Raspberry-Pi power wiring mods – Part 2 15

Raspberry Pi power supply modifications

Following from the last post, I have been looking at the Raspberry Pi power supply and powering a USB external hard drive (HDD). I also changed from the 850mA Motorola phone charger to a 2 Amp 12V/5V supply (just connected the 5V) that was left over from a dead DVR.

Power feed direct to the USB connectors

I moved the hard-wired 5V feed from the fuse and protection diode directly to the dual USB connector. This is where the HDD will draw its power from.

Raspberry Pi power supply modifications

This arrangement was easily able to start the HDD and the Raspberry Pi when they were both powered on at the same time ie. the HDD already plugged in. The problem was that when the HDD was plugged in with the Raspberry Pi already operating, it would reset and reboot. On one occasion it must have corrupted the SD card; I had to reload it and start again.

Add a Capacitor to improve stability

Raspberry Pi power supply additional capacitor

The solution to this reboot problem was to attach a 1000uF 6.3V capacitor to the Raspberry Pi across the existing 220uF on-board capacitor which is on the incoming 5V supply.

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15 thoughts on “Raspberry-Pi power wiring mods – Part 2

  • PMB Post author

    Hello Neil
    Good question. I’ve had another look at the schematic. It shows a bi-directional transient suppressor (D17) across the 5V rail inside the fuse. The fuse only works if powering from the micro-USB connector.
    D17, being bi-directional, won’t protect against reverse polarity. I don’t know how tolerant the R-Pi is of reverse polarity, but I suspect it’s not and would very quickly fail. Best to double and triple check everything before applying power.

    D17 looks like a SMBJ5.0 which according to the data has a maximum clamping voltage of 9.2, so it’s probably best not to rely on it to protect against any more than very-very short duration over-voltage spikes.

    A series diode is the best reverse polarity protection, but you have to take into account the forward volt-drop and apply a higher voltage. Can be a bit tricky at low voltages; depending on diode type, current rating and forward current. A power mosfet can be used for reverse polarity protection. All more difficult to implement on the R-Pi. Or you could attach a 5A schottkey Barrier diode across the top of D17. The problem with this is that you also need a fuse and it must be rated to protect the PCB traces between the power feed point and the diode.

  • Neil

    A follow up to my question above, if it does in fact mean you could fry the Pi, then is there any particular diode you would recommend? I don’t know enough about electronics to know which type to use.. thanks.

  • Neil

    Very nice & simple mod. One question though, since the reverse polarity protection is being bypassed, does this mean if you don’t get the polarity right then the RPi is cactus??
    (note: “cactus” is Aussie slang for “dead” 🙂 )

  • PMB Post author

    Rated at 1.2A, that’s 6 watts. I just measured my R-Pi power draw: it fluctuates a bit, but on average: 4W on its own and 5.4W with a TP-Link USB wifi dongle plugged in.

    I don’t think there are any drivers installed for the TP-Link wifi. So it’s probably just idle. The LED just flashes slowly rather than flickering like it does with a connection. It may use more power when operating and transmitting.

    You may be running a bit close to the limit of the power supply. Add to that the impedance/losses in the cable and on-board power route from the micro-USB connector to the large USB connectors, and the initial surge is enough to reset the R-Pi.

    Can you try a larger power supply? I’m still running the big one shown in the picture


  • PMB Post author

    It’s probably a momentary (very brief) voltage dip/sag that’s causing the reboot. What type of power supply do you have and what’s the current rating? The power supply may be struggling. Feeding through the micro-USB, you still have the other impedances in circuit on the board. I’ve found some smaller plug-pak type power supplies cannot reliably deliver their full load without significant volt drop from the supply and in the lead.

  • Alex

    I tried to add a 1000f 6.3V capacitor on my RPI, but i didn’t gained a lot of power stability. When i plug my wifi dongle, my RPI still reboot. I’m still using the micro-usb port, but i was expecting more stability… Do you have any idea what’s wrong ? (I placed the capacitor as in the photo of your post, with the gray stripe outward).

  • Edu

    I am still waiting for my raspberry to arrive. I want to use it with a HDD connected to it.
    I found your solution the most reliable of the ones I saw on the internet.
    Will It also work in a rev 2 board?


  • PMB Post author

    With a HDD or other heavy load plugged into the USB on the R-Pi, most of the power consumed is going out the USB. The idea is to feed power in at a point close to the load. The USB connector pins provide a convenient attachment point. The large capacitor added on-board helps hold the voltage up at the R-Pi when the USB HDD starts up drawing a higher peak current and preventing the reset.

    When feeding power in at the GPIO, the current for the HDD or USB load has to pass through the tracks on the R-Pi board to get to the USB connector. Connections and impedance will effectively increase power supply noise and voltage dips, and increase chances of unreliability.

  • Dan

    This is awesome! I’ve been looking at numerous ways to best power the PI and all of them seem to fall short (I hate the micro usb idea and the usb powered hub idea..). This takes the cake. Cant wait to try it out!