Strange USB cable - Page 1
Forum section: electronics / projects

author: dietert1
author_rank: Super Contributor
timestamp: August 27, 2025, 10:39:35 pm
content: Recently i made a little switch box that connects two JBL active speakers and a USB sound adapter to our Samsung TV. The idea was to use the 5 V output of the Samsung USB port to let a relay turn on and off mains for the speakers automatically when turning the TV on or off using its remote.In order to route the USB port through the box i bought a 1 m USB A-A cable at Sta. Efigenia for R$ 20 and a USB socket to solder into a small PCB. After wiring everything i ckecked the USB wires with a multimeter and found the wiring to be correct, except the white USB signal had an unexpected extra 18 Ohm resistance. Maybe a bad solder connection inside the A plug? Some impairment resistor to make the cheap cable inept for USB 2.0?I plugged the USB cable into an old USB hub that i can use for testing and heard the relay switch. When testing with the other devices in the intended setup the USB connection between the Samsung TV and the sound device did no longer work. The TV did not recognize the USB device.To check the USB cable i cut off the A plug and measured the resistances once more. It took an hour or so until i finally realized that the extra resistance of the white USB signal was inside and distributed along the cable. I looked at the wires under the microscope and found that the green wire is a strand of 6 wires and the white one is a strand of 7 wires. Apart from that they look similar. A rather special cable. I remember reading about strange cables, e.g. for scope probes, but not in the context of USB.Meanwhile i replaced that cable by another (even cheaper) USB 2.0 cable without that "feature" but with aluminium foil shield and now the switch box works as intended.Regards, Dieter

author: moffy
author_rank: Super Contributor
timestamp: August 27, 2025, 11:57:20 pm
content: It would be interesting to see what that white wire did to a pulse/square wave signal of say 1MHz, pulse response can be quite revealling.

author: thm_w
author_rank: Super Contributor
timestamp: August 28, 2025, 12:15:05 am
content: Doesn't make much sense for white cable to have 18 ohms resistance and green to be normal <1 ohm.I think its normal for these cheap USB cables to have higher than expected resistance:https://www.eevblog.com/forum/projects/what-is-an-acceptable-voltage-dropresistance-for-a-usb-cable/Though 18R is really high.If they sell "power saving power bars" in your country these can do what you've implemented here.

author: John B
author_rank: Frequent Contributor
timestamp: August 28, 2025, 12:29:58 am
content: The big concern here is isolation between mains and that USB port surely? I've never used those plated hole perf boards for mains stuff as it seems too easy for mains to track between conductive pads and make it's way to that USB port

author: dietert1
author_rank: Super Contributor
timestamp: August 28, 2025, 02:48:23 pm
content: Quote from: John B on August 28, 2025, 12:29:58 amYes, i agree that we should adhere to "good engineering practice" when showing something in this forum. Also it should be obvious that mains connected circuits aren't for beginners.The nearest point between USB and mains in my build is below the relay. The relay contacts provide a distance of 1/5" and meanwhile i managed to modify the board with an air gap there - by drilling the holes of the raster board with a 3 mm drill and then cleaning everything with a cutter. Now the isolation distance is 3 mm. Creep distance across other (unused) soldering points is also more than 3 mm.The relay HFD27 has a datasheet isolation spec of 1500 VAC between coil and contacts and 750 VAC between open contacts.Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: August 28, 2025, 03:46:01 pm
content: Quote from: thm_w on August 28, 2025, 12:15:05 amThanks for that hint.I found a master/slave power bar that we brought from Germany some years ago and tried to use it today. But it did not work, as the Samsung TV as master has a power saving burst mode when off. It takes energy from mains about once per second and the power bar is fast enough to operate its relay at each burst. The power bar has a sensitivity adjustment, but that did not really help. I would need to mod the power bar with a delay to make the scheme work as intended.In the end the JBL monitors ceased to work as their switching supplies did not like the repeating short mains pulses. Lucky enough they recovered after some minutes off.Regards, DieterEdit: boost -> burst

author: thm_w
author_rank: Super Contributor
timestamp: August 29, 2025, 12:39:54 am
content: Quote from: dietert1 on August 28, 2025, 03:46:01 pmInteresting, did not have that issue with my TV.Power barhas a simple slider option of ~8W or 15W trigger mode, sounds similar to what you have.