Life of a Teachers Assistant at RPI – Required Skills for the Job (Part 2)

Actually any dual channel scope can make differential measurements (using A-B mode). You just need to equalize the gain/offset of both channels.

One very common problem I often see, and is not generally taught in class, is how to properly compensate oscilloscope probes. People usually start using a scope without even checking to see if the probes are compensated. This can lead to erroneous conclusions about a circuits behavior which have nothing to do with the circuit and are entirely due to the probe. This applies especially to observing overshoot/undershoot on square waves.

The first thing one should do when using an oscilloscope is to connect the probe(s) to the calibration output on the front panel and adjust the compensating capacitor on the probe until the waveform is critically damped i.e., no overshoot/undershoot of the square wave.

As far as touching ICs to see if they're overheating, sooner or later you're going to to touch one that is really hot and you're going to burn your finger. This will happen if you spend any significant time debugging circuits. To avoid this lick your finger so there's a thin film of saliva on it before touching any hot components. This accomplishes two things. First if the component is above 100 deg C you will hear a sizzle and immediately withdraw your hand as a reflex. Second, the steam will act as a buffer before the component can actually burn your skin.

As a final note, one of the most important things you can impart to your students is a healthy respect for high voltage. And by high I mean anything over 60v. When working on live equipment ALWAYS know where the hazardous potentials are. Even if the equipment is not plugged in, high voltage capacitors can retain their charge for a long time. When I was much younger my parents were about to throw out an old (CRT) television and I thought I would cannibilize it for parts. I was snipping parts out of the HV flyback circuit when suddenly I found myself thrown across the room, my head had cracked the drywall. That CRT had retained a 20kV charge. The TV hadn't been plugged in for two months. Be careful with your assumptions!

As for touching components with your finger, we were never at a high enough voltage/current/power to where it would cause 3rd degree burns but could potentially make you withdraw your finger very quickly. Plus with a lab of about 20 kids, we don't always see or smell the smoke.

My assumptions were that you weren't going to be using things that plug into walls. Of course, there should be much more careful when working with higher voltages. I was merely speaking of smaller circuits (with 555 timers and 8 segment displays).

Ah the 'tongue tingle test'.. It's amazing how some people just won't believe you or try it even when you show 'em by doing it yourself!...they think it's going to kill them or it's a trick.
Now I wonder if this power lead is connected?...slurp yowl....

Del

What happened to instruments that make electrical measurements?---Although, to answer the last question of the original post--Deductive reasoning(and a snotty finger--I love short cuts when I troubleshoot too).

SteveM--Slobber? I remember an article in the Yakima Herald paper including pictures of a science instructor demonstrating this with his hand thrust into a crucible of molten aluminum. The hand was wet and the contents of the crucible was expelled and no serious burn, at least to his hand--the spectators may have burst into flame, but I am not sure.

KT,

I remember reading a similar article only it was molten solder.

As far as spectators bursting into flames, don't you hate it when that happens?

So it goes.

It could've been solder and may be the same one--I would guess that it was around twenty years ago.

As far as hating that--NOT IF THE SPECTATORS ARE EVIL NINJAS!

Poor guys family was killed--Damn ninjas.

Damn, he's got that poor apostrophe in the wrong plage....
Maybe he needs the money for punctuation lessons.

('Punch you Asians' lessons ?)

Del

Oh Del! There are 50,000 starving comedians out of work and you're cracking jokes--Have you no shame?-----GOOD ONE, though!