A sample text widget

Etiam pulvinar consectetur dolor sed malesuada. Ut convallis euismod dolor nec pretium. Nunc ut tristique massa.

Nam sodales mi vitae dolor ullamcorper et vulputate enim accumsan. Morbi orci magna, tincidunt vitae molestie nec, molestie at mi. Nulla nulla lorem, suscipit in posuere in, interdum non magna.

Theory Vs. Reality: First Comes the Assumpton Then It All Goes to Hell

This is a post where I could think of a lot of very funny names and titles, but when it came down to the actually writing one I chose the term I most often use in professional life.  “First comes the Assumption Then It all Goes to Hell”.  Very often the calculations and number crunching is right on, but the initial assumptions are not right or not complete.   There are many ways to fall into this trap.  This is a case of experience is the best teacher and I am still discovering new traps.   Tonight I am only going to talk about a couple of the traps, but there are many.

The first one:  Reality vs Theory.   This is a big one, not only on trying to engineer, design, and construct physical things, but also much of the political discussions.  For us home experimenters this is really a big problem because we often do not do one thing often enough to gain the experience to even know where reality diverges from theory.  Currently, I am learning about one with the Desulfator circuit.   When we learn theory we start out with simplified pure devices and then start adding complications.to start making the device look more like reality.

For example:  Way back there I talked about how a battery really can be thought about as an ideal battery with a series resistor and the resistor gets larger has the battery discharges.   In another example we started out talking about an ideal op-amp and then as we started looking at data sheets reality hit us in the face and we had to start thinking about non-ideal things such as input impedance,current and voltage offset. As we talked about those problems we found workarounds or more importantly, cases where the ideal assumptions were valid.  For example, op-amps are a lot easier to use if the resistors are kept in the 1K to 100K range and 10K is even better.

Things we draw as wires are not really just wires, they are actually small resistors, inductors, and capacitors. Sometimes they can even be antennas.   Things can get complicated very fast as we start pushing our personal envelope of knowledge and experience.   We have to learn when are these “gotchas” important and when are they not.

The one currently messing with my life is one that I have just became aware of.  I am an electrical engineer but I work with industrial equipment and not in the circuit board area.   In electronics in am a hobbyist and do not have the opportunity to gain experience day-to-day.   That is why I chose to learn Kicad by working with a circuit that has been designed and tested.   However, even with something as straight forward as that, sometimes life is not as simple as your initial assumption.

One of the capacitors is specified as an “Electrolytic, low impedance type”…. UH OH… there is some special requirements.  The first question is:   What exactly does that mean?   Time to do some learning!

A web search came up with a couple of interesting articles for professional electronic circuit design engineers.  It looks as if there is a special term called ESR, equivalent series resistance.  The first article I found was “Low ESR Capacitors:  Fact or Fiction“.  This article says to me there really is no fixed standard on what defines a low ESR cap and that means what might have been a black-white decision is now turning into one of shades of grey.  More reading!   This article by QuadTech helped define the problem.   Although I am getting an idea of why Low ESR is important and the divergence between real capacitors and the simple theoretical one we draw on our diagrams, the following article explained why it probably really is important in the desulfator circuit:  “Low ESR Capacitors“.  The desulfator is intending to pulse the battery quickly with a relatively high current pulse.   Resistance in the capacitor will do several things:  It will reduce the energy stored, it will reduce the energy delivered, and it will cause heating and premature failure of the cap.

All of this means that if the author of the article went to the trouble of writing this special note, I can assume it is important.

Now the next assumption:  I assumed when approaching this project I could easily find all the parts.   That is not the case.  My standard low cost suppliers does not even have a comment in either the catalog or data sheets about the capacitor being low resistance.    That means I am now looking at other suppliers.   The problem is those suppliers do not carry all the hobby parts such as blank copper clad board.   Now I will probably have to deal with two shipping costs.   That is part of it when you like to  piddle around with designing and building things.

One of the other traps is assuming finding the physical size of parts will be easy.   Some parts are easy, for example resistors do come pretty much in standard sizes for the small wattage resistors.   However some of the other parts are much more complicated.   For example, the inductors can be purchased with either vertical or horizontal mounting.  Luckily all the suppliers provide links to data sheets and other sources of dimensional data.   This is necessary as I start to lay out the board and is costing me the better part of a day sitting on the computer.

The will be more design decisions and traps to come, but that is all part of it.   Again, all of that is part of the effort of gaining experience and it is not experience that can be learned easily.  I have now looked at the ESR of 100 uF caps.   It runs from 5Ω down to 27mΩ.  Big difference!  And, that is only for the ones listed, many manufactures do not list their ESR.

Good Luck on your projects.


If you found this post to be enjoyable and interesting please consider subscribing to this blog using one of the methods on the home page or the e-mail subscription form also found there and at the bottom of each page.

Creative Commons License
Theory Vs. Reality: First Comes the Assumpton Then It All Goes to Hell” by Create-and-Make.com is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.


Print Friendly

Leave a Reply

You can use these HTML tags

<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>