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The “Not So Simple Guide” to choosing resistors - Part 5

Sat, 06 May 2023 17:07:28 GMT

Understanding tolerance parameters on resistor datasheets.

Hi fellow Electronics Engineers.

In this blog we’ll discuss how manufacturers represent tolerance on their datasheets and how to interpret these parameters for your design.

Initial tolerance

The first, and most obvious stage, when calculating the overall tolerance of a resistor is to consider the percentage that results from the manufacturer's ability to produce a resistor with accuracy. This initial tolerance (as it is often referred to) is intrinsic to the resistor technology. If you haven’t previously read The “Not So Simple Guide” to choosing resistors - Part 2 , this may be a useful starting point to introduce yourself to some of the different types of resistors that are available.

Note this may come as a shock to you, but nothing in the scientific universe is perfect. Yes, I have excluded the arts, as subjectively some works of art may be perfect. However, in the scientific realm, nothing is perfect, not even you cupcake!

When selecting a resistor from your preferred distributor, you will likely see an option that corresponds to initial tolerance. However, this is not the only factor to consider when choosing the appropriate resistor for your design.

Temperature Co-efficient

Remember back in high school science class how we learned that materials change their properties when exposed to heat? Well, resistors and their resistance properties are no exception. When you change a resistor's ambient temperature, its resistance changes too. Normally, a resistor manufacturer states that the initial tolerance is given at room temperature (25°C). However, if the temperature of the resistor increases due to environmental factors or self-heating, the resistance will also increase. This is called a positive temperature co-efficient or temp-co for short. Conversely, if you decrease the temperature of the resistor, the resistance will decrease.

Note not all electronic components have a positive temp-co, some have a negative temp-co i.e. as they heat up the parameter in question reduces. We will cover this topic in a later blog.

The temp-co is normally shown on the resistor datasheet as PPM ( P arts P er M illion). It is normal in the scientific community to represent dimensionless small units in such a way; 1 ppm is just another way of saying something is 0.0001 % of a value.

When you are designing a circuit; hardware; or electronic system, there are typically requirements for the operating temperate range it can deviate across. For instance, most consumer electronics are designed to operate in houses and have a sympathetic operating temperature range of approx. -10°C to +40°C. Since manufacturers gives the temp-co in ppm, you’ll need to calculate the error that can be attributed to change in operating temperature. Let’s say a resistor with a temp-co of 100 ppm operating at 75°C. That is a temperature delta of 50°C, which results in a temperature related inaccuracy of (50 x 0.01%) 0.5%.

Ageing Co-efficient

‘There is nothing permanent except change’ – Heraclitus

Time not only affects our bodies, but it also affects everything in the known universe, including our resistors. That’s right! A resistor we select on day one will have a different resistance value 10 years later. We call this phenomenon the ageing co-efficient, or age-co for short.

With reference to The “Not So Simple Guide” to choosing resistors - Part 2 we looked at different resistor technologies. Start with thick-film resistors, they're a good and low-cost option for most users, especially in digital applications like pull-up resistors. I can’t think of any reasons why you’d want to look any further. But if you're working in analog and need the gain of an op-amp to remain stable for a long time, like 20 years, then thick-film resistors might not be the best choice. For instance, a typical thick-film resistor, operating at an average temperature of 70°C, will have an extra 1% after just 1000 hours (we normally say 10,000 hours = 1 year). The next problem is the industry can’t agree on a mathematical model that allows us to extrapolate this trend. So we rely on statistical look-up tables for long term predictions, and I have personally seen data that suggests a 1% thick-film resistor after 25 years can increase up to 15%.

To understand why thick-film resistors undergo significant changes over time , we must consider the manufacturing process. Thick-film resistors are made by pasting a conductive material onto a ceramic substrate (similar material to a coffee mug), which is cost-efficient but relatively low-controlled.

If you require long-term stability, may I suggest considering a different technology such as thin-film resistors. They start manufacturing in a similar way to thick-film resistors, with a ceramic substrate, but the rest of the process is quite different. A dense, uniform metallic alloy layer is deposited onto the ceramic base under a vacuum, resulting in a resistive material thickness of approximately 250 Angstroms (1 Angstrom is 1.0E-10 m). This significantly improves all the sources of error that we have discussed in this blog. The initial tolerance for a thin-film resistor is more likely to be 0.1% instead of 1%, temp-co is commonly 25 ppm, and for age-co after 1,000 hours at 70°C, it is 0.05%. However, the downside is the cost. Thin-film resistors will set you back 10 – 20 pence each, whereas a thick film costs less than 5 pence.

