The “Not So Simple Guide” to choosing resistors - Part 1
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 electronics.
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 Bill of Materials (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.