Happy Halloween from Adafruit! Check out some scary #electronichalloween posts, the frightening Halloween section and spooky videos!

September 21, 2012 AT 1:00 am

Raspberry Pi – Increasing the USB power #piday #raspberrypi @Raspberry_Pi

Photo-2

Raspberry Pi – Increasing the USB power.

I bought a Logitech C920 webcam last week. It’s a very nice unit which outputs Full HD (1920×1080) video. The picture is really clear but it draws more power than the Raspberry pi can supply.

For me this power problem affects my network connection. It seems that the Network adapter is very picky when it comes to it’s supply voltage.

The Webcam seems to draw about 240mA at 5 volts when it focuses, while the Raspberry Pi is limited to 140mA per USB port and 1.1A total current draw.

The Raspberry Pi limits these currents by way of Polyfuses which are a fuse made of a special polymer. They “blow” like a fuse if too much current goes through them, but then over time “heal” and conduct again. Unfortunately due to their inherit design, before they “blow” their resistance increases as the current increases. This can cause a drop in voltage, commonly referred to as voltage droop.

Without going into too much more detail, this basically means that as a USB device approaches a 140mA current draw, the voltage going to the device will drop and in much that same way, if the total current approaches 1.1 Amps, the system voltage will drop.

I decided the best way to fix this problem is to bypass the Polyfuses on the USB port with wire links. This would mean that the 140mA limit was bypassed, but the total 1.1 Amp limit would still apply. Note that this more than likely voids the warranty on your Raspberry Pi.

You first need to find the PolyFuses on your Raspberry Pi. They are easy to find, they’re the two green “blocks” on the board between the USB port header and the activity LEDs.


Each Friday is PiDay here at Adafruit, be sure to check out our posts, tutorials and new Raspberry Pi related products. Have you tried the new “Adafruit Raspberry Pi Educational Linux Distro” ? It’s our tweaked distribution for teaching electronics using the Raspberry Pi.

998
Want a FREE Raspberry Pi? For limited time and while supplies last – Adafruit is including a FREE Raspberry Pi in all orders over $350 (not including shipping)


Check out all the Circuit Playground Episodes! Our new kid’s show and subscribe!

Have an amazing project to share? Join the SHOW-AND-TELL every Wednesday night at 7:30pm ET on Google+ Hangouts.

Join us every Wednesday night at 8pm ET for Ask an Engineer!

Learn resistor values with Mho’s Resistance or get the best electronics calculator for engineers “Circuit Playground”Adafruit’s Apps!



1 Comment

  1. Would you not be better just using a powered USB hub into the Pi and connecting whatever you want into that instead ?

Sorry, the comment form is closed at this time.