My phone charger consists of two parts (like every smartphone charger nowadays); the adapter and the Micro USB cable. Some time ago, it had an accident and the cable has interruption now. So I bought a new cable. On the adapter, it writes "DC OUTPUT: 5V 2.1A". Does the cable I use affect V and A values? (There is no such information on the packing of the new cable...) There will probably no problem while charging my phone and making data transfer, but I plan to use this charger with my new (and first) Raspberry Pi. I don't want to accidentally kill it.


closed as off-topic by Marcus Müller, Brian Carlton, PeterJ, Dmitry Grigoryev, uint128_t Sep 4 '17 at 16:19

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  • 1
    \$\begingroup\$ If you get a really cheap "charge only" cable with no data lines, the USB power negotiations will fail (google it) and the current will be limited. Apart from that, no. \$\endgroup\$ – winny Aug 31 '17 at 19:10
  • \$\begingroup\$ Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design. \$\endgroup\$ – Marcus Müller Aug 31 '17 at 19:11
  • \$\begingroup\$ Your belief that your new cable will have no problem with data transfers might be very unfounded. If you are afraid that a standard 5V AC-DC adapter will "kill" your Raspberry Pi and you have a difficulty to afford another one, it might be a better idea to put it back into the box. \$\endgroup\$ – Ale..chenski Aug 31 '17 at 19:15
  • \$\begingroup\$ @winny It is making data transfer without problem (as expected), so I think it has data line. \$\endgroup\$ – user24346 Sep 1 '17 at 3:28
  • \$\begingroup\$ @MarcusMüller Sorry for that, I tried to choose the most revelant... \$\endgroup\$ – user24346 Sep 1 '17 at 3:31

In short, yes, I've found important differences between charging cables.

The original USB spec was 500mA, so old cables will have thinner wires than the ones which came out after most smartphones were able to draw 2 amps. These thinner wires were alright for 500mA, but for 2A the voltage drop can be a nuisance.

Also, copper is expensive. Recently, I wondered why my phone didn't charge with a new cable I tried, so I cut the cable and stripped it. The power wires inside were mostly insulator, with just a few tiny strands of copper.

I guess the cable been "cost-optimized".

Another example, I have a USB soundcard which draws close to the 500mA limit. It is finicky about cables, some work, some don't. It's all down to how much copper the manufacturer paid for.

A crap cable won't kill your Pi. Worst case, it will lose power and reboot. Annoying, but not catastrophic.

A $2 charger, on the other hand, is another can of worms. Some are really dangerous, some don't use proper mains-rated components, some don't have proper creepage tolerances, etc. I've seen teardowns where the XY caps were just half-cent ceramic discs which are absolutely not rated for this use.

So it's all down to cost, as usual.

  • \$\begingroup\$ The new cable's thickness looks same with the old one's. I found an app called Ampere and tested both cables, all results seem identical. Maximum is 1880 mA with both cables, so I guess it supports 2A. I don't know about cable prices, just bought one from the nearest supermarket. But it doesn't look cheap. In fact, it is very similar to the old cable. Thanks for your answer! \$\endgroup\$ – user24346 Sep 1 '17 at 5:01

In short, no, the cable does not meaningfully affect the voltage and current.

A micro-USB cable is just wires, so the voltage and amperage will be almost entirely dictated by the charger. I say "almost" because there is some small resistance in the wires, and so there will be a small (negligible for most of us) voltage drop across the wires. If the cable uses 20 AWG wire for power, a three foot cable will have a resistance of roughly 150 mOhms (75 mOhms in the Vcc wire, 75 mOhms in the return trip through the ground wire), and a 2-amp current will result in a 300 millvolt drop - your device will see 4.7 volts instead of 5v, which should be just fine. Some chargers provide a bit more than 5v to counteract the drop so that you get something closer to 5V at your device.


In theory it affects the voltage measured at the end of the cable. Practically it will be no problem. Just be sure that the voltage is the same and the current is at least the same.

I guess that you will connect a Pi 3. Be aware that it is a little bit sensitiv on voltage. I tried one with a standard 5V adapter with sufficient current and it did not turn on.

With the original adapter (which is rated with 5.1V) there was no problem.

And no, the cable choice will not harm the Raspberry Pi as long as you select the correct voltage for the adapter.

  • \$\begingroup\$ Mine is "ZERO Wireless", so I guess it needs less power than Pi 3. Will try to find a 5.1V adapter if it doesn't work. Thanks :) \$\endgroup\$ – user24346 Sep 1 '17 at 4:50
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    \$\begingroup\$ @Armin Dont guess or make assumptions, ask before. \$\endgroup\$ – MatsK Sep 1 '17 at 6:39
  • \$\begingroup\$ 5V 2.1A adapter and a thick cable (same thickness with the original cable of the adapter) worked fine with Raspberry Pi ZERO W. I don't know much about GPIO for now (I want to learn), but keyboard and wireless mouse is working. \$\endgroup\$ – user24346 Oct 22 '17 at 4:15

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