Fine offset USB power

[ianboag]

Has anyone managed to power the sensor transmitter from a USB cable? I made up a connection - used three diodes to drop the 5v to 3 (give or take) but it all seems a bit intermittent. I figured the current must be diddly-squat given how long a pair of AAs last ....

[AllyCat]

Hi,

Yes, using forward diodes to drop the voltage is a BAD idea, because the average current can be so low, maybe only microamps, depending on the wind direction and anemometer turning.

I'd use a series resistor and a WHITE (or blue) LED as a shunt regulator (around 3 volts). But you may also need a large capacitor (perhaps 1000+ uF) for the surge of current (~20 mA) for 100+ ms, when the transmitter operates.

Cheers, Alan.

[Charlie]

Actually the whole idea is bad. If you truly want to run wires out to your wireless transmitter location, buy a cheap 3V power supply through eBay or similar. Running a long wire from outdoors to your computer is a great way to inject lightning into your computer to make the magic smoke leak out. Even if that doesn't happen, the additional noise injected is likely to cause problems with the computer (or the sensors!).
If you have a USB charger wall wart you really want to use, and you are happy to live with the risk and noise, then pick up a 3-terminal regulator from any electronics parts shop for about the same price as a couple diodes.

[ianboag]

I got one of those el cheapo buck power supplies which can give me a reliable 3v off USB.

I have a box on the bottom of the pole that contains the base station (FO), a USB cellular stick, and a (powered) USB hub. The four cameras at the top of the pole come back to the USB hub.

From the control point (where the PC lives) there is a 20m USB extender and 5v going to the hub.

The last connection on the hub goes to the buck thingo which makes 3v to go up the very top to the transmitter.

Hope that all makes sense ....