Kirchhoff’s Voltage Law (KVL) and how important is it? - Page 1
Forum section: electronics / beginners

author: gilligan
author_rank: Contributor
timestamp: April 03, 2019, 07:32:02 pm
content: 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.

author: TimNJ
author_rank: Super Contributor
timestamp: April 03, 2019, 07:47:26 pm
content: 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.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 03, 2019, 10:34:22 pm
content: 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.

author: rstofer
author_rank: Super Contributor
timestamp: April 03, 2019, 11:20:11 pm
content: 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.

author: rstofer
author_rank: Super Contributor
timestamp: April 03, 2019, 11:27:32 pm
content: 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.

author: aheid
author_rank: Regular Contributor
timestamp: April 03, 2019, 11:41:08 pm
content: Without Kirchhoff's Laws, electrons could go wherever they please. Just imagine what kind of mess that would be.Good thing Kirchoff came along and got his laws instated so humankind could start to enjoy the benefits of civilized electronic circuits.

author: LateLesley
author_rank: Frequent Contributor
timestamp: April 03, 2019, 11:56:28 pm
content: It becomes pretty important when you start having to workout resistor networks. It ome in with equivalent circuits, because remember some inputs and outputs of ICs are not ideal, and have resistance. In some circuits you have to take this into account. For example, if you make a resistor divider out of two 10MΩ resistors, and try to measure the voltage with a multimeter, you'll measure less than half the voltage. Why? Because the meter effectively becomes another resistor in parallel with one of the others. And you can use Kirchoffs laws to work out what the voltage would be in that case.Also, its incorrect to think that any IC will self limit any current flowing through it. While some might have protected outputs/inputs, its not always the case, and there's always ways which will blow them up. Many ICs have let out their magic smoke. So you do have to be careful. That's where datasheets come in, they tell you the limits.

author: Wimberleytech
author_rank: Super Contributor
timestamp: April 04, 2019, 12:30:05 am
content: At the risk of being trite, I say that Kirchoff's laws must be applied to the most simple of all circuits: a voltage source and a resistor, in order to understand the circuit operation. Never thought of it before, but maybe Ohm's law is just a special case of Kirchoff's law.The mechanics, as it were, of applying Kirchoff's law should be separated from the law itself.

author: iMo
author_rank: Super Contributor
timestamp: April 04, 2019, 09:41:11 am
content: Ohm's law comes from Maxwell's equations for electromagnetism. The Kirchhoff's circuit laws come from "graph theory" in mathematics. They make EEs more happy as the laws (or "rules") simplify their lives.EEs rarely use KCLs "in a manual calculation" today as the circuits they mess with are usually more complex and the impedancies and voltage and current sources involved are also "complex values". KCLs are mostly used by EE students during their examsThey use circuit simulators instead.

author: Wimberleytech
author_rank: Super Contributor
timestamp: April 04, 2019, 01:12:32 pm
content: Quote from: imo on April 04, 2019, 09:41:11 amI will not speak for you...don't speak for me...an EE.I use KCL often in my analysis of circuits. No, I don't write a matrix and solve for all currents. However, mentally, I am always applying KCL at a node.When you put a voltage source across a resistor, implicit in your calculation of current is that the sum of the voltages in that loop equal zero. That is KVL, and that was my point.I have attached a circuit. Show me how you solve for the current in R3 without KVL.

author: TimFox
author_rank: Super Contributor
timestamp: April 04, 2019, 01:17:21 pm
content: Ohm's law is independent of Maxwell's equations. It is just a linear approximation to the relationship between current density and electric field, which is quite accurate for solid conductors and not appropriate to semiconductor junctions or space-charge limited vacuum diodes. Objects that satisfy Ohm’s Law are called “ohmic”.