Resistor substitution box project - power calculation - Page 1
Forum section: electronics / beginners

author: the offset voltage
author_rank: Regular Contributor
timestamp: August 12, 2025, 09:26:32 am
content: Hi,I work on the resistor substitution box project from the book of Thomas L Floyd: Principles of Electric Circuits (self-study). This project belongs to Energy and Power section.The requirements of my projects are shown on Fig1.I have to calculate the power size of the resistors, draw the schematic and than design the PCB.To calculate the power rating of the resistors:1. Based on Fig1, to calculate the power rating, i use this formula: P = V²/R. As the minimum R is 10 Ohm and Vin=5V -> P = 2.5W, so i want to go with 5W for each resistors. it is ok?2. Also, it is ok to use SMD resistors in my case?3. The resistors are ok to be metal oxide or should i use wire wound or other type?Thank you.

author: BeBuLamar
author_rank: Super Contributor
timestamp: August 12, 2025, 09:33:15 am
content: It's expensive just for learning. You need a 9 position selector swtich. It's inefficient to switch in 1 resistor at a time. It's better to add resistors in series.

author: the offset voltage
author_rank: Regular Contributor
timestamp: August 12, 2025, 09:35:24 am
content: Yes, but i also learn PCB design in parallel and i find it more easy to me, co combine the theory from Principles of Electric Circuits book with PCB design projects.

author: tggzzz
author_rank: Super Contributor
timestamp: August 12, 2025, 09:37:52 am
content: Quote from: the offset voltage on August 12, 2025, 09:26:32 amCorrect so far as it goes.What is the maximum voltage that can be applied to a 4k7 5W resistor? Consequences?QuoteResistors will, of course, dissipate power, and the heat will raise their temperature.Hence you need to determine the temperature rise and whether that will be within the resistor's specified limits. That is left as an exercise for the student; the answer requires reading data sheets and application notes. Andthatis a lesson the student should apply for every component.

author: the offset voltage
author_rank: Regular Contributor
timestamp: August 12, 2025, 09:43:21 am
content: Hi,Regarding your question: "What is the maximum voltage that can be applied to a 4k7 5W resistor? Consequences?"- Using P = V²/R -> P = 5.3mW, so i think once i have selected the power rating for the smallest resistor, i should be safe on high resistors values with 5W, or I'm wrong?Also, regarding : "Hence you need to determine the temperature rise and whether that will be within the resistor's specified limits. That is left as an exercise for the student; the answer requires reading data sheets and application notes. And that is a lesson the student should apply for every component."- I will check, the book don't mention anything about this concern, thank you for the tip

author: tggzzz
author_rank: Super Contributor
timestamp: August 12, 2025, 09:57:24 am
content: Quote from: the offset voltage on August 12, 2025, 09:43:21 amSafe by a factor of 1000. Now look at the cost and size of 5W and 5mW resistors. "Engineers can do for $1 what any fool can do for $10".BTW, you can put 150V across a 5W 4k7 resistor, and it will stay within its power dissipation limits. But check the specified applied voltage limits and temperature limits.

author: the offset voltage
author_rank: Regular Contributor
timestamp: August 12, 2025, 10:10:22 am
content: Hi, I'm sure you want me to learn, but i find this "Engineers can do for $1 what any fool can do for $10" a lite bit to much fore someone who is starting to learn electronics at 15. But I’m sure you knew more than me when you were 15. Thanks

author: tggzzz
author_rank: Super Contributor
timestamp: August 12, 2025, 10:33:49 am
content: Quote from: the offset voltage on August 12, 2025, 10:10:22 amI didn't know you were 15I probably knew both moreandless than you at that age - but that's not important. What is more important is to want to learn and to keep learningfor the rest of your life. If you can do that, then you will always be able to enjoy discovering something new. Keeping a child-like (not childish!) sense of wonder is great fun.I'm at the other end of life, and the old saying is that the more you know, the more you realise how much you don't know. Corollary: watch out for people who make overly definitive statements, since they may be exhibiting theDunning Kruger effect. Experts usually make more nuanced less definitive statements, e.g. "... it depends on what you mean by X...".Keep up the good work

