# Solving by Inspecton

Discussion in 'General Electronics Chat' started by KL7AJ, Apr 28, 2009.

1. ### KL7AJ Thread Starter AAC Fanatic!

Nov 4, 2008
2,208
427
Someone once said that youth and enthusiasm is no match for old age and treachery. This is especially true in electronics...especially complex network "figure-outing." Being really bad at mathematics during my formative years, I was forced to think about things in a very practical, mechanical way. I always looked for physical meaning instead of mathematical analysis first. With a little practice and experience you may find that many complex network problems can translate to rather simple "thought experiments" or solving by inspection.

I always had a gift for really irritating my colleages by taking these intuitive leaps toward the right answer in a lot of electronics problems...especially R.F. ones. Now that I'm teaching (well, I've always taught electronics...but have only been getting PAID to do it for a few months...) I find that I can irritate my students the same way.

I presented a complex Smith Chart exam last week. It included three series sections of transmission lines, each of a different characteristic impedance. I used an odd value of termination resistance at the load end, and the problem was to give me the source resistance. I told them it was a Smith Chart test. What I DIDN'T tell them was that the problem could be solved in your head. The lengths of transmission lines I chose (1/4, 3/4, and 6 lambda, respectively, gave values that either repeated the load resistance, inverted the load resistance, or made the characteristic impedance irrelevant!) I loved watching my students sweat this out, meticulously drawing circles and arcs for about an hour and a half. But the best part was showing how it could be done in 15 seconds without a Smith Chart at all....if they'd paid any attention to my "small print" lectures.

I just love this job!

Eric

2. ### PRS Well-Known Member

Aug 24, 2008
989
36
I've done the same. I combine intuition with experiment. The other day I thought I had hit upon a plan to make cheap protoboards -- gluing pennies to plexiglass and then soldering the parts onto the pennies, each penny being a node.

But are pennies copper? Seems to me I had heard they weren't. So I melted a penny with a propane torch and it became molten and of a silvery color -- not copper! I did a little research and found out they were made of zinc with a copper coating.

I flashed back to a color photo in my old chemistry book where zinc is immersed in a solution of water and liquid copper. It went in bright and shiney, came out looking reddish brown like a penny. I suppose this is how they make pennies?