keith wrote:
On Thu, 09 Feb 2012 10:12:31 -0800, Austen wrote:

Sorry, FWIW, for the benefit of onlookers who dont know this unit, it
has TWO usb ports: One for power in the back and a separate data-only
port to plug it into the computer at the side. It buzzes until the
side usb port is connected to a computer, - it gives a splash-screen
and then as soon as the connection is recognised, the buzzing stops.

Only caught a few of the posts in this thread, so if I'm too far off
base I apologise. Many of the cheap power supplies are half-wave
(rectified one way only instead of through a bridge. They rely on a
capacitor to modify the intermittent "DC" to a (sort of) constant
voltage. It may be that the design of the Nuvi has this capacitor
within the body of the unit, so that if this capacitor has gone down,
the smoothing will be non- existent. If the buzz is mains frequency
this would support the hypothesis. If you can find a good quality
full wave power supply, and this eliminates or markedly reduces the
buzz, it would (almost) prove the case.

Keith

Surely all small low-voltage power supplies these days are switched-mode
with a tiny high-frequency transformer. This transformer provides isolation
of output vs input circuits, allowing use of a bridge. Using a bridge
rectifier at the supply input reduces the size of the smoothing capacitor
needed there to keep the switching running throughout the AC cycle.
Virtually no mains-frequency should get to the DC output. And if it did it
is usually described as a hum (even for US 60Hz) rather than a "buzz"...

Mike.
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