Tools measurement and tolerances - Electronics

Created; 05/02/2015, Changed; 04/03/2024, 11/01/2024

Previous page; Control Loops

When you assemble things from components, sometimes the completed assembly will not fit together, and other times the components will go together so well that the assembled item is superior to normal.  Electronics components are now made with very good tolerance control so that in practice the extreme (worst and best) cases of the situation do not apply, defective parts almost never enter a legitimate supply line.  It is usually the easiest and cheapest way to put a bigger margin in the design so that assembly is consistently fault free due to the accumulation of tolerances.  So instead of using a sum of squares, use the sum of worst case errors or some factor between the two if that is not practical.

If you want a good camera now pick a make and model and the next box off the shelf will be fine.  When my Granny bought her Rolliflex in the 1940s, mum tells me she made a lot of fuss to get the one demonstrated to her rather than another off the shelf.  The point must have been significant for granny to tell mum and for mum to tell me.  But the story is generally very true and well understood until about 1980 then anything purchased had become consistent that is anyone off the shelf is the same as any other off the shelf.  The first Zero Defects programme was launched by the big semiconductor manufactures such as Motorola about 1980.

• To qualify my point, most designs use IC's that must be operated within all tolerances for each parameter; temperature, supply and other voltages and currents etc.  Therefore, those significant could be stressed parts, which should be identified, checked theoretically and maybe practically individually.   If it was necessary to put a problem like this aside, ensure it is added to your To-do list.

• Complex resistor networks are not always necessary and have always been best avoided due to their poor long term stability for analogue conditioning.  Much closer tolerance resistors of 1% are common now, where 10% tolerance used to be are commonly used.  Digital solutions can be slower to develop, but are usually better. 

• To repeat the point, things have changed.  Much of the design work has been moved to the component manufacturer over the past 35 years.  People still design simple discrete component solution's because they are cheaper and do the job.

• There are exceptions such as a low power sensor IC may be operated above its maximum operating temperature but below its storage temperature but consequently at lower performance.  Manufactures will explain how to first predict and qualify an IC this way.  Take care, I was advised how to do this and warned that the IC was glued to the package and that the glue would soften and that was the reason that there was an absolute storage temperature limit. 

• A circuit design become part of a system where trade-off between cost and risk is relevant.  The PCB assembly and manufacture its testability, mounting rigidity, heat etc. do need to be either reckoned or calculated and analysed statistically. 

Best tools for the job

If you are working on a higher performance instrument design, there may be no accurate test instrument as good as the instrument you are designing.  In any case the required test equipment for the particular job may not be available or may change the way an instrument under tests functions such as when considering using an oscilloscope even with x10 attenuator probes severely loads a high frequency and high impendence circuits by its capacitance, so it is important to consider what parts of a circuit you place the oscilloscope probe.  Either way, spilt the system up into parts that you can test in different ways so that you can figure out logically the likely performance in the parameter you are interested in.  In this case, build up a pattern of cross-checks that give you a subjective confidence.  Continue to build up the system you are developing cross-checking in the way I have described, and reviewing what you had done previously until the work is complete, and you can say what you have achieved and how much confidence you have in its performance.

In conclusion, your best tool is your reasoning, and to develop an ability to look at things in alternative ways. Nothing is absolute, even the accuracy of a measurement, but by building up a pattern of measurement with tolerances you will come to a high degree of certainty.

Worst Case Design and understanding tolerances

This is about understanding shades of gray.  To start with, Sum the squares of the errors, to give you a figure of merit. You should, but it is not essential, square the sum of rooted numbers to give a meaningful value.

Super-position, I did not use this for solving resistor networks but to find the constants from a number of test points for a polynomial equation in the form y = a + b.x + c.x^2 + d.x^3 ... This way an equation could be found to fit a data set forming an X-Y graph.   Depending on the number of terms resolved using 2, 3, 4 or other by the number of points term determinants.  The problem was rationalised a little by summing the data, the squares of the data, the cubes of the data, but the number of points required grows exponentially so it is necessary to limit the number of terms and pick the most useful terms.  I subsequently learnt this is quantum maths that is used to solve multiple, variable problems like this.  But this was 40 years ago, and I have now forgotten much of what I developed then. 