author: golden_labels
author_rank: Super Contributor
timestamp: August 12, 2025, 10:46:11 am
content: the offset voltage:I believe the entire goal of this exercise is to optimize cost. Why else would they give you the table with resistor prices?Clearly, the author wants you to select the most affordable version and not just multiply two numbers.BeBuLamar:An engineer designs what the customer requires the engineer to be designed. The book is the customer in this case, and the book wants a set of individually switchable resistors. So much for the advice on a better design.

author: the offset voltage
author_rank: Regular Contributor
timestamp: August 12, 2025, 11:01:54 am
content: I understand, its clear to me.Now, i don’t know, should i use SMD resistors or TH?and what type should i use: metal, or wire wound resistors or other type?I don’t know base on what to select this components.Thank you,

author: xvr
author_rank: Frequent Contributor
timestamp: August 12, 2025, 11:29:35 am
content: This is a 5W resistor. Do you think it can be shrunk down to fit an SMD version?

author: MrAl
author_rank: Super Contributor
timestamp: August 12, 2025, 11:39:03 am
content: Quote from: the offset voltage on August 12, 2025, 09:26:32 amHello there,If this is just to be done in theory then 1/2 the power rating might work ok, but for a practical resistor box I think I would go with 1/4 power. That means that a 5 Ohm resistor at 5 volts would dissipate 5 watts, so go with a 20 watt resistor. Even with this overkill the body temperature could reach 100 degrees C which is 212 degrees F which is boiling temperature. That's not a problem in regular designs, but for a test instrument that would probably be a max. That also means it has to be mounted away from other parts and case walls and stuff like that.Now for a 10 Ohm resistor, we can divide that in half. A 10 Ohm resistor at 5 volts will dissipate 2.5 watts, so a 10 watt resistor should do it. That can still get pretty hot though so careful mounting is still necessary.Good resistor boxes will use 1 percent resistor values. You may not need this for your project though.I built a resistor box from a kit one time a long time ago. It came with 5 percent resistors. I replaced them with 1 percent resistors I purchased just for that project.The other requirement is that the switch has to be able to handle the maximum current. For this that would be 1/2 amp. I'd go with at least a 1 amp rated switch.There is also the question of the switching arrangement. If we connect the following resistors in series:1, 1, 2, 4, 8 Ohms, we can get resistances of 1 to 16 Ohms in 1 Ohm steps by placing a single SPST switch in parallel to each resistor except for the first 1 Ohm resistor. We can then selectively bypass either resistor to get the lower values of resistances.

author: tggzzz
author_rank: Super Contributor
timestamp: August 12, 2025, 11:40:21 am
content: Quote from: golden_labels on August 12, 2025, 10:46:11 amPartly.Too often the customer wants the impossible and/or doesn't know/articulate what they want and/or doesn't distinguish between "must have" "want to have" and "would be nice to have" attributes.Engineers (and to a lesser extent technicians) often have to elicit such required information from the customer.No, that isn't relevant inthiscase, but the OP will benefit from being aware of the general phenomenon, so they recognise it when they encounter it.

author: MrAl
author_rank: Super Contributor
timestamp: August 12, 2025, 11:40:55 am
content: Quote from: xvr on August 12, 2025, 11:29:35 amThey make SMD higher power resistors but that does not mean they are the same size as 1/4 watt SMD resistors (ha ha).