The icing on the cake regarding thin-film, is that industry has established a model using the Arrhenius equation developed by Dutch chemist Svante Arrhenius, that allows us to predict long-term ageing trends for thin-film resistors.

Note this is only an approximate model as in reality the ageing effect is really a random process. Often, we describe this phenomenon as the ‘random walk’. However, this method is normally acceptable for most engineering calculations.

The equation clearly shows that the long-term stability of a resistor is heavily influenced by the average operating temperature. In this example a temperature delta of 30°C results in an age drift factor of 2.

There are several other resistor technologies available that offer higher accuracy at a higher cost. For instance, a bulk metal foil resistor can have an initial accuracy of 0.005%, a temp-co of 1 PPM/°C and a life-stability of 0.015% (70°C, 10 khrs). However, these specialised resistors can be expensive costing approx. £40 each (as of 2023).

Environmental effects and overload

By now, it shouldn’t be much of a surprise that your resistor’s tolerance will be impacted if it’s exposed to any of the following environmental factors (this list is non-exhaustive): thermal-shock, vibration, mechanical-shock, voltage overload pulses (including electrostatic discharge), and power overload pulses.

Once again, it's important to note that different resistor technologies behave differently when exposed to these stimuli. Therefore, it's crucial to consider the technology of the resistor and its behavior in the environment before committing to it.

To worst-case, or not to worst-case, that is the question.

As we have discovered, or reminded ourselves, there are many sources of inaccuracy in a simple resistor. One might think that the correct way to proceed now is to sum up all the sources of error and accept the total. Summing all the error sources we have discussed in this blog would give us the following formula: -

The aforementioned is commonly called a worst-case analysis.

Note if you hear any engineers referring to this as a ‘worse’-case analysis, please correct them as it is called a worst-case analysis. Please remember the dictionary defines worst as ‘of the poorest quality…’, whereas the word worse is defined as ‘of poorer quality…’. Therefore, something can be worse compared to another. However, when something is as bad as it can get then it is the worst it can be.

However, I would encourage you to think about whether it is reasonable to sum up all sources of error. This is both a statistical improbability and quite often ‘breaks’ a perfectly good design. Therefore let us look at an often used statistical analysis tool: -

Considering how the normal distribution (or Bell-Curve) is found, we can refine the worst-case analaysis using a RSS ( R oot S um S quare) technique, to give us a more practicable tolerance model for the resistor: -

Hopefully, now you have a good starting point that will aid you in understanding how to calculate resistor tolerance for your design and how changing manufacturing technology can improve circuit accuracy.

For now, I bid you farewell and hope you are looking forward to the next instalment.

Until the next time... ut vis vobiscum

The “Not So Simple Guide” to choosing resistors - Part 4

Wed, 12 Apr 2023 07:17:53 GMT

Understanding voltage parameters on resistor datasheets.

Hi fellow Electronics Engineers.

To date in this resistor selection blog series we have discussed EIA decade values , physical properties and power ratings of resistors. Now let us look at how manufacturers represent voltage on their datasheets and how these parameters should be interpreted for your design.

Rated Voltage

Another quick reminder, a Volt is the electrical unit for potential difference between two points in a circuit. In SI units it is how much energy ( J ) per unit of charge ( C , the coulomb).

Note In case this is the first time you have read SI units, here is a description of what SI means, courtesy of the National Institute of Standards and Technology ‘The International System of Units (SI), commonly known as the metric system, is the international standard for measurement. The International Treaty of the Meter was signed in Paris on May 20, 1875, by seventeen countries, including the United States and is now celebrated around the globe as World Metrology Day’.

Please take note, the physical size of the component is a key driving parameter to dimensioning what most resistor manufacturers describe as ‘rated voltage’ (this is not the only parameter, but we will discuss later). This is basically what voltage the resistor can electrically accommodate across its two terminals, with either a static D irect C urrent (DC) operating voltage, or a R oot M ean S quare (RMS) A lternating C urrent (AC) voltage.

