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enter image description hereI am using an Edimax EW-7811Un 150Mbps 11n Wi-Fi USB Adapter with my RPi B model to connect internet.

Interestingly, when I power the RPi with an USB cable via my laptop, the adapter works. However, if I power it with 5V (L298 5V output) external source, it does not.

One reason could be the different amount of current from two sources, but I am not sure. What could be the issue with this?

Thank you.

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    Could you list the current out for both power sources? – Darth Vader Jun 19 '16 at 21:06
  • Honestly, I do not know. The first source is the mini USB port of the RPi that connects to laptop. The other one is a motor driver's (L298) 5V output. I will try to find more information. – alamaranka Jun 19 '16 at 21:12
  • Can you post a photograph of the Pi connected to the motor driver board? – goobering Jun 19 '16 at 21:29
  • Added on the post. – alamaranka Jun 19 '16 at 21:41
  • I have doubts as to whether the 5V regulator on that board is capable of supplying the 2-2.5 amps needed by a Pi running a WiFi adapter. It would be worth checking it for a printed part number. Just in case you're not sure, in the photo above, the regulator is on its back, slightly obscured behind the round capacitor on the right hand side. It looks like this. Google the part number for a datasheet and check its voltage output. – goobering Jun 19 '16 at 23:36
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Your post raises several issues.

  1. You mention "the different amount of current from two sources". This is a misconception common on this site. Power sources SHOULD provide 5±0.25V and often list a current. This is the MAXIMUM current that can be safely drawn without causing the voltage to drop below its rated voltage. (Of course, very many supplies do not actually meet their published ratings.

  2. You are attempting to run the Pi on a computer USB port. These are rated at 500mA, and should only supply this if negotiated by the device, which the Pi cannot through the µUSB port (because it has no data connection). Again many computers just supply 5V, without any control, but it is inadvisable to rely on this.

  3. You are running an old model B Pi. These actually run on a few hundred mA (the 2-2.5A quoted by @goobering relates to the newer Pi 2 & 3, and even they do not actually NEED anywhere near this) This model can run from a USB port - just. Even so it is likely to be unreliable, especially if you are trying to power WiFi. These models have very low USB current capabilities (100mA for the release 2 of the Model B, lower for the earlier models).

It is far from clear how you are attempting to power the Pi from the motor driver. You need to measure the voltage it ACTUALLY delivers.

  • Do you think what is the best way to power RPi for remote control purposes? – alamaranka Jun 21 '16 at 20:27
  • @AliPala I don't know what you mean by "remote control purposes". Any well regulated 5V supply rated at 1A should power a Pi Model B with a single WiFi dongle. This can be via the normal connector or alternatively through the 5V pins. This depending on the current the dongle draws many need a powered hub. This model can also be "back-powered" via USB. – Milliways Jun 21 '16 at 23:52
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You need to check the spec of the 5v regulator on your motor driver module. Your laptop is likely supplying 200-500mA, but depending on the regulator on the motor driver it could be as low as 50mA or 100mA (which wont drive the Pi + Wifi).

There'll be a 3 pin 7805 somewhere on the board - find out full IC model number and google that. You can also just check the package type - surface mount versions usually only supply around 50mA, TO92 packages around 100mA and TO220 around 500mA

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  • As I know there is a light which blinks when the Edimax wifi adapter is operating, is it blinking?

  • If its not, then its much clear that the motors are taking most of the current from the source and so there's not much left for the pi to power up the wifi adapter.

  • To be sure, You can measure the current rating by a multimeter (in series), and then check whether it is enough or not from specification.

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