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I have a self made Isolation Board which consists of 2 Relay Switches and provides a kind switching function to cut off the power supplied to a Sensor Device on the other side of this Board.

The sensor device is battery operated and in common scenarios will take the power from the Raspberry Pi 2 via the Isolation Board.

Current Specifications in general for Raspberry Pi-2 and the world standard for USB-2.0 is rated approximately ~500mA. This creates a problem since the Sensor is battery operated and the ratings on it are 800mAh.

This means when the battery of the sensor is completely drained out and if I would like to get the Raspberry Pi to start charging it for me-

  • The sensor would try to get more current from the USB port of the Pi (>500mA). This creates an electrical malfunction and the Raspberry Pi HANGS. Hence I cannot do anything with the Pi.

Possible Solutions already tried:

  • I am already using a Standard 2A Power Adapter of a very decent company to fulfill the main power supply needs hence Checking the Power Adapter option is not in question

  • I have also tried doing the usb_max_current = 1 int the config.txt file for increasing the current at USB. This too fails and the problem still persists. The Schematic of the Pi splits the current evenly to all the 4 ports and hence getting a single port with 1A current is not possible and it would also not comply to the USB standards

Are there any other ways to tackle this situation? The only possible way is limiting the current from the Sensor board to close it to ~450mA but that could be the last resort for this problem.

Can I use adapters that have higher current ratings? e.g. 3 or 4 A current ratings and even if I do use them will they help to overcome this problem?

Clear Inferences

  • the Sensor battery is high current hungry Lithium Polymer (LiPo) battery and they will draw massive currents when the sensor is completely discharged.

  • another current hungry device are the Relays which are on the Isolation Board but somehow how they aren't the real devil here.

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  • Why not add a separate power supply for the relays and sensors? You could also try using a powered USB3 hub which has a power per port spec of 900 mA.
    – Steve Robillard
    Commented Apr 24, 2016 at 17:05
  • This seems to be an ultimate resort. But I am still trying to see if I can somehow tweak the Pi to get the work done.
    – Shan-Desai
    Commented Apr 24, 2016 at 17:06
  • not likely, for the reasons you have already mentioned.
    – Steve Robillard
    Commented Apr 24, 2016 at 17:08
  • So this does mean that complying to USB standards is the what we can conclude when it comes to observing this problem from the Raspberry Pi-2? Also I have been trying to change the charging circuit of the LiPo battery where according to the datasheet, current limiting is possible but sadly no luck. Hence I had to revert back to the Pi
    – Shan-Desai
    Commented Apr 24, 2016 at 17:11
  • Just get a powered USB HUB. You can get 2A, 3A 5A ... Also you said The Schematic of the Pi splits the current evenly to all the 4 ports ? You cannot split power (Amperes) up like that. Since Pi 1a to 3 the USB ports have changed allot, from polyfuses, to no fuses and now not really sure. Best is NOT to use the USB to power any thing, just for data. Check the Y-Cable answer too. But really Powered USB HUBs are the way to go.
    – Piotr Kula
    Commented Oct 31, 2016 at 11:56

2 Answers 2

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Easy fix to a common problem. Just use a USB "Y" cable that draws current from 2 USB jacks to supply higher powered USB devices. These cables can be purchased almost anywhere USB cables are sold. Check eBay for lowest prices.

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  • You can also chop the power only one off. You will see a red and black cable. Red +5v black -5v(negative) - You can wire that up to a dedicated power rail. But keep in mind that this cable can cause backpower via the data plug too.
    – Piotr Kula
    Commented Oct 31, 2016 at 11:58
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USB ports on the pi are limited to 400mA total: Link and it's probably a bad idea to try to draw more than 100mA from each. Exceeding that could lead to the dark side :)

You also have to watch out for back-feeding the pi from the USB ports if you have a powered USB device. A schottky diode is highly recommended if you cannot disconnect power from your device's usb ports.

What I would do is put the hefty power supply on the board, make sure there is no back-feeding through the USB port, and also use that to power the Pi (either sacrifice a μUSB cable or just solder +5V and GND to the Pi itself.

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  • That link is massively outdated - it refers to a model B which are no longer available. It has NOTHING to do with the Pi2 (or B+). I don't know why the Foundation doesn't update its Documentation.
    – Milliways
    Commented Apr 24, 2016 at 23:24
  • A Schottky Diode is all that is saving me at the moment. The back current from the LiPo is what causes the Relays to click and the Pi to go crazy. Hope to find a way around this soon.
    – Shan-Desai
    Commented Apr 25, 2016 at 12:48
  • -1 - Sorry but modyfing the PCB it self is not a good idea. And also where did you conjure up those numbers 400mA? The USB ports can supply max 1A. If it one device or all four, the total should not exceed 1A. Which means that 1 USB port can supply 1A as long as its only 1 device drawing that power and you are using a 2.5A power supply.
    – Piotr Kula
    Commented Oct 31, 2016 at 12:01
  • Not recommended? By whom? Anyone who modifies their device without knowing what they are doing gets what they deserve; the rest of us can modify it all we want. Who are you to determine what is and what is not recommended?
    – JayEye
    Commented Nov 28, 2016 at 21:48
  • The numbers are from the (potentially outdated) documentation for the Pi. Instead of complaining here, how about you fix the documentation?
    – JayEye
    Commented Nov 28, 2016 at 21:49

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