I just started with electrics a few months ago, but now I am obsessed but still have MUCH to learn. What I am doing is essentially trial and error.

I needed a longer USB cable to a printer so I took a USB cable I had, cut it close to each end of it and then took the thickest 4 cables I could find and connected them to the USB connectors/ends. My idea was simply the thicker the better, the lower resistance the better. But it didn't work! The computer did not recognize the printer.

I read some more on USB and then I saw something about a need for 90 ohm impedance (imped... what?). Read some more and learned that impedance is sort of like AC resistance. After that I connected a 10 ohm resistor to D+ and a 20 ohm resistor to D- (not 90 ohm I know but that's all I had). Then I tested the cable again and it miraculously worked! I was quite confused that lower resistance is not always better in a cable.


My question is if my thinking is correct when I think that when measuring a DIY USB cable with a multimeter the resistance (sort of impedance) should be as close to 90 ohms as possible?

Some additional info added after asking

  • Cable length is about 2.5 meters (which is less than maximal 5 meters).
  • I measured with a multimeter before and after the adding of the resistors and + connected to +, - to -, D+ to D+, D- to D-, so I think the "wiring" is/was correct. It is not entirely unlikely that the connection was loose though and something fell out, but not that I noticed.
  • My extended cable works almost all the time except that sometimes when starting the printer then a USB network card connected to the same computer (as the printer) stops working/is disconnected, so I have to remove then reinsert the USB network card and it works again (so no permanent damage ;) ). And sometimes (after a long time of inactivity) it my computer looses connection to the printer (at least it doesn't print on command) and that is fixed by switching off the printer then switching it on again.
  • \$\begingroup\$ No. It is superficial. Search for impedance matching. Cables wil have zero or some mOhms resistance \$\endgroup\$
    – User323693
    Jul 9, 2015 at 13:23
  • \$\begingroup\$ What this question really needs is an answer about cable characteristic impedance and transmission-line effects ... \$\endgroup\$
    – brhans
    Jul 9, 2015 at 13:23
  • \$\begingroup\$ @Umar: I dont understand. Me adding the resistors and then it suddenly is not because of the reasons I think (that is that impedance gets closer to 90 ohms)? \$\endgroup\$ Jul 9, 2015 at 13:27
  • \$\begingroup\$ @brhans SE answers are not intended to be books. \$\endgroup\$
    – Matt Young
    Jul 9, 2015 at 13:27
  • \$\begingroup\$ @brhans: That sound complicated (for me). I have read a little bit on that but what I have found is currently above my level. \$\endgroup\$ Jul 9, 2015 at 13:28

3 Answers 3


USB cables are not simply "4 wires". They have certain properties that need to be met so that the data signals can travel through the cable. Your thick wires will not have the correct properties causing the data signals not to travel properly, instead they are suppressed and reflected resulting in the USB connection to fail.

  • \$\begingroup\$ Thank you. This is/was probably my fundamental misunderstanding. The most of the cabling I've done is with jumper wires on an Arduino and then I have not needed to think about different cable characteristics, and also I have not ever read anyone talk about characteristics of jumper cables in (simple) Arduino projects. I mean if using a breadboard then there will sort of be splices all over the place. I assume that cable characteristics is sometimes almost entirely irrelevant, but sometimes it is very relevant (for example USB cables...) depending upon what the cable is being used for. \$\endgroup\$ Jul 9, 2015 at 14:13
  • \$\begingroup\$ Exactly, USB works at signal frequencies where cable properties start to matter. The signals from/to an Arduino are significantly lower in frequency and distances are smaller so there cable properties matter much less. \$\endgroup\$ Jul 9, 2015 at 14:33

You may find this previous question helpful - Custom USB Impedance question.

Essentially, your problem is that impedance is not resistance (in the most general sense). Rather, resistance is impedance at DC and slightly higher frequencies. Since USB2 sends data at 480 Mbit/sec (peak) you need to consider questions of inductance and capacitance at GHz frequencies. Not only that, these issues have to apply over the entire cable length, so they become questions of geometry and spacing, rather than just adding a couple of components.

  • \$\begingroup\$ Thanks! I have not thought at all about capacitance and inductance of the cabling. \$\endgroup\$ Jul 9, 2015 at 15:17

For your USB cable, you need a differential impedance of 90 ohms between the lines and a 45 ohms on single-end wire impedance.

I think it is a hard project for a beginner, a more simple project could be to implement USB track on a PCB, but still that is pretty hard and not trivial.

For PCB, you can use the Saturn PCB toolkit that let you calculate differential impedance.

  • \$\begingroup\$ Thanks. Mhm. So the 90 ohms thingie is BETWEEN cables (i other words: the cables SHOULD affect each other and not be totally isolated from each other). So if impedance is measured on a correct USB cable the impedance on a single cable (in isolation) (btw just D+, D- or also the power lines?) should ideally be 45 ohms? And, just guessing here, the BETWEEN the cables should be as in a coaxial cable, that is I can't just add 90 ohm resistor between the cables. But... I could try for fun with a 90 ohm resistor connecting D+ and D- the cable and see what happens and if it would work better. \$\endgroup\$ Jul 11, 2015 at 14:29
  • \$\begingroup\$ Differential impedance is determine by the cable geometry, width, distance between the cable, the conductor choice, the dielectric between the 2 cable. It is cause by high frequency, and it takes a lot more than just basic electronics knownledge. \$\endgroup\$
    – MathieuL
    Jul 11, 2015 at 14:32
  • \$\begingroup\$ It is a complex problem that require simulation with software like FEMM or Matlab. Plus, you need good knownledge about high frequency design. \$\endgroup\$
    – MathieuL
    Jul 11, 2015 at 14:33
  • \$\begingroup\$ Aha "differential impedance". I see. At least now I have a better understanding of how NOT to do it and why it doesn't work perfectly. \$\endgroup\$ Jul 11, 2015 at 14:38
  • \$\begingroup\$ Impedance isn't resistance, impedance refer to V-I relation in a dynamic system ( frequency). Resistance is the V-I relation in a static system ( DC ) . \$\endgroup\$
    – MathieuL
    Jul 11, 2015 at 14:39

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