Remember, normally the manufacturers datasheet ‘Rated voltage’ does not include any de-rating (none of the parameters do as they are normally stated as absolute maximum values). i.e., it would be wise to make sure you have some design margin to ensure the component does not go anywhere near its maximum rating.

De-rating is common practice in many fields of engineering and helps ensure the design is reliable. I will detail good de-rating practices in a future blog. However, for now, I would suggest you choose components that have at least 30 % margin. For example, a 100 V component should never have any constant voltage greater than 70 V applied across it.

Permissible voltage

The last aspect to consider with respect to datasheet interpretation, is ‘Permissible voltage’. This is slightly different to the rated voltage, as now the datasheet introduces a time element. For instance, you may see a component that is rated for 75 V (non-derated) operating voltage, but also has a permissible voltage rating of 100 V for 1 minute. This dimension relates to something we often call the ‘transient operating condition’.

In engineering you will commonly see / hear the terminology static conditions, interlaced with transient conditions. With respect to voltages, a static condition generally refers to operating conditions that are stable, think about the voltage from a bench power supply. Now think about the observed voltage of a lighting strike in the time-domain (if captured on an oscilloscope). This is an example of a transient condition.

Note For a rule of thumb in electronics engineer we refer to events lasting longer than 1 second as static conditions. If the event lasts less than 1 second, then conversely, we refer to this as a transient condition..

Often designers must consider both operating scenarios. Therefore, the component manufacturers normally provide information about how the component can be ‘pushed’ for short durations. In later blogs we will explain the physics of what happens and how we can model / analyse these transient conditions.

What is the difference between DC Voltage, AC Voltage and Transient Voltages

As a slight deviation from the task of exclusively imparting knowledge on how to select a resistor, let us make sure you are comfortable with some basic understanding regarding how we describe different voltages.

Okay, let us start with a DC voltage source, this should be the easier one for you to comprehend. There are many common voltage sources including power supplies; linear voltage regulators; IC (Integrated Circuit) outputs and batteries etc. All these devices operate as a constant-voltage sources (also known transimpedance), i.e., considering a battery, if you place a load across the battery terminals, the voltage should remain constant (ignoring the battery discharging, or internal resistance causing the voltage to drop etc.).

However, for an AC voltage, e.g., your mains outlet in your house, this voltage is a sinusoidal waveform (as it is represented by the mathematical function sin, that you may remember from high-school trigonometry). This is because the voltage was generated from a rotating machine (we will cover this in detail in a separate blog). As the machine rotates, the voltage that is generated fluctuates.

Normally when an AC voltage is written we write it as its RMS value. There are many sources available online to show you how to derive RMS (of which I am sure you are capable of reading about), but for brevity, if you want to convert to or from a peak (pure) sinewave to its RMS voltage then here is a quick example

Note A word of caution, the derivation in the video shows that to calculate the RMS voltage from a peak voltage you simply multiply by 0.707 (or from an RMS to a peak, multiply by 1.414). Remember this is only true for a sinewave, any distortion or other waveforms will not yield the same results.

Engineers don’t normally use the mathematical average function when representing sinewaves as they have the same amount of area under the curve on the positive half (integration) of the cycle as it does on the negative half-cycle. Therefore, if you were to average these two halves out the answer would be zero. Try telling that to every person who has experienced an electric shock from a home appliance!

Note for the eagle-eyed you may have noticed in engineering textbooks that you see voltages represented in both upper and lower case, which may look contradictory to the note in power ratings . This is intentional as when we switch to lower-case in our mathematic notation (i.e. v instead of V), we are informing the reader that there it is a function of time (transient) as opposed to a static condition. For instance you may charge a capacitor from a DC source, the voltage of that source would be #.## V, but as the capacitor charges with an exponential time function (to be discussed in a future blog), it would be normal to say that the voltage at any given point in time is #.# v. To summarise, DC voltages and RMS voltages use upper case, transient voltages use lower case, what could be simpler?

Throughout Europe, houses commonly have 240 Vac electricity. To be precise, that is 240V RMS. The actual peak voltage is 340 volts.

