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 Surface Mount Device (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.