The resistor network calculation was a lot of work, and I developed programs written in BASIC that by trial and error found the best preferred value set of resistors to best meet the requirement.  Later solution was to breakup and solve smaller networks and do any arithmetic in the microcontroller.  So what I needed to do could be done using mental arithmetic, a calculator, pen and paper, entering preferred values. 

So to sum up; nearly all circuits with resistor networks can be split down to two resistors networks which come down to a voltage or a current source with an impedance, which you then load with the next resistor in the network.  In fact, if you can avoid resistor networks that is even better, resistors age and have temperature coefficients.  Spreadsheets are very useful for comparing and changing a simulation. 

My point is that the best lesson in tolerating and getting a feel of circuit function and actual behaviour is to do the maths, until you feel it, without having to go through everything to prove it.  Once you are, there, use a simulator rather than spreadsheet if you want. 

Often working out the solving equation is difficult, but there are often short-cuts that suffice for now and may be satisfactory.  

For example, 4 = 2 x 2 but if multiplication is difficult 2 + 2 addition may suffice.   I can not think of an example where this case has been useful.

A small angle triangle which has two long sides can be treated as a right angle or an equilateral triangle, and you can use sine instead of tangent, to solve your problem. 

An attenuator attenuating by 10% so pass 90% of the signal, but if the signal passes through two attenuators then roughly 80% using addition of the signal remains is a good approximation, although 0.9 x 0.9 is 0.81 (81%) is the correct answer. 

Look for shortcuts and use them, but also check their validity.   Shortcut approximation can be applied in computing where simple arithmetic is much faster than carrying out more complex maths, such as inside an interrupt routine. 

During World War two fast calculation for gun ranging was carried out using tables but synchros and servos systems were developed which only used addition to calculate the ranging of big guns. 

If you do start with a simulator, have a guess and do some arithmetic to support your guess first, use the simulator to test your guess.  If there is a disparity, prototype it and see which is correct and analyse it to understand it.  I have never used a simulation tool for anything I have designed and had put into production. 

I recommend that you download and use any spreadsheet or other models that an integrated circuit or other component the maker provides, such as for a switch mode power supply coil maker and controller manufacturing, provides they are well tested and good.  But give yourself some margin, the IC makers tend to specify to just the limit.

I find thermal modelling from data sheets difficult.  An IC won't dissipate much without an area of copper.  Hardly anything can be dissipated if the IC is faced down, face up is better, and with the board stood vertically is best.  I have found exposed tinned or silvered copper better than with solder resist covering a copper area.  Tinned or silvered copper does go mat white quickly, which could mean the board tends to improve with age slightly.  I have heard others contradict my observation on removing solder resist, so try it for yourself. 

A mat black anodised heat sink is 50% more efficient than an untreated aluminium heat sink.

---------------------------------

You can find components on a PCB that are emitting high frequency radio interference.  I found as I expected that the linear regulator was oscillating.  xx78xx regulators oscillator at low amplitude 55MHz, but I doubt that those are made now?  A solution I was told of was to fit bead chokes in the input and output leads.

-------------------------------------------------------------------------------------

Cannon AE-1

This camera uses a very low power Microcontroller made by Texas Instruments, with Ferranti's IIL (I squared L) low voltage transistor Intellectual Property. 

My father bought his first AE-1 in 1976 with the 50mm lens when the camera was first launched.  The camera was particularly popular with amateur photographers for at least a decade.  The A-1 was launched later and was the professional version.

I find this camera has everything to hand and only features that you need, and it does all that you expect of it well.  The design was not compromised in order to persuade you to purchase the next model but to keep the commercial pressure, on any remaining German or Nikon camera supremacy.  A classic camera, surely?

This camera made photography consistent.  To achieve the correct exposed pictures.  The AE-1 cost about £250 and by comparison a teenage electronics technician would be earning £19 to £24 weeks before deductions.  The camera has a low power Texas Instruments microprocessor in which one of the significant Intellectual Property parts was that it used Ferranti I2L (I squared L) low voltage transistor integrated circuits.  These I2L was developed in 1973 and probably continued being used in watch IC's for the next 20 years?

The right-hand camera; Clockwise from the top.

• Cannon 50mm, F1.8 lens - Normal perspective.

• The cap covers the flash connector - I use the hot shoe.

• Little silver button - Increases the exposure by 1.5 stops.

• Red dot on the side of the lens - With the lens unlocked, turn the lens so that the red dot lens is at 12-o-clock to fit or remove the lens.