Note Peak voltage is often abbreviated to pk. Also, peak-to-peak voltage is often written as pk-pk.

Back to our blog thread with respect to voltage. When considering what resistor is suitable for your circuit application, make sure you consider the peak-voltage, otherwise you may inadvertently choose a component that is not cable of handling the circuits voltage.

More to follow on this subject, but for now I would also consider placing multiple series resistors when connecting to line, so that if one component fails short-circuit, the rest of the circuitry (or the user / operator) doesn't inadvertently become exposed to the 'high' voltage.

For now, I bid you farewell and hope you are looking forward to the next instalment.

Until the next time... ut vis vobiscum

The “Not So Simple Guide” to choosing resistors - Part 3

Sun, 26 Mar 2023 21:00:00 GMT

Understanding power dissipation on resistor datasheets.

Hi fellow Electronics Engineers.

Note Let me state for the record, I wish to make no illusion about the field of engineering. If you genuinely wish to pursue design engineering, as a rewarding career, you will need to develop (if you haven’t already) both a strong ability in maths and a passion for physics. However, I believe, that today’s society has so much untapped talent, who have been scared away by academia, as those individuals have school results that are not in the top 1 or 2 % of academic achievement. Many of these students believe they do not have a place at the table of engineering. This, in my opinion, must stop immediately. Industry is (and it will only get worse) in dire need of an army of worker bees. From my experience the backbone of industry is seldom Ivy league university folk. Fundamentally, If the services and products our engineering community develop is primarily from the 1% or 2 % portion of society, who were fortunate enough to come from an Ivy league institution, then due to homogeneity, we will stagnate at a societal level. We must encourage and embrace all those who whish to join us, and I mean all ( including the 1% or 2% people). If someone tells you that you can’t, prove them wrong and show them that you most definitely can!

Now let us continue from the previous blog, where we discussed ‘ How to choose resistors based on physical properties and intended manufacturing methods .’

For now, I would like to continue the same thread of how to select a resistor.

Calculating power

Just a quick reminder, the unit of Power in SI (international system of units) is the Watt and it is equal to 1 Joule (the SI unit for energy) per second. A Watt is basically the amount of work done per unit of time.

Note The observant amongst you will notice the words Watt and Joule have their first letters in capital letters. That is because these words are named after famous scientists. Yes, there was a Mr James Watt, he was an inventor whose steam engine was essential for the industrial revolution. Also, there was a physicist called Mr James Joule. However, there was no Mr Kilo nor a Mr Gramm/e (not that history assigns acknowledgement too anyway) so we don’t write capital K we write lower case k, for instance kg not KG.

I freely admit that so far in this blog series, I have shied away from giving you Ohms Law. However, just to make sure you have a source at your fingertips, I remember when I first started my journey as an electronics engineer, I found this style of graphical representation very useful.

Mastering the transposition of Ohms Law should place you firmly on your path to developing the skills necessary to calculate what the power dissipation of components in your application circuits is.

In due time you will also learn how to use more advanced analysis methods like Kirchhoff or Thevenin equivalent circuits, or maybe a simulation package may aid more complex circuits. However, these more advanced analysis techniques are the subject for future blog(s), and I would not recommend you attempt to understand these techniques before you have a comprehensive grasp of Ohms Law. Also, I would encourage you to not solely rely on simulations, it’s important that you understand what the simulation output is attempting to convey, after all, G arbage I n G arbage O ut (GIGO).

Note: Power is mostly dissipated as heat in a component. Power is NOT consumed! Cake is consumed. Power, by the law of energy conversion (yes physics is fundamentally important), can neither be created or destroyed, it can only be transformed into another form of energy. Therefore, anyone who asks you what the ‘power consumption’ of a component is fully deserves an eyeball rolling!

Selecting a components power rating

From the working example above, we now know we require a component with a minimum power rating of 8.7 mW.

Hopefully, by now you have grasped the power the component is dissipating will (mostly) be dissipated as heat and before we can say what the power rating of the component will be, we must consider what temperature rise we want the selected component to be capable of operating with.

To understand what the target temperature should be, you may want to consider the following: -

A) What the maximum ambient temperature of the overall assembly is.