author: tggzzz
author_rank: Super Contributor
timestamp: August 12, 2025, 11:47:50 am
content: Quote from: the offset voltage on August 12, 2025, 11:01:54 amFundamentally any resistor with the right power rating will do. However, some will be better/worse than others.Is there a requirement that they are all the same type? Consider using a mixture of wattages and types, to minimise the overall size and cost. Such tradeoffs are normal engineering practice.Wirewound resistors are likely to be available in higher wattages, and can be more tolerant of short term overload. Since the internal construction is similar to a coil/inductor, they will probably have worse high frequency response. (But "high" and whether it matters depends on the application)PTH resistors will be easier for an amateur to assemble than SMD resistors.

author: tggzzz
author_rank: Super Contributor
timestamp: August 12, 2025, 11:49:21 am
content: Quote from: MrAl on August 12, 2025, 11:40:55 amOf course you still have to be able to get the heat away from the resistor, which will be more difficult for a smaller SMD resistor.

author: BeBuLamar
author_rank: Super Contributor
timestamp: August 12, 2025, 11:49:51 am
content: Quote from: golden_labels on August 12, 2025, 10:46:11 amSuch a subsitution box isn't very useful and it's a waste of money to actually build one. Just design it and that is good enough for learning.

author: tggzzz
author_rank: Super Contributor
timestamp: August 12, 2025, 12:42:19 pm
content: Quote from: BeBuLamar on August 12, 2025, 11:49:51 amOh, I don't know.Learning how to set an objective (e.g. make and assemble a PCB), and then achieve it is a useful skill in its own right and looks good on a CV when applying for jobs. Yes, the result will be terrible in many ways, but that isnotimportant. What is importantisto work out how you would do it better next time. That looks good during job interviews.Plus it is fun when you get something wrong or have a slip-up - and have a learning experience. Even better if magic smoke is involvedTo the OP... Go for it. Make mistakes. LearnAnd most importantly, "Have fun, safely".

author: the offset voltage
author_rank: Regular Contributor
timestamp: August 12, 2025, 12:53:41 pm
content: Hi all, i see some concerns that this is not that practical.For me, to be honest i care more about the steps on how to calculate the components, selection of the components than to the design of the PCB - soldering and testing of the project etc. As now i learn theory and PCB design in parallel, i am happy to spend some of $ on the components and than for the PCB manufacture, taking into account that i am building and learning at the same time.

author: tggzzz
author_rank: Super Contributor
timestamp: August 12, 2025, 01:27:30 pm
content: Quote from: the offset voltage on August 12, 2025, 12:53:41 pmIt is practical.Whether it is worth doing is a value judgement that only you can make.QuoteDo you actually have to make a PCB, or are there other ways you could get 90% of the benefit?There are many prototyping techniques thatdon'trequire a PCB, see some of them here:https://entertaininghacks.wordpress.com/2020/07/22/prototyping-circuits-easy-cheap-fast-reliable-techniques/(But please don't use solderless breadboards for this!)Commercial decade boxes usually don't contain PCBs: they connect resistors directly between the switch contacts (not SMD, obviously!). Here's one example:https://www.eevblog.com/forum/metrology/teardown-standard-resistors/msg4045519/#msg4045519Note there are several different types of resistor

author: the offset voltage
author_rank: Regular Contributor
timestamp: August 12, 2025, 01:35:27 pm
content: Hi,Thank you.I have at home a breadboard, but now i learn also PCB design, and the applications projects from the book of Floyd fit's very well for me. And i will continue to design the PCBs for all the applications projects from Floyd book, also the author recommend to get and design the PCB. Thank you for your help

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 12, 2025, 06:04:52 pm
content: Quote from: xvr on August 12, 2025, 11:29:35 amI had a look around and was a bit surprised at the power rating of some SMT resistors. For exampleRFPR3722J50R0L180S9is rated for 150W and apparently this is no exception. Size is "3722" (in Banana units) or 9.5*5.7*1mm in metric. This would definitely needs a good heatsink to get rid of that heat.But 5 Watt is not very special for SMT. 5W SMT resistors usually do have longer pads then the "standard" resistors, and the extra pad area is used to conduct heat away into the PCB, which then acts as a heatsink.

