USB 5 Volt Booster

Most people have USB hubs connected to their computers and more than a few of those would have noticed that not all devices work with them, This is usually because there's not enough voltage coming out of each port on the hub. The four port hub I was using for instance only provides about 4.5V from each of its ports instead of the required 5V. This is usually solved by powered hubs using an external power supply that plugs into the wall to supplement what comes from the PC.

Five volts is supplied from the computer but it turns out that by the time it's been routed through a diode inside the hub thats job is to protect the computer this drops to below the acceptable mark needed by some devices. When using a wall wort power supply with one of these hubs this power isn't diverted through a diode. I however wanted to power my hub from the 5 volts coming out of one of the floppy drive power connectors inside the PC. This was so that when the computer is turned off power wouldn't be being drawn by a wall wart power supply.

I modded the HUB with this USB extension plug and 5V in plug.

        The simple version of the project. Just a 5V out socket.           

I soon discovered this wasn't going to be as easy as I thought. My frirst attempt was the simple connector in the pictures. It turns out that when the PC is off, the 5 volts USB stand-by routes through the hub and back into the PC, and into internal disc drives, the motherboard and anything else powered by 5 volts. I thought the fix would be easy. Just put a diode in series like the hubs do which only allows power to flow in one direction. The problem with this is that when power is routed through a diode there is voltage drop. It turns out the diodes with the least voltage drop are Schottky diodes. I obtained some of these that were rated high enough to deal with the load they would be subjected to and only had a rated voltage drop of around 0.25 volts. I put this in-line and this stopped the voltage feed back problem when the PC was off but only boosted the output from the hub from 4.5 to 4.8 volts. This still wasn't enough to make my external USB powered hard disc happy.

The only reliable option was going to be get something to drop the PCs' internal 12 volt supply to about 5.25 volts and then by the time it went through the diode it would be 5 volts so I set about making a module to do just that.

Looking around at various single chip voltage regulators I came across the National Semiconductor LM317 which looked like just the ticket. On its spec sheet it had various example circuit for different applications. I chose the one closest to my application and made a couple of very slight changes.

The voltage regulator project

LM317 in a TO-220 package

The circuit as suggested on the document.

Slight modifications to provide a variable adjustment.


The LM317 in the TO-220 package will cope with a 1.5A at a minimum, typically will deal with 2.2A and can cope with 3.4A momentarily.

A heat sink is definitely needed to bleed off the type of voltage in this situation but I underestimated the size that I'd need in the first version of the module and so had to change things up quite a lot in order to not require a fan.


All soldered up and nowhere to go

The completed circuits' underside showing the protection diode.


The heat sink that deals with the waste heat from the voltage regulator.

The completion of the first version.


After assembly it was time to test the contraption and set the output voltage. I connected it up to a 'disposable PC' just in case. This was after all the first test of this design. One thing to note. It's important not to run this circuit without a load so I made a dummy load made from LEDs and resistors. I set its output to 5.08V to make sure it had enough omph to power external laptop hard drives and such.

The heat sink that deals with the waste heat from the voltage regulator.


The Second Version.

As I mentioned the first heatsink wasn't up to the task so I thought I'd see what was available in the local Jaycar store. I grabbed the black one pictured but this also ran a little hotter than I would've liked under full load. I finally settled on sawing off the end of a Pentium II CPU heatsink. Because it is exposed and could potentially come in contact with the computers chassis it needs to be electrically isolated from the voltage regulator so I used mica insulators which also provide heat transfer as well.


Once intstalled in the PC, out of any air flow the heatsink under load is completely satisfactory. And now I can happily attach any USB compliant device to the hub sitting in front of my screen and it'll work.

Copyright (c) 2009 Greg Newsome all rights reserved.