B) Internal heat rises caused by the dissipation of local components (maybe not on the PCB you are designing, but inside the same chassis).

C) If there are any maximum component operating requirements for the design.

D) If you have a maximum temperature difference delta (∆) of any component.

Here’s some advice where you may be able to find answers to consider.

A) The maximum ambient temperature is normally given to us as a requirement or maybe relates to some industry recognised standard like ETSI or MIL-STDs (the subject of future blogs). i.e., electronics that operate in the home often are specified to work across the temperature range of 0 °C to 40 °C. However, some aerospace designs need to work from -55 °C to 125 °C.

B) A basic method for this is given in the example below. More advanced methods include dedicated thermal simulation software (more to come in the future).

C) Quiet often there may be a need to keep a component temperature below a limit. For instance, the circuit you are designing may be specified to work up to 40 °C. However, if it is a hand-held device (think smartphone) and when you turn it on, the device self-heats to 80 °C and causes 1 ^st degree burns, do you think that is acceptable?

B) The operating temperature is intrinsically linked to how reliable that component will be. More to come in future blogs, but the life of a component is halved for every 10 °C.

As a starter for 10, considering the above points you may think this design you are working on, has to work on an office desktop (or lab) so up to 40 °C ambient sounds ok. Then the components around it (heatsinks, or maybe fans on etc) mean the component could be 50 °C before any power is dissipated. Finally, everything considered you don’t want this component to get hotter than 15 °C delta.

Back to the example resistor calculation where we had to manage a power dissipation of 8.7 mW. Reality is for a 1/4 W rated resistor operating at room temperature you are on a home run. However, if this was a 100 Ω, then that would be 1 W of power. It should be obvious that attempting to set a resistor to 4 x overstress is unacceptable, in this instance you would be required to fit a resistor with a larger power rating.

Note: do not attempt to put 10 volts across a 100 Ω 1/4 W resistor! If you do, you'll quickly see it start to smoke. The smell is very distinctive. Also, if you ignore the previous advisory, please don't hold the resistor in your hand while you do this! If you are the kind of specimen that ignores both advisories, then I suggest we let Darwin sort you out.

Selecting the correct power rating for your design

As an example, let’s attempt to finally nail down the power rating of the component for our resistor. Below is an extract from a resistor datasheet: -

Let us start with an example of a typical manufacturer ‘standard electrical specification’

The first highlight is orange, the manufacturers offer a 0.25 W resistor. Great start!

Second highlight in red, this component works all the way up to 165 °C. Therefore, you think your job is done and you are done with respect to thinking about power. If you were looking for a resistor that needs to dissipate 8.7 mW, then I would tend to agree with you.

I would always encourage you do a stress analysis (to be discussed in a future blog). Therefore, assuming you want to be thorough, I will attempt to take you through to the point where you have checked all there is to check.

The next item to look at is further into the datasheet: -

On face value, the resistor has a maximum power rating of 0.25 W (highlighted in red). However, notice in the heading column the manufacturer is giving you a little clue that all may not be that simple with the wording ‘POWER RATING P70 W’.

Let us look at what the manufacturer is telling us here: -

From this graph we can see that the component can accommodate its full rated power up to 70 °C. However, once you start going over component ambient temperatures (not assembly ambient temperature) the component maximum power capability has to be reduced.

That is because of the self-heating of the component. For instance, if the component ambient is 125 °C, then the 0.25 W resistor can only dissipate 0.1 W, before the component temperature is actually 165 °C.

Therefore, if you know your component is required to work in elevated operating temperatures, please ensure you check the component is capable of dissipating the power you think it is.

Finally on this graph, we will return to it later when we start to unpick basic thermal modelling of our designs. As this is a blog, I will park that as a teaser for now.

Extra De-rating for improved reliability

Please remember, the datasheet ‘Rated Power’ does not include any de-rating (none of the parameters do as they are normally stated as absolute maximum values) required to aid product reliability.

Assuming you would like the design, you produce, to last longer than a few minutes (I will explain how temperature directly links to reliability in a future blog), it would be wise to make sure you have some ‘design margin’ to ensure the component doesn’t go anywhere near its maximum rating.

