About AC-DC Adaptor–Heavier is better

So i heard from a long time ago – when choosing a AC-DC adaptor – heavier is better. Really?

For a circuit guy – like myself – I think what the saying is really trying to tell you that AC-DC adaptor with transformer built in is better. For this I can’t agree more... from low level signal perspective.

For a +5V AC adaptor without transformer – each and every node inside the adaptor circuit is considered “LIVE” which means it is dangerous – the AC outlet voltage 230V for example is directly switched by silicon IC, and filtered to give a output of +5V. See schematic below:

(picture from some online forum.. sorry that can’t remember where I got it exactly, been with me for some time)

For a +5V AC adaptor with transformer – generally the transformer will scale down the line voltage to much managed able 12VAC or so, before passing it through bridge rectifier to and further drop down to +5V. So the secondary side of the transformer is not “LIVE” and much safer to mess around. Note that since the switching is done at much lower voltage level – the electrical noise will be much less. As shown below:


There are considerations on which is better – costing, how “clean” it is..., eventually it is up to the application and individuals to decide. For me, when I build something that requires mV or nA level of accuracy – those with transformer are definitely my choice of adaptor. Other than that, it does not bother me much.

I guess at the end of the day, knowing the fundamental of different AC adaptors allow an engineer to decide what is good enough for the application, and this especially important when dealing with low level analogue signals.

3D Perspective of Test Pen – And How It Works

I have always been fascinated by how a test pen works – since 12 years old, heck, i even tried to power up one with 2 AA batteries in series at age of 12!

As i found it tough to explain how it works, i been having this idea to draw it out in 3D, so finally i did it! Since this is related to high voltage stuffs, do read the disclaimer at the bottom of this page.

Look at the picture below, we have a test pen that light up when in contact with a "LIVE" or "HOT" wire, what's the magic behind it?


See picture below for the “BIG” picture. Through the body capacitance the AC current completed its path to Earth wire or Neutral wire, and light up the neon light inside the test pen.

In the picture, note that there are wires being routed inside the wall, floor, or ceiling, and in some cases the metallic piping under the floor will provide the return path for AC current as well.

See diagram below for the equivalent circuit:


Lots more info at link below:
http://en.wikipedia.org/wiki/Test_light

Be thankful of AC transmission line

Recently I came across some pictures during Edison’s day, where the power line was in DC. Since there are lots of power loss (see Why Transfer Power At High Voltage), lots of cabling were required for power transmission. And the end result? Below is the picture that I managed to Google. At that time, in some part of the city people literally cannot see the sky!

Picture from wiki : http://en.wikipedia.org/wiki/Electric_power_transmission