1m distant from the camera.
So if you know the scene width you want, and its distance from the camera, you can find out the focal length of the lens you need by rearranging the above expression into: f = s x (D/W) If you want to cover a scene 3m wide at a distance of 5m from the camera, for example, this will give a D/W figure of 5/3 or 1.666. Therefore if you have a camera with a 1/3”
CCD imager, where s = 4.8mm, you’ll need a lens with a focal length of 4.8 x 1.666, or 8mm. On the other hand if you want the same scene width using a camera with a 1/4” imager, where s = 3.6mm, you’ll need a lens with a focal length of 3.6 x 1.666, or 6mm. To save you having to work these figures out for yourself every time, we’ve already worked out the viewing angle and scene widths for most of the common combinations of CCD imager and lens focal length. Output wiring Most CCD video cameras deliver standard CCIR/PAL composite video which is suitable for feeding straight into
the direct video input of standard TV sets, video monitors and VCRs. The video signal is typically 1V peak-to-peak at 75 ohms impedance, which means that coaxial cables of the same impedance and up to about 20m long can be used to
deliver the signal to the TV/monitor without any serious degradation. Where the camera must be used further away from the monitor, there are two main alternatives to coaxial cable. One is to use video baluns (wideband balanced-tounbalanced transformers) to couple the video signal into Category 5 twisted-pair cabling, as used for computer data
networks. This approach allows the use of Cat-5 cabling up to 600m long for B&W video signals, and 300m long for
colour signals.

Video baluns are available to handle either the video signal alone, or the video and two audio signals together. In bothcases they’re passive devices and need no external power. The other main way of sending the CCD camera signals
over a longer distance to the TV/monitor is to use small UHF video/audio transmitter and receiver units  the type of system used to reticulate cable TV video and audio around a home. This approach can give good results at distances of up to 100m or so. Where the camera signal is being sent only to a TV receiver, a simpler approach is to use a low cost RF modulator unit of the same type used for video games. This allows the camera signal to be tuned in on a suitable vacant channel, and doesn’t require the use of a separate receiver or demodulator unit. The useful range can be up to about 30 metres.

CMOS cameras
Although they’re nominally standard ICs, CCD imager chips have to be made using different processing steps from
those used in most other ICs, to produce their array of charge-containment regions. This makes them relatively
expensive, and has also made it difficult for manufacturers to combine them with the necessary auxiliary circuitry to

produce a complete camera on a chip’. Because of these shortcomings, IC designers have recently put a lot of effort into designing imager chips using standard CMOS processing technology, with the aim of replacing CCD imagers. To date they’ve had only limited success, and although CMOS cameras have begun to appear their performance usually doesn’t compare all that well with the CCD type. The image resolution is usually quite modest, and they have a relatively high noise level and image lag’ compared with CCDs. It’s likely that these drawbacks will be overcome in the future, though, and CMOS imagers and cameras will probably replace CCDs eventually. But for the present, CCD imagers and cameras deliver very good performance and value for money. CCD CAMERAS, LENSES & ACCESSORIES STOCK EDC stocks a very wide range of CCD cameras  from naked-board and very compact enclosed types to types built into darkened plastic domes, very compact bullet’ shaped cameras and a door viewer’ type with a wide angle fish-eye lens, all the way to pro-style cameras which take standard interchangeable CS lenses. Most styles of camera are available in either B&W (CCIR) or colour (PAL) versions , and many include a built-in microphone and audio preamp. There are models which include built-in IR illumination, and a dome’ model which has built-in pan and tilt servo motors for remote positioning.
Needless to say EDC also stocks a broad range of accessories for the cameras, including interchangeable lenses  including an auto-iris lens for situations where the camera must cope with a very wide range of lighting levels. There’s also an IR illuminator, internal and external mounting brackets, rugged camera housings for external or internal use, replacement and extension cables, Cat-5 video and AV baluns, monitors, camera switchers and video processors, AV transmitter/receiver sets and RF modulators. Everything needed for just about any kind of CCTV system!