De-rating is common practice in many fields of engineering and helps ensure a design is reliable (I will detail good de-rating practices in a future blog). However, for now, I suggest you choose components that have at least 30 % margin. For example, a 0.25 W component should never have any constant power greater than 0.175 W.

Also note, this reliability de-rating is extra to any thermal de-rating you may have identified following the guide earlier in this blog.

Later in the blog series, when you understand how to perform basic thermal modelling and stress-analysis, we will check what the worst-case operating temperature of the component is in your design.

Until the next time... ut vis vobiscum

The “Not So Simple Guide” to choosing resistors - Part 2

Mon, 06 Mar 2023 21:37:57 GMT

How to choose resistors based on physical properties and intended manufacturing methods.

Hi fellow Electronics Engineers, I hope you are all well and enjoying your journey of discovery.

Why am I doing this?

Following on from the previous blog, The “Not So Simple Guide” to choosing resistors - Part 1 , where we discussed the ‘Standard EIA Decade Values Table’, I would like to continue the same thread of how to select a resistor, this time focusing on physical properties and intended manufacturing methods.

I trust you are comfortable with using the internet in general, and maybe already have a favourite online component distributor. Intentionally, I won’t mention my go to distributor, as I don’t want to open the gates of hell debate on which one is ‘best’, I will only say which ever works for you, or your company, is the best answer here.

Note the term ‘best’ is quiet often used in society as a qualitative. It will serve you well as an engineer to constantly remind yourself that engineering is an extension of the school of sciences. Likewise, the whole world of science is governed by quantitative. i.e. something we can agree on like a test or measurement procedure, as opposed to something we subjectively say based on its perceived quality. Therefore, when someone says to you, “do what’s best”, be aware this is very much open to interpretation.

If you open your favourite component distribution webpage and ask it to take you to their passive component resistors section, then you will most likely have almost 1 million components available to choose from. With the previous blog, we know to search for resistors of value 11.5 kΩ. Depending on the distributer you are using, you should now have reduced the offerings to approximately 500 components.

Therefore, my experience tells me that the next step in realising the required component should be to consider the physical form of the component.

Through hole vs Surface Mount Device vs Chassis Mount

As is with most passive and active electronics components, we are normally offered these in a few physical groupings.

1. The ‘through hole’ set (sometimes called axial or radial - we will explain in a future blog).

This is the group that has leads intended for placing through a hole in your Printed Circuit Board (PCB) or prototyping board.

2. The S urface M ount D evice (SMD).

This group is intended for surface mount soldering.

3. The ‘Chassis Mount’ form factor.

This group is intended to be bolted (or fixed) to the assembly housing.

Note One of the most daunting things I found, when I joined the engineering industry, was the prolific use of acronyms. However, I would encourage that if a colleague says something, including an acronym, that you don’t fully understand, then don’t be afraid to ask for clarification. From my experience, most engineers are only too pleased to help you unlock the knowledge. Therefore, if in doubt ask! Finally, remember you can’t have too many TLAs – Three Letter Acronyms and FLAs – Four Letter Acronyms.

When you consider the physical group, you are selecting according to appropriateness for your design , as ever with engineering, you must consider which properties are to be traded against. The more you perform these ‘Trade-Studies’, the more you will start to (or already) understand that some properties are mutually exclusive.

One of the most obvious in this instance of trades, is how increasing the physical size of a resistor, also allows us to increase the maximum power dissipation property of the component. However, if your conceptual ideal resistor is a physically small component (as that best suits your available packaging volume) but requires a large power handling capability, I would recommend that you stay off your nannas extra-strong cough medicine as it’s playing havoc with your cognitive reasoning.

Through Hole

Whilst not as common as SMDs are for mass manufactured assemblies, I find them extremely useful for prototyping circuits. Although I am sure you will gain the skills, (if you haven’t already done so) to allow you to construct your own prototypes using SMD parts, I assure you, for low component count circuits, which are relatively low-tech circuits (low frequency; low power etc), a ‘Vero-board’ or even prototyping stripboard and some through hole resistors can work wonders in the proof-of-concept evaluation phase of a development project.

Through Hole Sizes

Normally, through-hole resistors are described purely on their power ratings. For instance, if you are talking to a veteran hardware engineer, and they request that you to fit 1/4 W resistors. They are probably asking you to use a through hole resistor. That means all decisions about physical size and voltages are decided by that one request.

