problems with usb module (two additional usb on the pc)  - Page 1
Forum section: electronics / beginners

author: CharlotteSwiss
author_rank: Frequent Contributor
timestamp: August 01, 2021, 02:30:27 pm
content: I have an old desktop pc (2005), for teaching I was carrying out measurements with the multimeter in one of the two front USB ports, but with the test lead I think I made a short circuit between the 5v pin and the gnd (usb frame); I smelled the smell of burning.Analyzing the small Nec front card (it has two USB and audio connections), and looking closely at the path of the two USB 5v (from pin 1 of the port to the red cable that will go to the MB), I see that we have in each track: a capacitor in parallel (both measure about 10nF, they seem ok to me), and then we have a resistance in series (R8 on the left and R7 on the right); without unsoldering them, I measure 1.2Kohm on the left one and 0.2ohm on the right one. At pin 1 of the two ports ok I still get the 5v.But having smelled the smell of burning on that card, something must be broken: visually I thought it was the R on the left, while the right one looks better. But by measuring the continuity on the left I do not have (in fact the R8 is 1.2Kohm), while on the right if I try the continuity on R7 I hear the beep ... and in fact I only have 0.5ohm.In your experience, how much should the value of these resistors be and therefore which of the two should be ok and which fault? They didn't screen print anything on the body ..Thank youCharlotte

author: CharlotteSwiss
author_rank: Frequent Contributor
timestamp: August 02, 2021, 11:03:04 am
content: Looking at the appearance of the two resistors on the two 5v power lines of the two usb (R7 and R8) I begin to think that perhaps R7 is the one that measures the correct value (0.2ohm) while the one on the left R8 measures 1.2Kohm maybe later the short circuit caused by me in one of the two usb ports. I do not understand instead the purpose of these two resistors positioned in series before pin 1 (5v) of the usb port: the 5v supplied by the motherboard via the red cable should already be stable, perhaps this resistor on the usb module is used when turning off the pc to to discharge the capacitor in parallel which is just before the resistor? I find nothing about it..

author: perieanuo
author_rank: Frequent Contributor
timestamp: August 02, 2021, 11:42:56 am
content: r7/r8 color suggests they are inductances

author: wraper
author_rank: Unknown
timestamp: August 02, 2021, 12:10:10 pm
content: They are ferrite beads, not resistors. I expect normal resistance to be < 0.1 Ohm

author: CharlotteSwiss
author_rank: Frequent Contributor
timestamp: August 02, 2021, 02:21:25 pm
content: thanks perieanuo and Wrapernow I understand why there was nothing written on it like the other resistances; he confused me the writing R on the PCB.So I think R7 ferrite bead is ok, and the smell of burning was on R8 ferrite bead which in fact now marks me a resistance of 1.2Kohm.Unfortunately I don't have any spare ferrite bead at home.The solutions could be:-I pass the old cards I have and see if I recognize any smd.-Or I have 1ohm resistors, but I don't think they are good ..-Or I could make a bridge and eliminate the ferrite bead R8 .... but I don't know if it's a smart solution ..Thanks guys for the valuable suggestionChar

author: wraper
author_rank: Unknown
timestamp: August 02, 2021, 02:25:34 pm
content: Quote from: CharlotteSwiss on August 02, 2021, 02:21:25 pmIt will work just fine. The worst that may happen EM emissions may become a tad worse.

author: CharlotteSwiss
author_rank: Frequent Contributor
timestamp: August 02, 2021, 02:53:28 pm
content: Quote from: wraper on August 02, 2021, 02:25:34 pmthanks wraper, you have been a gentleman.R7 ferrite bead I leave it, while I unsalt R8 ferrite bead and in its place I make a bridge of tin or copper.I learned something even today, happy

author: CharlotteSwiss
author_rank: Frequent Contributor
timestamp: August 02, 2021, 07:34:00 pm
content: On an old board I found a couple of smd, I believe they are ferrite bead (they measure about 0.1ohm); they were on the contacts of a serial port.I unsoldered and soldered them instead of the two ferrite beads on the usb board: being larger in size, I put some tin on the pitches, and then heated with hot air they soldered. The positioning is not the best, but the conductivity is good.Now I'll see when I attach these two USB ports to the old pc if they work properly ...

author: CharlotteSwiss
author_rank: Frequent Contributor
timestamp: August 02, 2021, 09:40:46 pm
content: Connected the two additional usb ports to the pc, everything is ok, both ports work correctly.Problem solved, I'm happy.Thanks Wraper and Perienanuo