Resistor substitution box project - Schematic/PCB Review - Page 1
Forum section: electronics / beginners

author: the offset voltage
author_rank: Regular Contributor
timestamp: August 16, 2025, 12:23:28 pm
content: Hi,Following:https://www.eevblog.com/forum/beginners/resistor-substitution-box-project-power-calculation/https://www.eevblog.com/forum/beginners/resistor-substitution-box-project-rotary-switch-knob/I have finished the schematic and the pcb layout. As i have to send now the Gerber to the pcb manufacturer, i will like if possible to have a short review before.I use this rules:- Minimum clearance: 0.3mm;- Trace Width: 2mm;- Holes: 3.1mm to use 3mm screw, with plated trough hole.Note:1. All of the resistors are 5W, i know that tggzzz user will not be happy, but i preferred to have only one footprint type, and looks more nice to me like this.This is the series i am using:https://www.mouser.at/c/passive-components/resistors/wirewound-resistors/wirewound-resistors-through-hole/?m=Vishay&series=ALSR2. For the J1 connectors, i did a mistake when i order it, it is an 19 pin connector, and it has only 10 pins populated, i wanted a 10 pin connector, but i did a mistake when ordering.Connector:https://www.mouser.at/ProductDetail/571-1-282843-0This is my second PCB design.Thank you

author: mariush
author_rank: Super Contributor
timestamp: August 16, 2025, 02:51:52 pm
content: Your specifications say the maximum voltage will be 5v.Your lowest resistor value is 10 ohm - that means the maximum amount of power dissipated by the resistor would be 2.5 watts.The higher value resistors will dissipate less heat.Your resistors value are 10 22 47 100 220 470 1k 2k2 4k7 ... at 22 ohm you already have P = 25/22 = 1.09w , at 47 ohm you have P = 25/47 = 0.53w At 1k you're looking at 0.025w already.I would suggest going with 3w resistors for the 10-470 ohm resistors, and with 1w rated resistors for 1k and higher.For the 10 and 22 ohm resistors, I would suggest paralleling two 3w rated resistors to get the heat spread across two resistors and get a 6w rating for free.For example, a couple 20 ohm resistors in parallel will give you 10 ohm, here's example :https://ro.mouser.com/ProductDetail/Vishay-Draloric/PAC300002009FAC000?qs=sGAEpiMZZMsPqMdJzcrNwl5vxkMOsrq%2F%2FqGZ3SKXFtM%3DFor the 22 ohm resistor, a 42 ohm resistor in parallel with a 47 ohm resistor will give you 22.18 ohm , which well within the +/- 5% the problem text wants.42 ohm :https://ro.mouser.com/ProductDetail/Vishay-Dale/RW79U42R0FB12?qs=sGAEpiMZZMsPqMdJzcrNwvkKzHhqxI%2Fazg51klBQOsg%3D47 ohm :https://ro.mouser.com/ProductDetail/Vishay-Dale/RS02B47R00FE70?qs=sGAEpiMZZMsPqMdJzcrNwnPpI93fAKplY3hN604yAJM%3DFor 100, 220 and 470, a single 3w rated resistor would be enough.Rotary switches with a lot of positions will typically be rated for a low amount of current.For example this 10 position one that costs $20 is only rated for 200mA :https://www.digikey.com/en/products/detail/grayhill-inc/56D36-01-1-AJN/98652Instead of spending a lot of money on such switch, I'd suggest just changing to on/off switches, as this would also allow you to parallel resistors to get various values.You can get on/off (rocker) switches for cheap, for example these ones are 20 cents a piece :https://www.digikey.com/en/products/detail/adam-tech/SW-R3-1A-A-1-0/15284400

author: the offset voltage
author_rank: Regular Contributor
timestamp: August 16, 2025, 03:09:49 pm
content: I already ordered the components, and i know that the power of the resistors is higher then needed, but its ok taking into account my goal. As for layout, its ok what i have done?I just realized that i didn’t put the name of the project on the top side, i have to update it.