There are more exotic through hole components like vitreous enamel, carbon film or even resistors designed to be used in military applications that are made to MIL-PRF-39017/55182 (I will cover MIL-PRF components in a future blog).

Through Hole Arrays

Another popular variant of through hole resistors is called the ‘array’. An array is where one package may contain several resistors. Arrays are useful when there isn’t a lot of space in a design, and you are investigating ways to increase the packing density of your assembly. Also, sometimes arrays have their terminals combined (tied together). An example of where a tied array is useful may be when you must fit a lot of pull-up resistors (I’ll explain the various names associated with resistors in a future blog) to a design e.g., a data-bus of a microprocessor.

Surface Mount Device (SMD)

In modern automated assembly lines, most PCB assemblies use SMD devices (not just resistors). Fundamentally, (not the only reason, but we will discuss this later), this is because SMD devices are ideal for automated assembly. This is because they are normally supplied as 7 or 13-inch diameter reels that are specifically designed for loading into machines referred to as ‘pick-and-place’ machines, which are the mainstay of PCB assembly lines globally.

SMD Sizes

When we are describing which SMD resistors to fit, we normally talk about the length and width of the component in inches. For instance, an ‘0603’ resistor has the dimensions of 0.06 x 0.03 inches, or in metric, 1.6 x 0.8 mm.

I recommend if you intend to prototype using SMD deceives you don’t go below an 0603 (there are 3 smaller sizes I know of 0402, 0201, 01005). This is because its relatively easy to fit or change an 0603 resistor by hand. Conversely an 0402 (or smaller) is exponentially more challenging.

Note For the younger engineers reading this blog, you may question why a lot of what you are looking at in the world of electronic engineering is still specified in imperial units like inches and ounces (oz). That is primarily because America dominates this sector in standardisation and many component designs, were or are owned by American companies. I understand you have possibly spent your entire life, up 'till now, using the metric system, but the reality is you will also need to get to grips with imperial system.

Like their through hole counterparts, SMD resistors are also available in array packages, with and without tied connections.

Chassis Mount

Physically the largest group, chassis mount resistors are the go-to part if large amount of average, or transient power requires dissipating. This doesn’t mean they can’t be mounted to a prototype stripboard or even a PCB as they often are. However, they are normally hand assembled, which is something to consider if you intend the design to be a high volume assembly.

Time to decide…

Over to you now readers. I hope you have been thinking about what the intended assembly method of the component is, and what power dissipation you believe the component must be capable of handling.

Until the next time... ut vis vobiscum

The “Not So Simple Guide” to choosing resistors - Part 1

Wed, 22 Feb 2023 20:37:31 GMT

How to choose the right resistor for your engineering design from experienced hardware electronics designer and Technical Director, Adam Poole.

Whether you are just starting out as a budding hobbyist dabbling in electronics hardware engineering or a veteran industry lifer who has honed their craft for decades, it doesn’t matter, there is always an opportunity to learn something new in the field of electro nics.

Why am I doing this?

Having served industry for more than 30 years, I am pleased to say, ‘I’m still learning’. However, I do feel a debt on my shoulders. The debt is to those who helped me, by mentoring and training me, so I think it is well over-due that I gave something back to the industry I love. I am also extremely fortunate in that I served an old school industrial apprenticeship in parallel with studying to gain my qualifications. One thing that stood out was that University was great at teaching me how to analyse a design, but at no point was I really taught how to design, which I believe is essentially a creative skill and not a technical one.

Therefore, if you will let me, I would like to spend some time passing on the collective wisdom I’ve gained throughout my career. If you are an experienced hardware designer and wish to add to this wisdom, please do drop me a line I’d love to hear your thoughts.

Without further ado, I’d like to start with resistor selection. I can’t think of a simpler yet more essential starting point than the humble resistor.

Populating a BOM

If I were to state, from my experience, that your average B ill o f M aterials (BOM), is likely to be ~50% resistors, then I hope you agree that it is essential that all electronics hardware engineers must understand how to select these components in detail.

