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Saturday, 23 March 2013

Thinking In Terms of Current

Some years ago, there’s this movie called “Lost In Translation”, now, I have not watched it, but that reminded me of how I used to viewed or analyse circuit diagrams. During my early days – I cannot think or view the circuit in terms of current flow, as such everything being translated into voltage terms. Since DMM is handy for measuring circuit voltages – things looked good.
But as years goes by – when I got better in circuit understanding – I realize that by translating current into voltage (using resistance, impedance, equations), there are valuable circuit insights being lost in translation. For example – if you don’t think in terms of current flows – you cannot understand how decoupling works shown in previous posts:
To show you what I meant, from my earlier post about load line:
There is this BJT amplifier shown as below:
In this circuit – assuming that it is in linear region, collector current changes with base current directly, in fact Icollector = hfe * Ibase, where hfe is the current gain of BJT. Yes, Vout might be our parameter of interest, but it really is just a “byproduct” of this action: Vout = Vdd – (Ic * RC).
So for beginner – I would like to say when we view or analyse circuit – it is worth the effort to try to think in terms of current – it will open a whole new world.

Friday, 1 March 2013

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: