When things are not what they seem

[KeepItSimpleStupid]

I guess I learned my lesson in school when I challenged teach and won on a number of occasions and won. I would get answers like, your not supposed to know that yet or you have to answer the questions within the context of the course.

I've been hanging out in a very specialized group mainly as a spectator, but learning lot of stuff.

One thing that was talked about was a device that was home built that had three 1K resistors in series. Everyone was warned that you CANNOT replace the three resistors with a single 3K resistor, nor change from a leaded to an SMD part.

It turns out that in this particular application, the parasitic inductances and capacitances form a filter.

So, remember when you learned that resistances in series add? True in most cases, but not this one.

I've learned that you have to place yourself in whatever world your working in and learn the terminology for that world. What you learn is generally contingent on your ability to grasp the concepts or prior knowledge.

Be warned.

 

[jpanhalt]

Good story. From the field of chemistry, I could share a few others. In one case, a particular luciferin synthesis had to use "new" glassware. In another case, an assay using HPLC required that a 100-uL, not 50-uL syringe be used, even though the volume being injected was less than 50 uL.

The answer to the first riddle was never pinned down. The second answer was due to a difference in construction of the two sizes of Hamilton syringes, which led to increased carryover with the smaller syringe.

I am sure every field has similar gotcha's. The real fun is when you solve the puzzle of "why."

John

 

[Nigel Goodwin]

One thing that was talked about was a device that was home built that had three 1K resistors in series. Everyone was warned that you CANNOT replace the three resistors with a single 3K resistor, nor change from a leaded to an SMD part.

It turns out that in this particular application, the parasitic inductances and capacitances form a filter.

So, remember when you learned that resistances in series add? True in most cases, but not this one.

True in every case - including this one - but NOT true for capacitors and inductors in complicated circuits, which is what this is really about.

But what is missing of course is context - there's no context for the three resistors, their exact layout, or even mention of EXACTLY what type of resistors they were - presumably the original discussion provided that context?, making the reasoning fairly obvious (although it's still a horrible idea )

 

[RichTheDude]

Most resistors typically have a resistive region, capacitive, and finally inductive region.

Capacitors also typically have an inductive region, where the series L dominates. In high speed digital decoupling, various sizes of C and packages are used to lower the impedance across a wide bandwidth (smaller packages typically cope well at higher frequencies).

Need to be up into the realms of RF or high speed digital to worry about it mind .

 

[picbits]

Often resistors are used in series to increase their overall voltage handling rather than trying to find one single high voltage resistor. Again this is why you shouldn't replace them with surface mount or a single resistor under certain circumstances.

 

[KeepItSimpleStupid]

The context was the design of a very fast pulser for calibrating scopes. The mechanism employed was avalanche breakdown of a transistor. Rise times were measured in the picosecond range. An area where even lead length matters.

It wasn't this https://www.eevblog.com/forum/blog/eevblog-306-jim-williams-pulse-generator/ thread, but the same topic.