It may appear condescending to be asked, ‘do you know how to select a resistor’? However, I can honestly say the first time I was tasked with populating a BOM and pinning my hat on one manufacturer’s part number, my gast was well and truly flabbered at how many parameters were on datasheets. Not to mention the number of manufacturers offering resistors or even the different types of resistors. Having just completed my undergraduate degree at the time, I started to realise that my journey as a designer was only just beginning, and I still had a lot to learn.

Ohms law is your best friend

I will assume you are already aware of Ohms law, or at least can find another source for this elementary physics. If you don’t know it though and aren’t comfortable with transposing this equation, I suggest you practice, as not many days go buy where I don’t use it in one form or another.

What is an Ohm? it’s a place where a Volt lives

How to choose the right resistor for your electronics design?

The part of the resistor selecting journey I would like to start from is the point where you know the resistance value of the component from either by calculating it or sourcing it from a reference design or a textbook you have been following. For example, you might be designing a rudimentary low-pass filter (we’ll talk about filters in another blog) and the answer to your equation returns the required resistance value of 11.765 kΩ.

Resistor Manufacturers use Standard EIA Decade Values

Armed with this resistor value, you jump into your preferred distributor catalogue, and you search for the exact resistance value, but you are surprised to find zero search results. So, what went wrong you ask? Nothing went wrong, it’s just that resistor manufacturers make resistor values in accordance with ‘Standard EIA Decade Values Table’ and the values are normalised. If you click on the image below embedded in the image, you should be looking at my go-to page. You may also hear this colloquially referred to as the ‘E-series’, which I assure, in this instance, has nothing to do with a premium German car manufacturer or 90s UK rave culture.

Why do manufacturers do this?

Manufacturers do this because it is intrinsic to the tolerance of the component you are selecting .

For instance, if you look at the small extract of the table above you can see that the number 11.765 doesn’t exist, the closest value corresponding to E192 series is 117. This basically means the closest resistor value you can ‘easily’ and commercial obtain is 11.7 kΩ. However, when you continue to look at what tolerance you require (we’ll cover tolerance in a future blog) you will see that the E192 series aligns with more expensive ‘precision’ resistors. Therefore, use this series at your peril, as it will drive cost into your design and become a supply-chain issue as those resistors will unlikely be available as ex-stock.

How to choose a resistor if you cannot find the correct value?

In this example, my advice would be to round the value down to 11.5 kΩ. This allows us to select a value from the E48 resistor range. This is commercially a much easier component to source and so 9 times out of 10 you will be grateful you made this compromise.

Why does the E-series exist?

Finally, let’s look at why the E-series exists. Focusing on the E24 series of components. If I told you this was the series that aligns to resistors that have a 5% initial tolerance, you could then work out that a 10 kΩ resistor with a 5% tolerance has a maximum value of 10.5 kΩ. i.e. if you took one of these components and measured it with a ohmmeter (or DMM) you could see any measurement ranging from 9.5 kΩ to 10.5 kΩ. The same applies for the 11 kΩ components, measure one of these out of the box and you could see any measurement from 10.45 kΩ to 11.55 kΩ. Notice how the ranges of values overlap? Therefore, in the E24 series it makes no-sense to offer ‘in-between’ values. Now the E96 becomes a necessity if we look at 1% tolerance and finally the E192 is for 0.1% components.

As you can prove for yourself, with a little research, when you jump from one tolerance range, say 1%, to a higher tolerance component of 0.1% you will increase component cost. That’s normally for 2 reasons:

The component is manufactured using a more complex process.
They aren’t produced in as much quantity and therefore have less economies of scale.

Note on the word ‘precision’.. In timely honour of one of my university professors who used to jump around the lecture hall at the mere utterance of, in his opinion, the misuse of the word precision. From the dictionary he would retort, ‘the word precision reflects something that is minutely exact’. i.e., it is absolutely correct with no error! Therefore, the way it is used throughout the electronics and scientific community isn’t literally correct. Thanks Prof. H. I hope you are pleased this wisdom is being passed on, if only the readers could mentally replay your not so merry jig!

My advice is always to try and stick with E48 resistors wherever possible, as your experience grows (and hopefully by reading future blogs) you will learn when and where it is appropriate to move upwards, but E48 is a good starting point.

I hope this advice helped, more to come on resistors and other components soon.