Kirchhoff's Law & Resistors - Confused!

[DigitalInfinity]

Hi There,

I'm studying digital electronics at school, and it seemed like a good idea to join this forum.

I've run into a slight confusion, which I'm hoping someone can clear up for me. We have a book that states resistors are used to limit current flowing through a load. However, Kirchhoff's 1st Law states that the amount of current flowing into a node must equal the current flowing out of it, i.e. the current remains constant throughout the circuit. I understood this to mean that it's the voltage that drops, not the current, but this book states that the resistor can be used to limit the current as well as the voltage.

Have I misinterpreted Kirchhoff's 1st Law?

[retiredfeline]

What does node mean to you?

[DigitalInfinity]

Anything on the circuit? Like if I have two resistors and an LED I'd have 3 nodes?

[retiredfeline]

See the first diagram illustrating one node. https://en.wikipedia.org/wiki/Kirchhoff's_circuit_laws

[Andree Henkel]

Hi There,...I've run into a slight confusion, which I'm hoping someone can clear up for me.

We have a book that states resistors are used to limit current flowing through a load.

correct

However, Kirchhoff's 1st Law states that the amount of current flowing into a node must equal the current flowing out of it,

also correct

 i.e. the current remains constant throughout the circuit. I understood this to mean that it's the voltage that drops, not the current, but this book states that the resistor can be used to limit the current as well as the voltage.[/quote]

Have I misinterpreted Kirchhoff's 1st Law?

Seems like. Each node at a point in time has a current flowing in and the (as this point in time) exactly same amount of current flowing out. There is nothing written that these currents are "constant". Kirchhoffs law also applies, when the currents change over time, for instance when driven by an AC Supply. And when you increase the resistors in circuit the currents will be smaller, but the sum of current into a node and out of if will still be zero.

[Brumby]

Kirchhoff's current law is simply descriptive - stating that the sum of all currents flowing into a node (a single point in a circuit) is equal to the sum of all currents flowing out of that node.  **

This does NOT determine what the actual values of the currents are.  Those are a function of voltage and resistance (and any other components).

These two ideas cross paths when you get into solving a network problem and the algebra takes over.  (But watch out!  Keep an eagle eye on the signs of your currents.)


** The water analogy is good here....  Imagine three or four pipes joining at a single fitting.  Kirchhoff's current law states the amount of water flowing into the fitting is the same as the amount of water flowing out.  With water pipes, that is rather obvious - assuming no leaks, of course.

[rstofer]

Kirchhoff's Voltage Law video
https://youtu.be/Bt6V7D5av9A

Kirchhoff's Current Law video
https://youtu.be/OYerdzZPSI0

These are very elementary videos.  Things get a lot more complex when we have voltage and current sources and especially when the sources are AC.

Notice Dave's use of Kirchhoff's Current Law
https://youtu.be/7FYHt5XviKc

KCL is specifically used at 5:00 here:
https://youtu.be/c6aKfEJn8gs

You can hardly get out of the bed in the morning without Kirchhoff's Laws.

[audio]

Resistor Normally Impede the flow of current.

Higher resistance allows smaller current through it and smaller resistance allows higher current through it.

Yes it is going to be used as current limiter.

hope this would make you clear.

[timenutgoblin]

Kirchhoff's 1st Law states that the amount of current flowing into a node must equal the current flowing out of it, i.e. the current remains constant throughout the circuit. I understood this to mean that it's the voltage that drops, not the current, but this book states that the resistor can be used to limit the current as well as the voltage.

Yes, the current flowing into a resistor is equal to the current flowing out of the resistor. However, Ohm's Law states that V = I x R which implies that there is a voltage drop - not a current drop - across the resistor.

According to Kirchoff's (Voltage) Law, the sum of the voltages across the load and the resistor must be equal to (i.e., the same) as the supply voltage for the circuit.

If you're driving an LED using a digital logic gate that operates at 3.3V (logic high) and the LED operates at 1.8V then the current-limiting resistor limits the maximum current for the LED and drops the voltage from 3.3V to 1.8V required for the LED. The 3.3V at the output of the digital logic gate acts as a voltage supply for the circuit. As you can see the sum of the voltage drops across the resistor and the LED is equal to supply voltage provided by the output of the digital logic gate.

[radiolistener]

We have a book that states resistors are used to limit current flowing through a load. However, Kirchhoff's 1st Law states that the amount of current flowing into a node must equal the current flowing out of it, i.e. the current remains constant throughout the circuit.

The current flowing into resistor equals the current flowing out of resistor. Because resistor limits both currents.

All looks good. What is not clear?

[andy3055]

OP: Seems like you are confused about the meaning of the term "Node." It is not a component but a point where two or more components are joined together.

[Capernicus]

Resistors drop the current flowing through a load, but they actually are included with the total resistance of the load, with the load.
Ohms law describes resistors.
Kirchoffs Law is about the wires, and junctions of the resistors.

Theres 2 basic setups for networks of resistors, and one drops the voltage, and one drops the current.
If you want to divvy out power to transistors from a raw power source, you need to know this.

Give yourself some time,  I works out current division all by myself, but I needed help to understand voltage division.

If you want some more hints, post again.

[gf]

I've run into a slight confusion, which I'm hoping someone can clear up for me. We have a book that states resistors are used to limit current flowing through a load. However, Kirchhoff's 1st Law states that the amount of current flowing into a node must equal the current flowing out of it, i.e. the current remains constant throughout the circuit. I understood this to mean that it's the voltage that drops, not the current, but this book states that the resistor can be used to limit the current as well as the voltage.

Voltage across a resistor and current through a resistor are always proportional to each other (V=I*R, or I=V/R). That's Ohm's Law.

If you apply a fixed voltage V to a resistor, this will result in a current (in the amount of I=V/R) flowing through the resistor. In this case the resistor determines (or "limits") the current drawn from the voltage source ¹⁾. OTOH, if you feed a fixed amount of current through a resistor, this will result in a voltage drop (in the amount of V=I*R) across the resistor. There can't be a current flowing though a resistor w/o voltage across the resitor (and vice versa).

Things are getting more complicated, though, when neither a constant voltage nor a constant current is apllied to the resistor, but when the resistor of interest is part of a complex circuit, where other components in the circuit also influence the voltage across the resistor and/or the current through the resistor. V=I*R still applies, but neither V nor I is a priori known then. Then you need to analyze the whole circuit ²⁾ in order to find either the current through the resistor, or the voltage across the resistor. Once the current through the resistor is known, the voltage across the resistor can be easily calulated as V=I*R, or vice versa, once the voltage across the resistor is known, the current through the resistor can be easily calculated as I=V/R.

And yes, the same amount of current which flows into the first pin of the resistor, also flows out from the second pin of the resistor, or in other words, the current flows through the resistor.

¹⁾ The term "limits the current" is usually used when the resistor is connected in series with another component, when this other component would draw a large amount of current when connected alone to the same voltage source. Due to the series resistor, the current though the component (and through the resistor) gets limited then.

²⁾ That's where Kirchoff's Laws come into the play as well. Based On Ohm's and Kirchoff's Laws you'll setup N equations with N unknowns for the circuit, and solve for various currents and voltages in the circuilt.