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The relationship between voltage, current and resistance is called Ohms Law, so named after its discovery by German
physicist George Simon Ohm in 1827. Ohm discovered that there is a relatively linear relationship between the potential difference applied to the ends of a conductor, and the current that is flowing through it. The parameter that relates voltage and current is defined as the conductors resistance. From this comes the following formula: R = (V/I)
where R is the resistance, measured in ohms (W) V is the potential difference in volts (V) I is the current in amperes (A)
As you can see from the formula, the resistance of a conductor which passes one ampere with a PD of one volt applied
between its ends would therefore be 1 ohm.

The simple formula can be transposed thus:
V = I x R or I = (V/R)
which can be very handy!
POWER IN A CIRCUIT (DC)
When electrical power is dissipated in a circuit, heat is created. The amount of heat is expressed in watts  whether its from your electric kettle, a radiator or even your hifi amplifier.

Heres the basic formula:
P = V x I
where P = power in watts
V = potential difference in volts
I = current in amps
Just like Ohms law this can be transposed to:
V = (P/I) or I = (P/V)

POWER IN AC CIRCUITS
The calculations above only relate directly to DC circuit conditions, although they can be applied to a circuit carrying
AC provided that:
1. The RMS values of the voltage and current are used (see below); and
2. There is virtually no capacitance or inductance, to produce significant phase shifts. If there is significant capacitance and/or inductance, power calculations become a bit more complicated as well see shortly.

MEASURING AC WAVEFORMS
Most circuits encountered in electronics are carrying AC, and we often need to measure AC waveforms with a view to
calculating circuit component values, signal levels etc. There are several ways of describing an AC waveform, and each one has a use, depending on what you need to know. AC signals are by definition different from one instant in time
to the next. A pure AC signal swings about a zero voltage axis, going positive one moment and negative the next. This means that its average value over a complete cycle is zero, because the positive and negative sides of the waveform cancel out. However the positive-going part of the wave can still deliver energy, and so can the negative part. There is plenty of energy, for example, in a 240V AC power point! So we have to find an alternative way of describing this energy-delivering aspect of AC, other than the average value. The parameter we use instead of the average value is the RMS or root mean square value, which is found by squaring the instantaneous values of the AC voltage or current, then
calculating their mean (i.e., their average) and finally taking the square root of this  which gives the effective value of the AC voltage or current. These effective or RMS values dont average out to zero, and are essentially the AC equivalents of DC voltage and current.
RMS POWER MEASUREMENT
When we use the effective or RMS values of AC voltages and currents, and we have a circuit which is resistive (i.e., with no phase shifts due to capacitance or inductance), we can simply multiply the voltage and current together to give the power dissipated  just as we can for DC. For example an AC waveform of 100V RMS applied across a resistive load (say 10 ohms) would draw the equivalent current (10A RMS) that wed get from 100V DC, and the load would dissipate the same amount of heat energy: 1000 watts. Another name for this RMS power is continuous effective power .
Note that although its not too difficult to calculate the RMS value of regular repetitive waveforms like sine, sawtooth,
triangular or square waves, its much more difficult to do so with non-repetitive waveforms such as a music signal with
non-repetitive peaks. This is why, for example, amplifier power ratings are calculated and measured with sinewave signals. Although typical amplifiers normally dont handle sinewaves, these waveforms do provide a standardised way to measure and rate amplifier performance.