• The black button - Preview the exposure and is the same as pressing the shutter release down halfway.

• The Slide switch on the body - is for stop down metering use with manual lenses.  Dad told me not to use it, it will damage the camera, but I have used it with a Signor 105mm, F2.8 lens.  Reading the quaint Japanese English manual carefully first.  You can adjust the aperture stopped down until the needle in the viewfinder lines up with the mark.  (The manual lens has a ring to open up the aperture for focusing - remember to close it back down before taking the photo)

• Silver button on the side of the camera - Lens lock - Press to release the lens.  I read that this lens mounting is noted for its lack of wearing compared to later models such as EOS.

• The camera base you can see the tripod/flash bar mount, The film wind back release button, The coupling for film motor accessory.

Left Camera;

• 35-105mm F3.5 macro-zoom lens.  These were made from about 1985 and use a significant Polaroid moulded lens technology (IP of 1980) to make aspherical glass lens elements.  A traditional spherical lens focuses on a radius, not a flat plane of the film.  I think the glass was ground and polished in the traditional way to make a spherical lens, then the glass was heated and pressed into a mould to reshape the glass to aspherical shape.

• The battery is 6V and is under the cover at the side of the lens.

• The speed setting ring under the wind on leaver can easily be moved whilst looking at the meter in the view finder for aperture priority photography or stop down manual lens photography.  But the camera is a shutter speed priority camera with an automatic lens.

• You lift another ring and rotate it to set the film speed.  That is a bit fiddly.

• The wind on wind back and film loading is free of binding and as good as it gets for loading the 35mm film cassette reliably in any 35mm camera.

• The black button is the shutter release, you can screw a shutter release cable into it.  Press the button halfway to down to preview and lock the exposure with the meter in the viewfinder.

• The leaver is set to lock - unlock - self-timer, which causes the LED to flash for several seconds.

• The button in the other side of the flash hot shoe is battery check.  Look in the viewfinder at the meter needle.

• There is a circle with a line through it mark which indicates where the film is and where you would measure the focal distance from if you were to set the focus that way. It is important for close up large aperture photography.

• Wind the film back handle when you have come to the end of the film - first press the button on the base of the camera to release the film.

• Camera from the back has a clip to hold part of the film box with the film rating and type.  The viewfinder has the meter, manual warning LED, exposure warning LED, Focus, clear screen, Fresnel ring and split image.

Significance difference between these two AE-1 cameras is that there is no difference between them.  They were different to British, European, American products in the respect that any two of the same product would be the same by comparison. 

I took the photo with a digital Cannon IXUS 60 - that camera takes comparable quality photos.  The camera's flash has done well, I had zoomed to x3 and photo edited to crop and turn the picture black and white.

-------------------------------------------------------------------------

Important points on testing;

1. Consider the symptoms and what might be the cause,

2. Test your theory and avoid disassembly,

3. Half split, see if the signal gets halfway or gets from halfway to output,

4. Repeat half splitting until you identify the likely cause,

5. Fix the fault.

Useful Links;

I have variously been recommended and received criticism of the following modelling tools (which are free or limited demo's); The schematic and circuit board layout is the most significant factor and CAD that you can work with is important.  I use power supply component maker's modelling tools, particularly for wound components they are very good.  I use spreadsheet or perhaps basic program based tools provided by manufactures of inductive or integrated circuits such as for switch mode power supply design, these are very good.

Conversely, the discrete circuits on the previous pages could be modelled rather than prototyped, and I can see a benefit with the high voltage circuit, if you wish to see how fast it can operate accurately.  That was not a requirement, though.

• LTspice http://www.analog.com/en/design-center/design-tools-and-calculators.html?domain=www.linear.com#LTspice - The user in not tied to LT products.  This tool supports any manufactures spice models that are not encrypted and conform to spice standards.  The tool is less flexible about adding parts but works well enough and the tool is very well-supported.

• Tina-TI http://www.ti.com/tool/tina-ti - It is not the case now that a user must have one TI part on their circuit.

• Tina http://www.tina.com/ - The not Texas Instruments version of the tool.  The demo version does not allow outputs or any work to be saved. 

• SIMetrix http://www.simetrix.co.uk/site/index.html -  This has been recommended to me over the other two above.  There is a free version that is very good.

To discuss electronics, see; Blog page Electronics 

Next Page; Software and Digital