About Debugging Guidelines

In debugging – sometimes I got carried away with my “instinct”, although mostly it get me to the right conclusion in a short time – it is definitely too much of “black magic” to the newbies.
So my humble advice to the beginners is to think about every circuit or IC as a functional block – define the boundary yourself or from a schematic – make sense of the circuit does. To state the obvious, in a typical circuit block:

• will have a function name in the schematic - this contains important info regarding what the block supposed to do
• will generally has input, output, supply and control signals

identify which is which will generally give you a sense of how to debug the block

Once you know that the input/output relationship does not hold (assuming that you understand what the block supposed to do), here are some of the steps that might helps

1. Bias the circuit so that the block will have known state (or consider overwrite the input from a known source), use DMM to confirm all the DC pins, starting with supplies pins.
2. Confirm the input output relationship – if it is not correct, break this block into smaller block and repeat the process (take a look at binary search debugging post: http://electroniccircuitdesignsharing.blogspot.com/2013/02/debugbinary-search.html).
3. If the block is already an IC, check for part number, orientation, cold solder join, missing solder joints before “accusing” it as faulty part.
4. remember that sometimes certain failure mode needs more than a DMM to be used (see http://electroniccircuitdesignsharing.blogspot.com/2012/06/why-oscilloscope-is-needed-for-circuit.html) , so if DMM cannot give you convergence, you should consider to use another instrument (depending on what this block supposed to do).

so good luck with your troubleshooting!

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

Where and how does the current flow when the load resistor is being replaced by realistic next stage?

What’s the current loop for a step positive voltage at vin?

The flow:

  

Note that there’s no ground current involve other than the interconnect of 2 decoupling capacitors. – current does not always flows in ground interconnect