POWER IN AC CIRCUITS WITH REACTANCE

If a circuit carrying AC has capacitance and/or inductance as well as resistance, these dont dissipate power themselves but the added capacitive or inductive reactance produces a phase shift between the voltage and current. This means that the power dissipated in the circuits resistance can no longer be calulated simply by multiplying the RMS voltage and current. Doing this merely gives a quantity known as the circuits volt-amps (VA). The real power in the circuits resistance can only be found by multiplying the RMS voltage across it with the proportion of the RMS current flowing through it which is in exactly the same phase. This works out to be: PREAL = (VRMS x IRMS) x cos(f) where f is the phase angle between the voltage and current.
This real power will generally be smaller than the VA figure, because the VA turns out to include the energy that is simply stored in the circuits reactances during one half of the AC waveform, and returned during the other.
PEAK & PEAK-TO-PEAK VALUES
There are other ways of describing an AC waveform besides the RMS value, which are sometimes useful. For example its often necessary to know a waveforms half-wave peak level (to calculate wiring insulation and capacitor voltage rating requirements for example) or its peak-to-peak level, which is simply the total swing between the positive and negative-going peaks of the waveform.

MEASURING AC VOLTAGE & CURRENT
Many digital multimeters do not measure the RMS value of AC voltages directly. Often they simply measure the peak value, and calculate the equivalent RMS value  assuming a sine waveform. This calculated value is the one displayed. Older moving-coil meters tend to measure the half-wave average value, but are made to indicate the equivalent RMS.
Of course some DMMs do in fact measure the RMS value of voltages and currents, and these True RMS reading meters
are generally the best type to use if you really need to know the RMS value  especially for non-sinewave voltages and
currents. However youll find that such meters tend to be somewhat more expensive than the regular type.  Incidentally, its worth remembering that a True RMS meter will also include the contribution of any DC voltage and current which may be present along with the AC. Happily you can still get a fairly accurate idea of the RMS value
of a sine waveform, knowing one of the other measurements such as the half-wave average, peak or peak-to-peak value. his can be done by calculation, or using the handy table on this page. As you can see its also possible to work out the RMS value of a few other symmetrical and regular waveforms, such as square and triangular waves, knowing their peak, average or peak-peak values. The important thing to bear in mind, though, when using this type of table is that you do need to know the exact basis on which your meters measurement is made. For example if it really measures the peak value, and then calculates and displays the equivalent sinewave RMS figure, this means youll need to use the table differently from the situation where it measures the half-wave average and calculates the sinewave RMS figure from that. So take care, especially if youre not sure exactly how your meter works.

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All semiconductor devices have some electrical resistance, just like resistors and coils, etc. This means that when power diodes, power transistors and power MOSFETs are switching or otherwise controlling reasonable currents, they dissipate power  as heat energy. If the device is not to be damaged by this, the heat must be removed from inside the device (usually the collector-base junction for a bipolar transistor, or the drain-source channel in a MOSFET) at a fast enough rate to prevent excessive temperature rise. The most common way to do this is by using a heatsink.
To understand how heatsinks work, think of heat energy itself as behaving very much like an electrical current, and
temperature rise as the thermal equivalent of voltage drop. We also have to introduce a property of materials and objects known as thermal resistance, which behaves in a very similar way to electrical resistance: the more heat energy flowing’ through it, the higher the temperature rise across it. As you might imagine metals like copper and aluminium have very low thermal resistance, while air tends to have a relatively high resistance. So do many plastics and ceramic materials.

It turns out that there’s a thermal equivalent of Ohm’s Law, which describes the way heat energy behaves in something like a power transistor: T(j-a) = Pd x Rth(j-a) Here T(j-a) is the temperature rise of the transistor’s heatproducing junction, above that of the ambient’ temperature (i.e., that of the surrounding environment  roughly air temperature
‘); Pd is the power being dissipated; and Rth(j-a) is the total thermal resistance between the junction and the
surrounding ambient. Usually T(j-a) is measured in degrees Celsius, Pd in watts and Rth(j-a) in degrees C-per-watt (°C/W). So as you can see the relationship is just like Ohm’s Law (E = I x R), except for the units being used and the fact that we’re talking about heat. For example, we can say that if the total Rth(j-a) is 6°C/W and our transistor.s junction is dissipating 20 watts, its temperature will rise by (6 x 20) = 120° above the ambient temperature.

