Electronics > Beginners
Kirchhoff’s Voltage Law (KVL) and how important is it?
gilligan:
So, I've been around electronics all my life... my father repaired TV's and I don't remember when I didn't know how to solder.
But here I am at 42 years old and learning electronics way more indepth than I ever had. I guess my dad just wasn't a good teacher (thank you youtube (and Dave!)).
I'm reading about Kirchhoff’s Voltage Law and it makes total sense, but I'm also curious at how important it is and when is it not needed to calculate things like current limiting resistors and such?
So for example, LED... pretty important or it will blow up, that makes sense and you have a pretty healthy margin of error there. Then there is transistors, this becomes similar to LED's.
But what about like IC's? If I have an IC I assume I don't need to limit current. It will essentially limit itself right?
That I guess is where I get confused in when and where the EE meets the Iron. ;)
TimNJ:
Cool you've taken up learning electronics!
Using KVL to find the current limiting resistor in a LED circuit is a good practical example, but KVL isn't strictly limited to those types of circuits. I've used KVL the most when designing discrete transistor circuits. When you bias a transistor amplifier, you may want to account for the PN junctions in a BJT or the voltages at different points in your bias network. And, anytime you throw a zener in the circuit, you wind up using KVL.
An IC will only draw as much power as it needs. When operating in a steady state, you can sorta kinda model an IC as a resistor. On the other hand, once you exceed the forward voltage spec of an LED (or any diode), it effectively becomes a short circuit! That's what you need to limit the current there.
CatalinaWOW:
You are right, the KVL isn't too important when evaluating two component circuits. But as the component count goes up it gets more and more important, along with its brother the KCL. How much you will use them depends entirely on what piece of electronics you focus on. If you stick with digital (logic components, gate arrays, microprocessors and the like) you can almost completely ignore these laws. But if you work a lot with analog circuits you will find uses frequently and needs often.
One interesting and often overlooked application is in evaluating grounding systems. Again, doesn't matter if high accuracy or low noise isn't important. Just hook the grounds up any old way. But when trying to get max dynamic range or low system noise these kinds of analysis will be your friend.
rstofer:
Watch Dave's Op Amp video for an example of KCL. In fact, you just about can't analyze Op Amp circuits without KCL.
Around 18:00, he essentially uses KCL to show that the input current through the input resistor flows through the feedback resistor.
rstofer:
And don't forget to include Thevenin and Norton. In no particular order, you need: Ohm's Law, Kirchhoff's Laws, Thevenin Equivalent Circuits and Norton Equivalent Circuits.
These 4 (counting Kirchhoff as 2) are the basis for everything in electronics. Sure, it gets a lot deeper later but you can't even get started without these 4.
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