EasyEDA uCurrent design with JLC STM assembly available parts - Page 1
Forum section: electronics / projects

author: horvbrun
author_rank: Newbie
timestamp: April 01, 2020, 10:41:24 am
content: Hello, I am new to this forum. I entered in contest of building a low power device and now I have a problem with measuring the current my device pulls. Due to limited funds I decided to make my own uCurrent based of Daves rev 5. schematic and parts available on JLC PCB STM assembly service. And I ran into few problems, there are no precision resistors so I had to pick first tolerance next to specified, instead of having 0.05% tolerance and 10ppm temperature coefficient resistors I picked 0.1% tolerance and 25ppm temperature coefficient resistors and they didnt have MAX4239 (high bandwidth) so I picked MAX4238(low bandwidth). Will these choices greatly affect device ability to measure current? And I wasnt able to find DP3T L203011MS02Q switch that Dave used in his design so would like to use JS203011AQN or JS203011CQN but I am not sure whether to use them or not because the datasheets don t look the same. L203011MS02Q has 4 neighbour pins connected and JS203011AQN , JS203011CQN have their pins different, my question here is can I plug in switches I found without changing pin order or schematic?Datasheet of switch Dave used L203011MS02Qhttps://dznh3ojzb2azq.cloudfront.net/products/Slide/L/documents/datasheet.pdfDatasheet of switches I would like to use and available JLC assembly service :JS203011AQNhttps://dznh3ojzb2azq.cloudfront.net/products/Slide/JS/documents/datasheet.pdfJS203011CQNhttps://dznh3ojzb2azq.cloudfront.net/products/Slide/JS/documents/datasheet.pdf

author: thm_w
author_rank: Super Contributor
timestamp: April 01, 2020, 08:41:53 pm
content: dont cross posthttps://www.eevblog.com/forum/eda/easyeda-ucurrent-design/Higher tolerance and tempco means it could be less accurate, not a huge deal.MAX4238 = 1MHz GBPMAX4239 = 6.5MHz GBPSo your bandwidth might be around 50kHz, vs the stock 300kHz. Up to you if this stuff matters.Looks like L203011MS02Q is silver plated, 4A rated DP3T switch.The one you linked is limited to 0.3A which is not very high. See if you can find something physically larger.

author: horvbrun
author_rank: Newbie
timestamp: April 02, 2020, 10:40:41 am
content: Sorry, I found out there is a better subforum for my topic after I posted it already and I wasn t able to delete it.I don t mind slight drop in accuracy and drop in bandwidth. And I don t mind the lower current rating and smaller dimensions of switch because I wouldn t measure currents higher than 200mA.I contacted RS Components, Digi key and mouser customer support and EVERYONE told me they are the same switches (working principle) and even when I showed them schematic of uCurrent rev 5 they told me to go ahead and that I could switch them without changing pin order.I ve done my research and I will be posting it below.So there is actually two types of DP3T switches and it seems no one recognizes the difference.1. DP3T WITH common pin is described as having 2 inputs and 6 outputs that are divided between two switches that are operated with one slider. Those 2 switches don t have any electrical contacts in any position of operation so we can say that DP3T has 2 galvanically separated switches with each having 1 input and 3 outputs.2.DP3T WITHOUT common pin is almost the same as above but with one main change. There is no common pin in three positions of operation. Each position connects 2 neighboring pins. Example, one side of switch has 4 pins from 1 to 4, with three possible positions connecting pins 12 in positions 1, connecting pins 23 in positions 2 and connecting pins 34 in positions 3I hope this saves time for someone that stumbles upon the same problem.

author: thm_w
author_rank: Super Contributor
timestamp: April 02, 2020, 08:45:14 pm
content: Quote from: horvbrun on April 02, 2020, 10:40:41 amI wouldn't expect customer support to have much knowledge of switches, thanks for posting the drawings.Also forgot the switch resistance is 70mR vs <10mR, which will not matter much if you are only using 200mA.