What is an acceptable voltage drop/resistance for a USB cable? - Page 1
Forum section: electronics / projects

author: HwAoRrDk
author_rank: Super Contributor
timestamp: February 17, 2017, 09:44:15 pm
content: I was trying to use my Atmel AVR ISP MkII (clone) today, but just couldn't get it to play ball. It was just locking up whenever I asked it do anything. After tearing my hair out trying all sorts of things like re-installing drivers, etc. on a hunch I decided to measure the +5V voltage level on the device. It was about 4.2V!Pretty sure that's too low to function properly. So I started trying to trace the cause of this voltage drop.Eventually, I narrowed it down to the USB A-to-Mini-B cable the device uses. For some reason - despite apparently working just fine for a number of years - this cable is suddenly dropping far too much voltage.I measured the resistance of the VBUS and GND conductors, and I got readings of 6 and 2 Ohms. Too much? This cable is only 3ft/0.9m long. What should the specs be for such a USB cable?To try and compare against something, I also measured a 3 meter USB A-to-B cable, and it showed as 0.5 Ohms for both conductors (assuming my cheap-o meter is measuring anywhere accurately at that level). This is more like what a proper USB cable should measure, right?I think I'll file this particular mini-B cable under 'T' (for Trash) and get a new one.And can anyone recommend a good quality but cheap brand of USB cables?

author: sleemanj
author_rank: Super Contributor
timestamp: February 17, 2017, 11:46:49 pm
content: Yes, too much. The minimum acceptable voltage for USB is 4.4v, so with your 8 Ohms in the wires you can't be drawing more than 75mA assuming you do indeed have 5v on the supply end (which you probably don't), that's low even by Chinese standards.Cheap cables intended for data or very low current only can get away with high resistance (thin or very poor) wire on the power lines.Since you have different resistance on VBus and Gnd, I'd say the higher of the two has developed a higher resistance, and probably it was supposed to be 2 Ohms on each, which wouldn't be unusual for cheap cables.

author: DaJMasta
author_rank: Super Contributor
timestamp: February 17, 2017, 11:55:16 pm
content: It's probably worth checking the source voltage to be sure, it may be a hair under... but your .8V drop would need just 100mA if your round trip cable resistance is 8 ohms.I haven't had to buy cheap USB cables in a long while, but in the states for cheap good quality misc cables, I usually use monoprice. They also sell through amazon, now.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: February 18, 2017, 01:54:29 am
content: Yes, I did check my supply voltage at the PC end. It was 5.08V - nice and healthy, so no worries there.Can't think why this USB cable has suddenly degraded, seemingly overnight. It's not even slightly damaged or anything. Perhaps the half-life of the Chinesium the wire is made from has been reached.I'm aware of Monoprice, but although their cables are dirt cheap, sadly the international shipping prices to the UK are a joke. Although... as I write, I've just found a couple of UK eBay sellers with their cables at reasonable prices (albeit still over twice the dollar price), so I might try one of them.

author: james_s
author_rank: Super Contributor
timestamp: February 18, 2017, 08:11:15 am
content: It's probably a bad connection right at one of the connectors, may only be a single strand actually touching. I've been burned a few times by crappy USB cables, one of them was brand new. Spent way too much time troubleshooting before I thought to try a different cable.

author: rolycat
author_rank: Super Contributor
timestamp: February 18, 2017, 09:05:51 am
content: If you are in the UK, I've found JuicEBitz cables to be very good. They use 20 AWG power wires and aren't outrageously priced. They currently do a range in red which helps to distinguish them.This1M cable is £3.65 from Amazon.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: February 18, 2017, 08:19:20 pm
content: I found this tech note PDF which was quite informative. Thought others might find it useful.http://www.gbs-elektronik.de/fileadmin/download/manuals/TN_Choosing_USB_Cable.pdfThanks for the tip about JuicEBitz cables. Although the one you linked was Micro USB, not Mini.