FAQs / Cycle touring and bushwalking / USB (5V) power from different voltage sources

FAQs - Cycle touring and bushwalking

USB (5V) power from different voltage sources

5V from more than 5V

There are two basic options:

voltage regulator - LM7805; and
switchmode power supply - LM2576.

There is a third "option" - use a resistor to drop the voltage, but the resistor will have to get rid of the extra energy as heat and when using a power source like batteries, the voltage will drop as the batteries go flat. Both the voltage regulator and switchmode power supply will keep the output voltage (5V) constant even when the input voltage fluctuates.

Voltage regulator

Advantages:

wide input supply voltage (6-28V);
cheap;
easy to build - no external components required, 5 minute job.

Disadvantages:

inefficient - less than 40%, perhaps as low as 10%;
significant heat generated, so needs bulky heatsink for high current drain applications.

Switchmode power supply

Advantages:

higher efficiency (as much as 90%);

Disadvantages:

much more complicated circuit;
more expensive to buy and/or build.

The solution

Initially I used a LM7805 voltage reg (hot and inefficient) which I built into a USB cable.

5V LM7805 voltage regulator built into USB cable

Due to that being hot and inefficient, I ordered an Rx-Regulator or battery eliminator which is used on battery powered remote control models to eliminate the need for a separate Rx-battery pack.

The Rx-Regulator takes your input voltage and can output either 5V or 6V at up to 3A. Weight is only 9g. The particular type that I got also has a LED indicator that glows green when its powered and red when the supply voltage drops below 6V (indicating your LiPo pack is flat).

I soldered a suitable male plug to the power input and a femae USB connector the output.

Here's a view of the other side:

Update - December 2012

To get this power supply working properly with my Motorola Defy+ phone, you need to short the D- / D+ lines (or connect with 220Ohm resistor). If you only power the GND and +5V lines then the phone will not charge, it requires the D- / D+ lines to be connected to start charging.

Here's the complete setup, with 4 x 18650 bike light battery pack, USB breakout cable box with USB-A to USB-B micro, mini and Nokia round connectors, RX-Regulator wired to USB socket:

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