Ohm’s Law – Resistors for Fan Speed Part 3 - PCB Design - Page 1
Forum section: electronics / beginners

author: the offset voltage
author_rank: Regular Contributor
timestamp: August 09, 2025, 11:29:52 am
content: Hi,followinghttps://www.eevblog.com/forum/beginners/ohms-law-resistors-for-fan-speed/andhttps://www.eevblog.com/forum/beginners/ohms-law-resistors-for-fan-speed-part-2/i am now working on the PCB for the Resistors for Fan Speed project.I have a question regarding the Layer Stack Manager and Rules.I am using the layer stack manager values from Fig.1 (this values are used on my PCB design course).For the Rules i use (this values are also used on my course):Rules:- Clearance: minimum clearance: 0.3mm;- Width: preferred: 0.5mm; min: 0.3; max: 1mm;- Routing via style: via diameter 0.6mm; via hole size 0.3mm;I need your help with some info:- there is a calculation formula for the layer stack manager values? i don't know how to select the correct values.- for the Rules i understand that we have to follow the PCB manufacture capabilities, but, should i use any number from the PCB manufacture or there is some specific formula to use?Since this is a learning project, i understand that i can use whatever i want, but i just want to understand from where this values are coming.Thank you,

author: Psi
author_rank: Super Contributor
timestamp: August 09, 2025, 11:47:52 am
content: Quote from: the offset voltage on August 09, 2025, 11:29:52 amNormally you just send Gerber files to the PCB fab and maybe a BOM + pick-n-place if doing PCBA. This means none of the files you give them say anything about the layer stack thickness details. So whatever values you put in there don't matter except they change what the Altium 3D view looks like (PCB thickness, copper thickness). They also matter if doing any circuit simulations in Altium as they change resistance between pads and RF impedance.They might also matter if you plan to send the PCB Fab the actual Altium design files, but you don't normally do that.Normally the fab decides the core/substrate/mask/silk layer thickness stuff based on their equipment, what pcb stock thickness they have and based on what you've told them when you submitted your PCB. Namely the thickness pcb you want, the number of layers and required copper thickness for outer layers and for inner layers. (1oz 2oz etc..)1oz = ~0.0348mm thick copper2oz = ~0.0696mm thick copperQuote from: the offset voltage on August 09, 2025, 11:29:52 amThat's all pretty typical for a general circuit board anyone can make without having to charge extra. (PCBWay/JLCPCB)For some fine pitch chips, like 0.4mm/0.5mm pitch, you need to go to 0.2mm width/gap and 0.2mm vias. but if you don't need to don't use them and save some money. I always try to use 0.3 holes min unless i need 0.2mm

author: the offset voltage
author_rank: Regular Contributor
timestamp: August 09, 2025, 04:09:29 pm
content: But, based on what requirements are the values for Layer Stack Manager and the Rules selected? i am confused, i was expected to be some kind of formulas, not a plug and play values, i don't understand

author: Psi
author_rank: Super Contributor
timestamp: August 09, 2025, 11:21:11 pm
content: Quote from: the offset voltage on August 09, 2025, 04:09:29 pmCan you explain why you expected a formula, I don't understand why you would expect it to be done with a formula.(Unless you are asking about current flow in the PCB and how wide the tracks need to be? You didn't say anything about current)The PCB fab decides how they make your PCB layer stack to meet your requirements. 4 layer, 1oz etc..A lot of the time there are things about the PCB fabrication that the customer doesn't won't to worry about or need to know. The exact brand of glue or material used between layers etc..The PCB Fab has various types of PCB material/glues for various different thicknesses they could use. There are many combinations they could choose.They will normally select what is the cheapest at the time and what they currently have in stock. So long as the total adds up to the thickness of PCB you want it doesn't really matter for most people.If you are doing advanced high speed stuff, like RF, or you want huge volumes, then it might start to matter more and you might provide them a layer stack with all the thicknesses and material types you want used. Usually this is because you want the impedance of the signals on your PCB exactly the same because you need to tune the circuit to match the properties of the PCB.If they changed the layer stack from one batch to the next it might cause you problems. However even then you often just ask for 'impedance control' and give them the details of what you need and they still have control over the layer stack they just make sure the impedance is always where you want it.

author: the offset voltage
author_rank: Regular Contributor
timestamp: August 10, 2025, 11:02:32 am
content: "Can you explain why you expected a formula, I don't understand why you would expect it to be done with a formula."I was expecting to be some formulas at the base, not a selection based on ..."(Unless you are asking about current flow in the PCB and how wide the tracks need to be? You didn't say anything about current)"I am using a 12V / 80mA ventilator that will be powered from a 12V power supply (and speed will be controlled using some resistors and a rotary switch), it was mentioned here:https://www.eevblog.com/forum/beginners/ohms-law-resistors-for-fan-speed/.I think when i will join university, everything will be clear to me, but i want to build and learn in the mean time.Thank you,

author: Psi
author_rank: Super Contributor
timestamp: August 10, 2025, 11:24:26 am
content: Quote from: the offset voltage on August 10, 2025, 11:02:32 amThe copper thickness and track width you can use a formula to figure out based on current flow.If an engineer is concerned about the amount of current flowing on the PCB they normally check it using an online calculator. It's much faster than doing the math manually.The calculator tells you how many degrees C or F that the track temperature will increase based onthe current flowing, the track width, track distance and the copper thickness. Some of them will recommend a track width if you enter all the other info.However the basic design rules like clearance min/max, width min/max and via min/max are not calculated, they are rules you apply to yourself to make sure you don't accidently use something on the PCB that will be complex and costly or impossible. You always try to design the PCB to be cheap and only make it complex if you need to. Design rules keeps everything correct by not allowing you to do anything else.

author: the offset voltage
author_rank: Regular Contributor
timestamp: August 10, 2025, 11:46:45 am
content: Do you recommend any online calculator for PCB design?Thank you,

author: Psi
author_rank: Super Contributor
timestamp: August 10, 2025, 11:52:32 am
content: Quote from: the offset voltage on August 10, 2025, 11:46:45 amThere's lots of them for track width, even digikey has one and they go into the math too.https://www.digikey.co.nz/en/resources/conversion-calculators/conversion-calculator-pcb-trace-widthFor more complex things, and having multiple tools in one place, most engineers use something like the Saturn PCB Design Toolkit.It's an app you run locally and it has lots of tabs for lots of useful tools.It's free, you can see it and download it here.It's a bit overwhelming for newbies, there's lots of options.https://saturnpcb.com/saturn-pcb-toolkit/

author: the offset voltage
author_rank: Regular Contributor
timestamp: August 10, 2025, 12:14:09 pm
content: Great, thank you very much Psi