author: golden_labels
author_rank: Super Contributor
timestamp: August 12, 2025, 06:48:47 pm
content: I believe the point here is really the exercise, not making a good decade box. An exercise as set up by the author, with the intent purpose to drag the reader through a particular process. Perhaps useful in the upcoming tasks.the offset voltage:There are 5 W surface-mount resistors. But they’re pushing the limits. I can’t imagine any reason to go for them, unless you are forced to use SMD and are limited in space. Or, perhaps, have very specific parasitics requirements that only an SMD component can meet.Just use a THT resistor. Or multiple SMD ones to distribute power across them.

author: the offset voltage
author_rank: Regular Contributor
timestamp: August 12, 2025, 07:17:21 pm
content: Thank you,I will go with THT resistors, and i will calculate the power individual for each resistor, and i will add some extra 20% safety margin for each resistor.I will come back with a review for the schematic/PCB layout before i send the Gerber to manufacturer, as is better to have some extra eyes for a short review. As this is my second PCB i already learned some lesson from the first PCB that i have designed and i hope i should not have big errors.Thank you

author: TimFox
author_rank: Super Contributor
timestamp: August 12, 2025, 07:36:16 pm
content: Quote from: the offset voltage on August 12, 2025, 07:17:21 pmThat should work, but I would allow a 50% safety margin on resistor power, i.e. use a resistor rated for at least 2 times the expected power. When run at full-rated power, a THT resistor gets very hot! (Guess if I learned that the easy way or the hard way, at age 18.)

author: the offset voltage
author_rank: Regular Contributor
timestamp: August 12, 2025, 07:56:39 pm
content: I will do that,Thank you,

author: tggzzz
author_rank: Super Contributor
timestamp: August 12, 2025, 08:22:53 pm
content: Quote from: TimFox on August 12, 2025, 07:36:16 pmThereby preventing a learning experience involving magic smokeShame!

author: TimFox
author_rank: Super Contributor
timestamp: August 12, 2025, 09:22:42 pm
content: Quote from: tggzzz on August 12, 2025, 08:22:53 pmMy experience was smoke-free, yet painful. (Allen-Bradley vitreous enamel wire-wound resistor.)

author: tggzzz
author_rank: Super Contributor
timestamp: August 12, 2025, 09:28:22 pm
content: Quote from: TimFox on August 12, 2025, 09:22:42 pmSounds only too plausible. A rite of passage, similar to picking up a soldering iron by the wrong end.

author: MrAl
author_rank: Super Contributor
timestamp: August 13, 2025, 09:59:48 am
content: Quote from: tggzzz on August 12, 2025, 11:49:21 amHello,Yes that's with any power design. With SMD parts like this though, the PCB design would become more critical. It has to be done right. In fact, I would be more apt to go with two parts spaced away from each other so that the heat could be spread out over a larger PCB area.A single SMD resistor could probably require a copper PCB area of around 1.75 by 1.75 inches on top and bottom with good via design. This would take up more board space than a standard ceramic resistor unless we also add a bottom side heat sink. A decent heat sink may lead to around 1.1 by 1.1 inches of board space top and bottom. It may still take up more room though with that added heat sink.

author: tggzzz
author_rank: Super Contributor
timestamp: August 14, 2025, 09:24:50 am
content: Quote from: tggzzz on August 12, 2025, 01:27:30 pmIt won't affect the OP's choices, but here's a picture of the internals of a JJ Lloyd box I used to have. Not a high grade device, but perfectly usable.

author: BeBuLamar
author_rank: Super Contributor
timestamp: August 14, 2025, 12:15:10 pm
content: Using a PCB and using SMD componet is a practice to use those otherwise for a decade box even complex one there is no need for PCB nor SMD.

author: tszaboo
author_rank: Super Contributor
timestamp: August 15, 2025, 10:12:32 pm
content: Quote from: xvr on August 12, 2025, 11:29:35 amI've used 30W SMD resistors in D2PAK package.