This means if the ambient temperature rises to 38°C, the transistor.s junction temperature will reach (120 + 38) =
158°C. Fairly obviously, then, if we want to keep the temperature rise inside the .works. of a power transistor below its rated safe level, for a given amount of power being dissipated and for a given ambient temperature, the only way to do this is by reducing the value of Rth(j-a), the total thermal resistance between the device.s internal source of heat (usually called .the junction.) and the ambient. Now Rth(j-a) is really made up of at least two separate thermal
resistances, in series. One is the thermal resistance inside the device package, between the junction and its outside case, called Rth(j-c); the other is the resistance between the case and the ambient, Rth(c-a).
We can.t do much about Rth(j-c), but luckily in most power device packages this is fairly small anyway: typically between 0.7 and 4.5°C/W, depending on the package (see Table). But we can do something about Rth(c-a), and here.s where heatsinks come in. If a power transistor or MOSFET package is simply supported by its leads above a PC board and surrounded by air, heat energy can mainly flow from the case to the ambient only by two rather inefficient methods: radiation and air convection. As a result, the thermal resistance to ambient Rth(c-a) is fairly high . typically between 35 and 100°C/W. But if instead we bolt the device to a somewhat larger piece of metal, especially a stout piece of aluminium .heatsink. extrusion with fins, heat can flow much more easily from the case to the ambient: firstly through the aluminium and then to the air via the heatsink.s fins . which provide improved surface area to assist both radiation and convection. Although a heatsink generally provides a much lower thermal resistance to the ambient Rth(hs), when we use one we inevitably introduce additional thermal resistances, each in series with the heat flow: the thermal resistance of the contact between the case and the heatsink Rth(c-hs), which is largely due to tiny amounts of trapped air, and the thermal resistance of any electrical insulating washer (mica or plastic) we might need to use between the case and heatsink. Luckily we can minimise these last resistances by using a thin smear of .thermal compound., a special paste which has very low thermal resistance. This can reduce the total case-heatsink thermal resistance to around 1.5°C/W or less, even when a mica washer is used. Assuming you use thermal compound for the best .thermal joints., the main way you can reduce the total Rth(c-a) is by using a large enough and efficient enough heatsink . i.e., one with a low enough thermal resistance Rth(hs). Here you have to be guided by the data given by the manufacturer and supplier, as far as the exact Rth(hs) is concerned, but the general .rule of thumb. is the larger the heatsink, the lower its thermal resistance. All of the heatsinks sold by Jaycar have their thermal resistance clearly specified.

Once you know the maximum safe junction temperature for your power transistor or MOSFET (Tjmax), the power it’s
going to be dissipating (Pd) and the maximum ambient temperature it will be working at (Tamb), you can easily work
out the maximum total Rth(j-a) from this expression: Rth(j-a) = (Tjmax – Tamb)/Pd Usually for silicon devices, it’s reasonably safe to assume Tjmax is about 150°C. Similarly in many cases, it’s reasonable to assume that the maximum ambient temperature inside the equipment’s case will be about 50°C. So for a rough but fairly practical rule of thumb calculation, you can find the maximum total Rth(j-a) by dividing 100°C (150° – 50°) by the power being dissipated, in watts. You can usually find Rth(j-c), the junction-case thermal resistance of the power transistor or MOSFET itself, from the manufacturer’s data  or use the typical values for the various common packages. Then add the thermal
resistance of the thermal compound and/or insulating washer, again from the Table, and this will give you the total junctionto-heatsink resistance. Subtract this from the maximum Rth(j-a) figure, and you’ll get the maximum allowable heatsink resistance. You can then select a heatsink which will provide no more than this value of thermal resistance.
Of course if you have the room, it’s always a good idea to use a larger heatsink, with an even lower thermal resistance than your safe maximum. Your power transistor or MOSFET will then run even cooler.

