tradeoff: when to add a new value of resistor to BOM vs combining existing? - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: April 22, 2025, 07:56:56 pm
content: Suppose you have a simple design that only uses 1k, 10k, and 100k 0603 resistors (plus some transistors, capacitors, diodes, and whatnot - 30 unique items on the BOM. It has a total BOM cost of maybe $2.00. It's a small 24V module that is part of a larger system)Suddenly, to your absolute horror, you discover that your design requires a single 2.7k to 2.6k nominal (+/- ~5%ish tolerance) resistor. Do you add a single 2.7K resistor to the BOM and increase your unique part count, or do you make one using a network of 1k/10k/100k resistors? One can easily make a network equivalent to a 2.66k resistor (which is close enough) using qty 3x 1k resistors and a 10k resistor - and it would be good enough. The network uses 4 resistor but avoid adding another line to the BOM.It can feel silly to have just one instance of a particular value of resistor.What are the factors you use to decide when "adding an additional BOM item" is not worth it?Different situations have different thresholds for adding a new part vs reusing existing ones.Quantity, application, manufacturer preferences, etc.Sometimes for low volume products the effort (for a prototype house, or similar) to define everything, order the components and set up the machine greatly outweighs the cost of the components.Having 4 resistors instead of 1 increases the number of solder joints, possibly increases the failure rate in the field, and reduces yield from the factory.In high volume products I'd imagine that is it's undoubtedly worth adding an extra part to the BOM.For modestly complicated designs it is easy to get well over 100 unique BOM items, which sometimes seems like "too many".

author: thm_w
author_rank: Super Contributor
timestamp: April 22, 2025, 11:03:57 pm
content: Google has good answers:https://electronics.stackexchange.com/questions/472749/is-it-good-practice-to-substitute-a-resistor-by-parallel-and-series-ing-themhttps://electronics.stackexchange.com/questions/698872/reasons-to-use-combinations-of-resistors-parallel-or-in-series-looking-for-edhttps://www.reddit.com/r/AskElectronics/comments/2l7hcu/is_it_worth_the_effort_to_combine_common_parts_in/Don't overthink it and just use the 2.66k resistor.BOM unique items is what it is, get it working first and then consider going in and optimizing for a few cents savings.

author: bdunham7
author_rank: Super Contributor
timestamp: April 22, 2025, 11:44:34 pm
content: Quote from: incf on April 22, 2025, 07:56:56 pmCheck your math on that. Sometimes it is OK to combine parts like that and in some cases it may even be an improvement. For example, using 4 smaller resistors in a 2x2 rather than a single larger one will spread the power dissipation out a bit. Also there is the method of using multiple resistors in series instead of a single higher voltage part. I wouldn't in this case at any volume level as long as the value needed was a standard one.

author: incf
author_rank: Frequent Contributor
timestamp: April 23, 2025, 12:17:03 am
content: Quote from: bdunham7 on April 22, 2025, 11:44:34 pmThere are a *lot* of ways to connect 4 resistors. The simulator hasn't let me down yet with resistor networks.

author: bdunham7
author_rank: Super Contributor
timestamp: April 23, 2025, 01:19:12 am
content: Quote from: incf on April 23, 2025, 12:17:03 amYep! I missed one.

author: peter-h
author_rank: Super Contributor
timestamp: April 23, 2025, 08:03:34 am
content: I used to work with a guy who designed almost everything with 10k resistorsIt's not that difficult.But adding one reel to your stock is usually fine. What is dumb is what you see in some expensive products e.g. avionics where they sprinkly the whole E96 range all over the board. There isn't a pick and place machine with enough room for all the reels they use.

author: PGPG
author_rank: Frequent Contributor
timestamp: April 23, 2025, 08:15:35 am
content: You seem to not count assembling costs but only element costs.For many years (1988...2010) I was trying to do my best to not extend resistor list we use too much. We were using semi-automatic assembling device that had about 50 drawers to be filled with elements. The best would be to not have to change their contents when assembling any of manufactured by us small devices (each in small quantities - serie like 50 pcs).Now, as we are using assembly house I have made a list of standard values I simply use without stressing out.My list is denser than 1k, 10k, 100k

author: tszaboo
author_rank: Super Contributor
timestamp: April 23, 2025, 08:37:57 am
content: I used to do that, and I don't do that anymore.These resistors are like 10 EUR a reel. Assembly house charges you somewhat based on the number of solder joints.Plus I need to keep in mind the heat-up and power consumption of every single resistor. For others I need to worry about electrical clearances. So I end up with 2512 resistors, or 1206, or a 1206 with high power rating, so be it. I have a dozen other thing to worry about rather than trying to save 1 line item on a BOM.That being said, I try to use the same values if possible, but I just don't go out of my way of connecting them in parallel to reduce the BOM count.