Nuts and Volts - March 2015

display will show. However, it should work lower, with some caveats. Your question does bring up a very important point that needs to be addressed.

The ratings listed in datasheets are those that the component was designed for and tested to. If a part fails prior to reaching its maximum rating, then it is defective and — depending on return policies — should be replaced.

Fortunately, many components don't fail abruptly. They tend to deviate from their normal function. So, I expect my thermometer will need an adjustment to its calculated temperature as its gets colder. This can be done with software, so it's trivial to fix.

My flight computers and sensors produce output that changes gradually during a flight. Since I don't see an abrupt change in sensor output, I know there's no significant failure. There may still be calibration issues to deal with, but I expect they're minor.

Paul Verhage

Power to the USB

Thanks for the January 2015

PICAXE Primer column, even though I don't use PICAXE. It was an interesting look at USB that I've not looked into before. Out of curiosity, is there not a need to include any filtering caps on the supply? I'm sure that the supply from a PC is relatively clean, but what about the wall wart?

Andrew Retallack

I'm glad you found the January Primer article interesting, Andrew!

Your email prompted me to pull out my scope and run three separate tests. I connected a USB cable to the following three voltage sources, and took a look at the +5V power line in each case:

1. A USB port on my PC.

2. A powered USB hub.

3. A 5V (1A) USB wall wart.

To place a load on the supply, I connected a 1K resistor between +V and ground on the breadboard. As expected, the output voltage did vary slightly among the three power sources (from 5.2V to 5.4V), but otherwise, all three sources produced essentially the same result: There was a small high frequency ripple (about

18 mA 20 MHz).

I have been using an FTDI USB-to-serial cable for powering and

READER FEEDBACK Continued from page 7

Trenton Computer FestivalTM

The College of New Jersey, Ewing, NJ Saturday, March 21, 2015

9:00 am –– 5:00 pm

Registration/Flea Market open at 9 am, Talks start at 10: 15 am.

• Internet-of-Things • Android Wear talk by author Barry Burd • Project Tango • Android/iPhone Apps • Raspberry Pi Tutorial and Projects

Admission: $10.00 (advance) $12.00 at Gate Free Parking www.tcf-nj.org

The 2015 Trenton Computer Festival TM is sponsored by The College of New Jersey and its School of Engineering.

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