basics, heatsinks, mosfet, power

In its simplest form, an Amplifier’s gain is a ratio of output over input. Like all ratios, this form of gain is unitless. However, there is an actual unit intended to represent gain, and it is called the bel. As a unit, the bel was actually devised as a convenient way to represent power loss in telephone system wiring rather than gain in Amplifiers. The unit’s name is derived from Alexander Graham Bell, the famous Scottish inventor whose work was instrumental in developing telephone systems. Originally, the bel represented the amount of signal power loss due to resistance over a standard length of electrical cable. Now, it is defined in terms of the common (base 10) logarithm of a power ratio (output power divided by input power):

 

AP(ratio) = Poutput / Pinput

AP(Bel) = log Poutput/ Pinput

 

 

Because the bel is a logarithmic unit, it is nonlinear. To give you an idea of how this works, consider the following table of figures, comparing power losses and gains in bels versus simple ratios:

 

It was later decided that the bel was too large of a unit to be used directly, and so it became customary to apply the metric prefx deci (meaning 1/10) to it, making it decibels, or dB. Now, the expression “dB” is so common that many people do not realize it is a combination of “deci-” and “-bel,” or that there even is such a unit as the “bel.” To put this into perspective, here is another table contrasting power gain/loss ratios against decibels:

As a logarithmic unit, this mode of power gain expression covers a wide range of ratios with a minimal span in fgures. It is reasonable to ask, “why did anyone feel the need to invent a logarithmic unit for electrical signal power loss in a telephone system?” The answer is related to the dynamics of human hearing, the perceptive intensity of which is logarithmic in nature. Human hearing is highly nonlinear: in order to double the perceived intensity of a sound, the actual sound power must be multiplied by a factor of ten. Relating telephone signal power loss in terms of the logarithmic “bel” scale makes perfect sense in this context: a power loss of 1 bel translates to a perceived sound loss of 50 percent, or 1/2. A power gain of 1 bel translates to a doubling in the perceived intensity of the sound.

An almost perfect analogy to the bel scale is the Richter scale used to describe earthquake intensity: a 6.0 Richter earthquake is 10 times more powerful than a 5.0 Richter earthquake; a 7.0 Richter earthquake 100 times more powerful than a 5.0 Richter earthquake; a 4.0 Richter earthquake is 1/10 as powerful as a 5.0 Richter earthquake, and so on. The measurement scale for chemical pH is likewise logarithmic, a difference of 1 on the scale is equivalent to a tenfold difference in hydrogen ion concentration of a chemical solution. An advantage of using a logarithmic measurement scale is the tremendous range of expression afforded by a relatively small span of numerical values, and it is this advantage which secures the use of Richter numbers for earthquakes and pH for hydrogen ion activity.

Another reason for the adoption of the bel as a unit for gain is for simple expression of system gains and losses. Consider the last system example where two Amplifiers were connected tandem to amplify a signal. The respective gain for each Amplifier was expressed as a ratio, and the overall gain for the system was the product (multiplication) of those two ratios:

If these fgures represented power gains, we could directly apply the unit of bels to the task of representing the gain of each Amplifier, and of the system altogether:

Close inspection of these gain fgures in the unit of “bel” yields a discovery: they’re additive. Ratio gain fgures are multiplicative for staged Amplifiers, but gains expressed in bels add rather than multiply to equal the overall system gain. The frst Amplifier with its power gain of 0.477 B adds to the second Amplifier’s power gain of 0.699 B to make a system with an overall power gain of 1.176 B. Recalculating for decibels rather than bels, we notice the same phenomenon:

 

 

To those already familiar with the arithmetic properties of logarithms, this is no surprise. It is an elementary rule of algebra that the antilogarithm of the sum of two numbers’ logarithm values equals the product of the two original numbers. In other words, if we take two numbers and determine the logarithm of each, then add those two logarithm fgures together, then determine the “antilogarithm” of that sum (elevate the base number of the logarithm { in this case, 10 { to the power of that sum), the result will be the same as if we had simply multiplied the two original numbers together. This algebraic rule forms the heart of a device called a slide rule, an analog computer which could, among other things, determine the products and quotients of numbers by addition (adding together physical lengths marked on sliding wood, metal, or plastic scales). Given a table of logarithm fgures, the same mathematical trick could be used to perform otherwise complex multiplications and divisions by only having to do additions and subtractions, respectively. With the advent of high-speed, handheld, digital calculator devices, this elegant calculation technique virtually disappeared from popular use. However, it is still important to understand when working with measurement scales that are logarithmic in nature, such as the bel (decibel) and Richter scales. When converting a power gain from units of bels or decibels to a unitless ratio, the mathematical

inverse function of common logarithms is used: powers of 10, or the antilog.

