Over my years, I have noticed there are two sides to *engineering*.

1.Practical Engineering

2.Theoretical Engineering


Without flooding this post with copy/paste definitions of the terms, I’ll keep this very simple.

1.Practical Engineering ” To have knowledge of designs by years of working with them on a hands on practice.

2.Theoretical Engineering ” To have the knowledge of designs by years of learning and working more so with the formulas and mathematics of how a design is going to work without an actual working unit.


Let us say that we have two engineers,

a) Practical



Both engineers are faced with the same exact problem. ( We will keep this simple )

*Problem* –

“What resistance is needed to not burn out an LED when the  only able voltage source is 9vDC”

Both engineers may know this answer instantly because they have come across this issue a hundred times in their lives. Where as, if they haven’t, they may go about it two different ways.

*Both know the forward voltage of the LED is 2.2vDC and the rated current flow is 20ma. So with that information, they can both solve this question in their own way.*


(Resistance)= ( Volt source – Voltage of LED ) / the current draw of the LED


R=(Vs – vLED) / I LED

R=resistance V=Voltage I=Current

S0, with that said, the *Theoretical* engineer already has enough information to plug into the formula.

(9 – 2.2 ) / .0200

6.8 / .0200

R=340 Ohm Resistor is needed for that circuit


And in all honesty, there is nothing wrong with *Theoretical* engineering. Look how quickly that answer was generated. Let’s look at the * Practical* engineer and how he/she is going to tackle this same problem.



The *practical* knows he/she is going to be using one led and a 9v DC battery. All that’s missing is that resistor value. So, He/she is just going to grab a few things,



3)9 volt battery

4)Potentiometer *adjustable resistor* or just some random resistors they have laying around the house/shop.

Logic and practice is going to tell them to start with a high resistance value and work down from there, the multi-meter is going to be used to capture the actual current flowing through the circuit. So with a higher than needed resistance the current is going to be lower that the wanted 20ma. So while slowly reducing the resistance the *Practical* engineer will be able to slowly tune in the value.

With the *practical* setup the current is measured at 18.6ma with a resistance of 340 OHMs (*banded value combination*) after starting from 800 Ohm and working back. Those numbers show how it’s almost right. But, by actually being hands on, the *practical* found there were a few differences from the original information given.

The power supply was only putting out 8.4 v DC which changes the values a bit when in formula.

What is actually in front of the *practical* is;

V=8.4Dc      ILED=18.6     RES=335   VLED=2.3 ( all very realistic tolerances in LED and resistor manufacturing.

Doing the math-

(8.4 – 2.3) / .0186

6.1 / .0186

Equals 327 Ohms.

*Now keep in mind there are a few variables to overcome. When dealing with OHMS law, some days it just makes me wanna beat my head against the wall. But factor in the calibration of the test equipment,resistance in traces,wires,terminations, the actual battery strength or power supply output, then the overall tolerance of the resistor’s *actual* value,and the LED’s tolerance, there will always be some sort of variance from the math to the actual circuit. (unless tolerance is at a minimum )

But for the most part, both *practical and theoretical * engineers achieved the desired outcome, both in their own ways.


Is *Practical* beneficial to saving a design company time and money? Most often not. But has great applications when dealing with existing circuits that are changing over time.

Time is money and the quickest way to the finish line is what a company needs to do to stay competitive.


BUT……. when would the *Practical* engineer’s ways be beneficial?

Let’s say, Neither of the engineers knew the actual forward voltage and current draw of an LED.. Now, there are some missing numbers. And often this is when the two sides of engineering have their wins and losses.

A *Practical* will be able to use his/her way to actually test out the LED, determine the limits of the LED by visual and past experience. The *Theoretical* will most likely reach out to the *practical* or, order the right LEDS with their expectations written on the package. Some *theoretical* engineers will also maintain the ability to utilize the same practices as the *practical*, but it all comes down to one thing, ~Experience~