Converting decibels into unitless ratios for power gain is much the same, only a division factor of 10 is included in the exponent term:

Because the bel is fundamentally a unit of power gain or loss in a system, voltage or current gains and losses don’t convert to bels or dB in quite the same way. When using bels or decibels to express a gain other than power, be it voltage or current, we must perform the calculation in terms of how much power gain there would be for that amount of voltage or current gain. For a constant load impedance, a voltage or current gain of 2 equates to a power gain of 4 (22); a voltage or current gain of 3 equates to a power gain of 9 (32). If we multiply either voltage or current by a given factor, then the power gain incurred by that multiplication will be the square of that factor. This relates back to the forms of Joule’s Law where power was calculated from either voltage or current, and resistance:

Power is proportional to the square of either voltage or current

Thus, when translating a voltage or current gain ratio into a respective gain in terms of the bel unit, we must include this exponent in the equation(s):

 

The same exponent requirement holds true when expressing voltage or current gains in terms of decibels:

However, thanks to another interesting property of logarithms, we can simplify these equations to eliminate the exponent by including the “2″ as a multiplying factor for the logarithm function. In other words, instead of taking the logarithm of the square of the voltage or current gain, we just multiply the voltage or current gain’s logarithm fgure by 2 and the fnal result in bels or decibels will be the same:

The process of converting voltage or current gains from bels or decibels into unitless ratios is much the same as it is for power gains:

 

While the bel is a unit naturally scaled for power, another logarithmic unit has been invented to directly express voltage or current gains/losses, and it is based on the natural logarithm rather than the common logarithm as bels and decibels are. Called the neper, its unit symbol is a lower-case “n.”

 

For better or for worse, neither the neper nor its attenuated cousin, the decineper, is popularly used as a unit in American engineering applications.

 

REVIEW:

·         Gains and losses may be expressed in terms of a unitless ratio, or in the unit of bels (B) or decibels (dB). A decibel is literally a deci-bel: one-tenth of a bel.

 

·         The bel is fundamentally a unit for expressing power gain or loss. To convert a power ratio to either bels or decibels, use one of these equations:

AP(Bel) = log AP(ratio) AP(db) = 10 log AP(ratio)

 

·         When using the unit of the bel or decibel to express a voltage or current ratio, it must be cast in terms of the an equivalent power ratio. Practically, this means the use of different equations, with a multiplication factor of 2 for the logarithm value corresponding to an exponent of 2 for the voltage or current gain ratio:

AV(Bel) = 2 log AV(ratio) AV(dB) = 20 log AV(ratio)

AI(Bel) = 2 log AI(ratio) AI(dB) = 20 log AI(ratio)

 

·         To convert a decibel gain into a unitless ratio gain, use one of these equations:

AV(ratio) = 10 AV(dB) 20

AI(ratio) = 10 20 AI(dB)

AP(ratio) = 10 AP(dB) 10

 

·         A gain (amplifcation) is expressed as a positive bel or decibel fgure. A loss (attenuation) is expressed as a negative bel or decibel fgure. Unity gain (no gain or loss; ratio = 1) is expressed as zero bels or zero decibels.

 

·         When calculating overall gain for an Amplifier system composed of multiple Amplifier stages, individual gain ratios are multiplied to fnd the overall gain ratio. Bel or decibel fgures for each Amplifier stage, on the other hand, are added together to determine overall gain.

 

components, Decibels, device, Electronic